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PERF_EVENT_OPEN(2)                                                      Linux Programmer's Manual                                                     PERF_EVENT_OPEN(2)

NAME
       perf_event_open - set up performance monitoring

SYNOPSIS
       #include <linux/perf_event.h>    /* Definition of PERF_* constants */
       #include <linux/hw_breakpoint.h> /* Definition of HW_* constants */
       #include <sys/syscall.h>         /* Definition of SYS_* constants */
       #include <unistd.h>

       int syscall(SYS_perf_event_open, struct perf_event_attr *attr,
                   pid_t pid, int cpu, int group_fd, unsigned long flags);

       Note: glibc provides no wrapper for perf_event_open(), necessitating the use of syscall(2).

DESCRIPTION
       Given a list of parameters, perf_event_open() returns a file descriptor, for use in subsequent system calls (read(2), mmap(2), prctl(2), fcntl(2), etc.).

       A  call to perf_event_open() creates a file descriptor that allows measuring performance information.  Each file descriptor corresponds to one event that is mea‐
       sured; these can be grouped together to measure multiple events simultaneously.

       Events can be enabled and disabled in two ways: via ioctl(2) and via prctl(2).  When an event is disabled it does not count or generate overflows but  does  con‐
       tinue to exist and maintain its count value.

       Events  come  in  two  flavors:  counting and sampled.  A counting event is one that is used for counting the aggregate number of events that occur.  In general,
       counting event results are gathered with a read(2) call.  A sampling event periodically writes measurements to a buffer that can then be accessed via mmap(2).

   Arguments
       The pid and cpu arguments allow specifying which process and CPU to monitor:

       pid == 0 and cpu == -1
              This measures the calling process/thread on any CPU.

       pid == 0 and cpu >= 0
              This measures the calling process/thread only when running on the specified CPU.

       pid > 0 and cpu == -1
              This measures the specified process/thread on any CPU.

       pid > 0 and cpu >= 0
              This measures the specified process/thread only when running on the specified CPU.

       pid == -1 and cpu >= 0
              This measures all processes/threads on the specified CPU.  This requires CAP_PERFMON (since Linux 5.8) or CAP_SYS_ADMIN  capability  or  a  /proc/sys/ker‐
              nel/perf_event_paranoid value of less than 1.

       pid == -1 and cpu == -1
              This setting is invalid and will return an error.

       When  pid is greater than zero, permission to perform this system call is governed by CAP_PERFMON (since Linux 5.9) and a ptrace access mode PTRACE_MODE_READ_RE‐
       ALCREDS check on older Linux versions; see ptrace(2).

       The group_fd argument allows event groups to be created.  An event group has one event which is the group leader.  The leader is created first, with  group_fd  =
       -1.   The  rest  of the group members are created with subsequent perf_event_open() calls with group_fd being set to the file descriptor of the group leader.  (A
       single event on its own is created with group_fd = -1 and is considered to be a group with only 1 member.)  An event group is scheduled onto the CPU as  a  unit:
       it  will be put onto the CPU only if all of the events in the group can be put onto the CPU.  This means that the values of the member events can be meaningfully
       compared—added, divided (to get ratios), and so on—with each other, since they have counted events for the same set of executed instructions.

       The flags argument is formed by ORing together zero or more of the following values:

       PERF_FLAG_FD_CLOEXEC (since Linux 3.14)
              This flag enables the close-on-exec flag for the created event file descriptor, so that the file descriptor is automatically closed on execve(2).  Setting
              the  close-on-exec  flags  at  creation  time,  rather  than  later  with  fcntl(2),  avoids  potential  race  conditions where the calling thread invokes
              perf_event_open() and fcntl(2) at the same time as another thread calls fork(2) then execve(2).

       PERF_FLAG_FD_NO_GROUP
              This flag tells the event to ignore the group_fd parameter except for the purpose of setting up output redirection using the PERF_FLAG_FD_OUTPUT flag.

       PERF_FLAG_FD_OUTPUT (broken since Linux 2.6.35)
              This flag re-routes the event's sampled output to instead be included in the mmap buffer of the event specified by group_fd.

       PERF_FLAG_PID_CGROUP (since Linux 2.6.39)
              This flag activates per-container system-wide monitoring.  A container is an abstraction that isolates a set of resources for finer-grained control (CPUs,
              memory,  etc.).   In  this  mode, the event is measured only if the thread running on the monitored CPU belongs to the designated container (cgroup).  The
              cgroup is identified by passing a file descriptor opened on its directory in the cgroupfs filesystem.  For instance, if the cgroup to  monitor  is  called
              test,  then  a file descriptor opened on /dev/cgroup/test (assuming cgroupfs is mounted on /dev/cgroup) must be passed as the pid parameter.  cgroup moni‐
              toring is available only for system-wide events and may therefore require extra permissions.

       The perf_event_attr structure provides detailed configuration information for the event being created.

           struct perf_event_attr {
               __u32 type;                 /* Type of event */
               __u32 size;                 /* Size of attribute structure */
               __u64 config;               /* Type-specific configuration */

               union {
                   __u64 sample_period;    /* Period of sampling */
                   __u64 sample_freq;      /* Frequency of sampling */
               };

               __u64 sample_type;  /* Specifies values included in sample */
               __u64 read_format;  /* Specifies values returned in read */

               __u64 disabled       : 1,   /* off by default */
                     inherit        : 1,   /* children inherit it */
                     pinned         : 1,   /* must always be on PMU */
                     exclusive      : 1,   /* only group on PMU */
                     exclude_user   : 1,   /* don't count user */
                     exclude_kernel : 1,   /* don't count kernel */
                     exclude_hv     : 1,   /* don't count hypervisor */
                     exclude_idle   : 1,   /* don't count when idle */
                     mmap           : 1,   /* include mmap data */
                     comm           : 1,   /* include comm data */
                     freq           : 1,   /* use freq, not period */
                     inherit_stat   : 1,   /* per task counts */
                     enable_on_exec : 1,   /* next exec enables */
                     task           : 1,   /* trace fork/exit */
                     watermark      : 1,   /* wakeup_watermark */
                     precise_ip     : 2,   /* skid constraint */
                     mmap_data      : 1,   /* non-exec mmap data */
                     sample_id_all  : 1,   /* sample_type all events */
                     exclude_host   : 1,   /* don't count in host */
                     exclude_guest  : 1,   /* don't count in guest */
                     exclude_callchain_kernel : 1,
                                           /* exclude kernel callchains */
                     exclude_callchain_user   : 1,
                                           /* exclude user callchains */
                     mmap2          :  1,  /* include mmap with inode data */
                     comm_exec      :  1,  /* flag comm events that are
                                              due to exec */
                     use_clockid    :  1,  /* use clockid for time fields */
                     context_switch :  1,  /* context switch data */
                     write_backward :  1,  /* Write ring buffer from end
                                              to beginning */
                     namespaces     :  1,  /* include namespaces data */
                     ksymbol        :  1,  /* include ksymbol events */
                     bpf_event      :  1,  /* include bpf events */
                     aux_output     :  1,  /* generate AUX records
                                              instead of events */
                     cgroup         :  1,  /* include cgroup events */
                     text_poke      :  1,  /* include text poke events */

                     __reserved_1   : 30;

               union {
                   __u32 wakeup_events;    /* wakeup every n events */
                   __u32 wakeup_watermark; /* bytes before wakeup */
               };

               __u32     bp_type;          /* breakpoint type */

               union {
                   __u64 bp_addr;          /* breakpoint address */
                   __u64 kprobe_func;      /* for perf_kprobe */
                   __u64 uprobe_path;      /* for perf_uprobe */
                   __u64 config1;          /* extension of config */
               };

               union {
                   __u64 bp_len;           /* breakpoint length */
                   __u64 kprobe_addr;      /* with kprobe_func == NULL */
                   __u64 probe_offset;     /* for perf_[k,u]probe */
                   __u64 config2;          /* extension of config1 */
               };
               __u64 branch_sample_type;   /* enum perf_branch_sample_type */
               __u64 sample_regs_user;     /* user regs to dump on samples */
               __u32 sample_stack_user;    /* size of stack to dump on
                                              samples */
               __s32 clockid;              /* clock to use for time fields */
               __u64 sample_regs_intr;     /* regs to dump on samples */
               __u32 aux_watermark;        /* aux bytes before wakeup */
               __u16 sample_max_stack;     /* max frames in callchain */
               __u16 __reserved_2;         /* align to u64 */

           };

       The fields of the perf_event_attr structure are described in more detail below:

       type   This field specifies the overall event type.  It has one of the following values:

              PERF_TYPE_HARDWARE
                     This indicates one of the "generalized" hardware events provided by the kernel.  See the config field definition for more details.

              PERF_TYPE_SOFTWARE
                     This indicates one of the software-defined events provided by the kernel (even if no hardware support is available).

              PERF_TYPE_TRACEPOINT
                     This indicates a tracepoint provided by the kernel tracepoint infrastructure.

              PERF_TYPE_HW_CACHE
                     This indicates a hardware cache event.  This has a special encoding, described in the config field definition.

              PERF_TYPE_RAW
                     This indicates a "raw" implementation-specific event in the config field.

              PERF_TYPE_BREAKPOINT (since Linux 2.6.33)
                     This indicates a hardware breakpoint as provided by the CPU.  Breakpoints can be read/write accesses to an address as well as execution of  an  in‐
                     struction address.

              dynamic PMU
                     Since  Linux  2.6.38, perf_event_open() can support multiple PMUs.  To enable this, a value exported by the kernel can be used in the type field to
                     indicate which PMU to use.  The  value  to  use  can  be  found  in  the  sysfs  filesystem:  there  is  a  subdirectory  per  PMU  instance  under
                     /sys/bus/event_source/devices.   In each subdirectory there is a type file whose content is an integer that can be used in the type field.  For in‐
                     stance, /sys/bus/event_source/devices/cpu/type contains the value for the core CPU PMU, which is usually 4.

              kprobe and uprobe (since Linux 4.17)
                     These two dynamic PMUs create a kprobe/uprobe and attach it to the file descriptor generated by perf_event_open.  The  kprobe/uprobe  will  be  de‐
                     stroyed on the destruction of the file descriptor.  See fields kprobe_func, uprobe_path, kprobe_addr, and probe_offset for more details.

       size   The  size  of  the perf_event_attr structure for forward/backward compatibility.  Set this using sizeof(struct perf_event_attr) to allow the kernel to see
              the struct size at the time of compilation.

              The related define PERF_ATTR_SIZE_VER0 is set to 64; this was the size of the first published struct.  PERF_ATTR_SIZE_VER1 is 72, corresponding to the ad‐
              dition  of  breakpoints in Linux 2.6.33.  PERF_ATTR_SIZE_VER2 is 80 corresponding to the addition of branch sampling in Linux 3.4.  PERF_ATTR_SIZE_VER3 is
              96 corresponding to the addition of sample_regs_user and sample_stack_user in Linux 3.7.  PERF_ATTR_SIZE_VER4 is 104 corresponding to the addition of sam‐
              ple_regs_intr in Linux 3.19.  PERF_ATTR_SIZE_VER5 is 112 corresponding to the addition of aux_watermark in Linux 4.1.

       config This  specifies  which  event  you want, in conjunction with the type field.  The config1 and config2 fields are also taken into account in cases where 64
              bits is not enough to fully specify the event.  The encoding of these fields are event dependent.

              There are various ways to set the config field that are dependent on the value of the previously described type field.  What follows are various  possible
              settings for config separated out by type.

              If  type is PERF_TYPE_HARDWARE, we are measuring one of the generalized hardware CPU events.  Not all of these are available on all platforms.  Set config
              to one of the following:

                   PERF_COUNT_HW_CPU_CYCLES
                          Total cycles.  Be wary of what happens during CPU frequency scaling.

                   PERF_COUNT_HW_INSTRUCTIONS
                          Retired instructions.  Be careful, these can be affected by various issues, most notably hardware interrupt counts.

                   PERF_COUNT_HW_CACHE_REFERENCES
                          Cache accesses.  Usually this indicates Last Level Cache accesses but this may vary depending on your CPU.  This may  include  prefetches  and
                          coherency messages; again this depends on the design of your CPU.

                   PERF_COUNT_HW_CACHE_MISSES
                          Cache  misses.  Usually this indicates Last Level Cache misses; this is intended to be used in conjunction with the PERF_COUNT_HW_CACHE_REFER‐
                          ENCES event to calculate cache miss rates.

                   PERF_COUNT_HW_BRANCH_INSTRUCTIONS
                          Retired branch instructions.  Prior to Linux 2.6.35, this used the wrong event on AMD processors.

                   PERF_COUNT_HW_BRANCH_MISSES
                          Mispredicted branch instructions.

                   PERF_COUNT_HW_BUS_CYCLES
                          Bus cycles, which can be different from total cycles.

                   PERF_COUNT_HW_STALLED_CYCLES_FRONTEND (since Linux 3.0)
                          Stalled cycles during issue.

                   PERF_COUNT_HW_STALLED_CYCLES_BACKEND (since Linux 3.0)
                          Stalled cycles during retirement.

                   PERF_COUNT_HW_REF_CPU_CYCLES (since Linux 3.3)
                          Total cycles; not affected by CPU frequency scaling.

              If type is PERF_TYPE_SOFTWARE, we are measuring software events provided by the kernel.  Set config to one of the following:

                   PERF_COUNT_SW_CPU_CLOCK
                          This reports the CPU clock, a high-resolution per-CPU timer.

                   PERF_COUNT_SW_TASK_CLOCK
                          This reports a clock count specific to the task that is running.

                   PERF_COUNT_SW_PAGE_FAULTS
                          This reports the number of page faults.

                   PERF_COUNT_SW_CONTEXT_SWITCHES
                          This counts context switches.  Until Linux 2.6.34, these were all reported as user-space events, after that they are reported as happening  in
                          the kernel.

                   PERF_COUNT_SW_CPU_MIGRATIONS
                          This reports the number of times the process has migrated to a new CPU.

                   PERF_COUNT_SW_PAGE_FAULTS_MIN
                          This counts the number of minor page faults.  These did not require disk I/O to handle.

                   PERF_COUNT_SW_PAGE_FAULTS_MAJ
                          This counts the number of major page faults.  These required disk I/O to handle.

                   PERF_COUNT_SW_ALIGNMENT_FAULTS (since Linux 2.6.33)
                          This  counts  the  number of alignment faults.  These happen when unaligned memory accesses happen; the kernel can handle these but it reduces
                          performance.  This happens only on some architectures (never on x86).

                   PERF_COUNT_SW_EMULATION_FAULTS (since Linux 2.6.33)
                          This counts the number of emulation faults.  The kernel sometimes traps on unimplemented instructions and emulates them for user space.   This
                          can negatively impact performance.

                   PERF_COUNT_SW_DUMMY (since Linux 3.12)
                          This  is  a  placeholder  event that counts nothing.  Informational sample record types such as mmap or comm must be associated with an active
                          event.  This dummy event allows gathering such records without requiring a counting event.

              If type is PERF_TYPE_TRACEPOINT, then we are measuring kernel tracepoints.  The value  to  use  in  config  can  be  obtained  from  under  debugfs  trac‐
              ing/events/*/*/id if ftrace is enabled in the kernel.

              If type is PERF_TYPE_HW_CACHE, then we are measuring a hardware CPU cache event.  To calculate the appropriate config value, use the following equation:

                      config = (perf_hw_cache_id) |
                               (perf_hw_cache_op_id << 8) |
                               (perf_hw_cache_op_result_id << 16);

                  where perf_hw_cache_id is one of:

                      PERF_COUNT_HW_CACHE_L1D
                             for measuring Level 1 Data Cache

                      PERF_COUNT_HW_CACHE_L1I
                             for measuring Level 1 Instruction Cache

                      PERF_COUNT_HW_CACHE_LL
                             for measuring Last-Level Cache

                      PERF_COUNT_HW_CACHE_DTLB
                             for measuring the Data TLB

                      PERF_COUNT_HW_CACHE_ITLB
                             for measuring the Instruction TLB

                      PERF_COUNT_HW_CACHE_BPU
                             for measuring the branch prediction unit

                      PERF_COUNT_HW_CACHE_NODE (since Linux 3.1)
                             for measuring local memory accesses

                  and perf_hw_cache_op_id is one of:

                      PERF_COUNT_HW_CACHE_OP_READ
                             for read accesses

                      PERF_COUNT_HW_CACHE_OP_WRITE
                             for write accesses

                      PERF_COUNT_HW_CACHE_OP_PREFETCH
                             for prefetch accesses

                  and perf_hw_cache_op_result_id is one of:

                      PERF_COUNT_HW_CACHE_RESULT_ACCESS
                             to measure accesses

                      PERF_COUNT_HW_CACHE_RESULT_MISS
                             to measure misses

              If  type  is PERF_TYPE_RAW, then a custom "raw" config value is needed.  Most CPUs support events that are not covered by the "generalized" events.  These
              are implementation defined; see your CPU manual (for example the Intel Volume 3B documentation or the AMD BIOS and Kernel Developer Guide).   The  libpfm4
              library can be used to translate from the name in the architectural manuals to the raw hex value perf_event_open() expects in this field.

              If type is PERF_TYPE_BREAKPOINT, then leave config set to zero.  Its parameters are set in other places.

              If  type  is  kprobe or uprobe, set retprobe (bit 0 of config, see /sys/bus/event_source/devices/[k,u]probe/format/retprobe) for kretprobe/uretprobe.  See
              fields kprobe_func, uprobe_path, kprobe_addr, and probe_offset for more details.

       kprobe_func, uprobe_path, kprobe_addr, and probe_offset
              These fields describe the kprobe/uprobe for dynamic PMUs kprobe and uprobe.  For kprobe: use kprobe_func and probe_offset, or use  kprobe_addr  and  leave
              kprobe_func as NULL.  For uprobe: use uprobe_path and probe_offset.

       sample_period, sample_freq
              A "sampling" event is one that generates an overflow notification every N events, where N is given by sample_period.  A sampling event has sample_period >
              0.  When an overflow occurs, requested data is recorded in the mmap buffer.  The sample_type field controls what data is recorded on each overflow.

              sample_freq can be used if you wish to use frequency rather than period.  In this case, you set the freq flag.  The kernel will adjust the sampling period
              to try and achieve the desired rate.  The rate of adjustment is a timer tick.

       sample_type
              The  various  bits  in this field specify which values to include in the sample.  They will be recorded in a ring-buffer, which is available to user space
              using mmap(2).  The order in which the values are saved in the  sample  are  documented  in  the  MMAP  Layout  subsection  below;  it  is  not  the  enum
              perf_event_sample_format order.

              PERF_SAMPLE_IP
                     Records instruction pointer.

              PERF_SAMPLE_TID
                     Records the process and thread IDs.

              PERF_SAMPLE_TIME
                     Records a timestamp.

              PERF_SAMPLE_ADDR
                     Records an address, if applicable.

              PERF_SAMPLE_READ
                     Record counter values for all events in a group, not just the group leader.

              PERF_SAMPLE_CALLCHAIN
                     Records the callchain (stack backtrace).

              PERF_SAMPLE_ID
                     Records a unique ID for the opened event's group leader.

              PERF_SAMPLE_CPU
                     Records CPU number.

              PERF_SAMPLE_PERIOD
                     Records the current sampling period.

              PERF_SAMPLE_STREAM_ID
                     Records  a  unique ID for the opened event.  Unlike PERF_SAMPLE_ID the actual ID is returned, not the group leader.  This ID is the same as the one
                     returned by PERF_FORMAT_ID.

              PERF_SAMPLE_RAW
                     Records additional data, if applicable.  Usually returned by tracepoint events.

              PERF_SAMPLE_BRANCH_STACK (since Linux 3.4)
                     This provides a record of recent branches, as provided by CPU branch sampling hardware (such as Intel Last Branch Record).  Not all  hardware  sup‐
                     ports this feature.

                     See the branch_sample_type field for how to filter which branches are reported.

              PERF_SAMPLE_REGS_USER (since Linux 3.7)
                     Records the current user-level CPU register state (the values in the process before the kernel was called).

              PERF_SAMPLE_STACK_USER (since Linux 3.7)
                     Records the user level stack, allowing stack unwinding.

              PERF_SAMPLE_WEIGHT (since Linux 3.10)
                     Records  a  hardware provided weight value that expresses how costly the sampled event was.  This allows the hardware to highlight expensive events
                     in a profile.

              PERF_SAMPLE_DATA_SRC (since Linux 3.10)
                     Records the data source: where in the memory hierarchy the data associated with the sampled instruction came from.  This is available only  if  the
                     underlying hardware supports this feature.

              PERF_SAMPLE_IDENTIFIER (since Linux 3.12)
                     Places the SAMPLE_ID value in a fixed position in the record, either at the beginning (for sample events) or at the end (if a non-sample event).

                     This  was necessary because a sample stream may have records from various different event sources with different sample_type settings.  Parsing the
                     event stream properly was not possible because the format of the record was needed to find SAMPLE_ID, but the format could  not  be  found  without
                     knowing what event the sample belonged to (causing a circular dependency).

                     The PERF_SAMPLE_IDENTIFIER setting makes the event stream always parsable by putting SAMPLE_ID in a fixed location, even though it means having du‐
                     plicate SAMPLE_ID values in records.

              PERF_SAMPLE_TRANSACTION (since Linux 3.13)
                     Records reasons for transactional memory abort events (for example, from Intel TSX transactional memory support).

                     The precise_ip setting must be greater than 0 and a transactional memory abort event must be measured or no values will  be  recorded.   Also  note
                     that some perf_event measurements, such as sampled cycle counting, may cause extraneous aborts (by causing an interrupt during a transaction).

              PERF_SAMPLE_REGS_INTR (since Linux 3.19)
                     Records  a subset of the current CPU register state as specified by sample_regs_intr.  Unlike PERF_SAMPLE_REGS_USER the register values will return
                     kernel register state if the overflow happened while kernel code is running.  If the CPU supports hardware sampling of register state  (i.e.,  PEBS
                     on  Intel  x86)  and precise_ip is set higher than zero then the register values returned are those captured by hardware at the time of the sampled
                     instruction's retirement.

              PERF_SAMPLE_PHYS_ADDR (since Linux 4.13)
                     Records physical address of data like in PERF_SAMPLE_ADDR.

              PERF_SAMPLE_CGROUP (since Linux 5.7)
                     Records (perf_event) cgroup ID of the process.  This corresponds to the id field in the PERF_RECORD_CGROUP event.

       read_format
              This field specifies the format of the data returned by read(2) on a perf_event_open() file descriptor.

              PERF_FORMAT_TOTAL_TIME_ENABLED
                     Adds the 64-bit time_enabled field.  This can be used to calculate estimated totals if the PMU is overcommitted and multiplexing is happening.

              PERF_FORMAT_TOTAL_TIME_RUNNING
                     Adds the 64-bit time_running field.  This can be used to calculate estimated totals if the PMU is overcommitted and multiplexing is happening.

              PERF_FORMAT_ID
                     Adds a 64-bit unique value that corresponds to the event group.

              PERF_FORMAT_GROUP
                     Allows all counter values in an event group to be read with one read.

       disabled
              The disabled bit specifies whether the counter starts out disabled or enabled.  If disabled, the event can later be enabled by ioctl(2), prctl(2), or  en‐
              able_on_exec.

              When  creating  an event group, typically the group leader is initialized with disabled set to 1 and any child events are initialized with disabled set to
              0.  Despite disabled being 0, the child events will not start until the group leader is enabled.

       inherit
              The inherit bit specifies that this counter should count events of child tasks as well as the task specified.  This applies only to new children,  not  to
              any existing children at the time the counter is created (nor to any new children of existing children).

              Inherit does not work for some combinations of read_format values, such as PERF_FORMAT_GROUP.

       pinned The pinned bit specifies that the counter should always be on the CPU if at all possible.  It applies only to hardware counters and only to group leaders.
              If a pinned counter cannot be put onto the CPU (e.g., because there are not enough hardware counters or because of a conflict with some other event), then
              the counter goes into an 'error' state, where reads return end-of-file (i.e., read(2) returns 0) until the counter is subsequently enabled or disabled.

       exclusive
              The  exclusive  bit  specifies that when this counter's group is on the CPU, it should be the only group using the CPU's counters.  In the future this may
              allow monitoring programs to support PMU features that need to run alone so that they do not disrupt other hardware counters.

              Note that many unexpected situations may prevent events with the exclusive bit set from ever running.  This includes any users running a system-wide  mea‐
              surement as well as any kernel use of the performance counters (including the commonly enabled NMI Watchdog Timer interface).

       exclude_user
              If this bit is set, the count excludes events that happen in user space.

       exclude_kernel
              If this bit is set, the count excludes events that happen in kernel space.

       exclude_hv
              If  this  bit is set, the count excludes events that happen in the hypervisor.  This is mainly for PMUs that have built-in support for handling this (such
              as POWER).  Extra support is needed for handling hypervisor measurements on most machines.

       exclude_idle
              If set, don't count when the CPU is running the idle task.  While you can currently enable this for any event type, it is ignored  for  all  but  software
              events.

       mmap   The  mmap  bit  enables  generation of PERF_RECORD_MMAP samples for every mmap(2) call that has PROT_EXEC set.  This allows tools to notice new executable
              code being mapped into a program (dynamic shared libraries for example) so that addresses can be mapped back to the original code.

       comm   The comm bit enables tracking of process command name  as  modified  by  the  execve(2)  and  prctl(PR_SET_NAME)  system  calls  as  well  as  writing  to
              /proc/self/comm.  If the comm_exec flag is also successfully set (possible since Linux 3.16), then the misc flag PERF_RECORD_MISC_COMM_EXEC can be used to
              differentiate the execve(2) case from the others.

       freq   If this bit is set, then sample_frequency not sample_period is used when setting up the sampling interval.

       inherit_stat
              This bit enables saving of event counts on context switch for inherited tasks.  This is meaningful only if the inherit field is set.

       enable_on_exec
              If this bit is set, a counter is automatically enabled after a call to execve(2).

       task   If this bit is set, then fork/exit notifications are included in the ring buffer.

       watermark
              If set, have an overflow notification happen when we cross the wakeup_watermark boundary.  Otherwise, overflow notifications  happen  after  wakeup_events
              samples.

       precise_ip (since Linux 2.6.35)
              This  controls  the  amount  of  skid.  Skid is how many instructions execute between an event of interest happening and the kernel being able to stop and
              record the event.  Smaller skid is better and allows more accurate reporting of which events correspond to which instructions, but hardware is often  lim‐
              ited with how small this can be.

              The possible values of this field are the following:

              0  SAMPLE_IP can have arbitrary skid.

              1  SAMPLE_IP must have constant skid.

              2  SAMPLE_IP requested to have 0 skid.

              3  SAMPLE_IP must have 0 skid.  See also the description of PERF_RECORD_MISC_EXACT_IP.

       mmap_data (since Linux 2.6.36)
              This  is the counterpart of the mmap field.  This enables generation of PERF_RECORD_MMAP samples for mmap(2) calls that do not have PROT_EXEC set (for ex‐
              ample data and SysV shared memory).

       sample_id_all (since Linux 2.6.38)
              If set, then TID, TIME, ID, STREAM_ID, and CPU can additionally be included in non-PERF_RECORD_SAMPLEs if the corresponding sample_type is selected.

              If PERF_SAMPLE_IDENTIFIER is specified, then an additional ID value is included as the last value to ease parsing the record stream.  This may lead to the
              id value appearing twice.

              The layout is described by this pseudo-structure:

                  struct sample_id {
                      { u32 pid, tid; }   /* if PERF_SAMPLE_TID set */
                      { u64 time;     }   /* if PERF_SAMPLE_TIME set */
                      { u64 id;       }   /* if PERF_SAMPLE_ID set */
                      { u64 stream_id;}   /* if PERF_SAMPLE_STREAM_ID set  */
                      { u32 cpu, res; }   /* if PERF_SAMPLE_CPU set */
                      { u64 id;       }   /* if PERF_SAMPLE_IDENTIFIER set */
                  };

       exclude_host (since Linux 3.2)
              When  conducting measurements that include processes running VM instances (i.e., have executed a KVM_RUN ioctl(2)), only measure events happening inside a
              guest instance.  This is only meaningful outside the guests; this setting does not change counts gathered inside of a guest.  Currently, this  functional‐
              ity is x86 only.

       exclude_guest (since Linux 3.2)
              When  conducting measurements that include processes running VM instances (i.e., have executed a KVM_RUN ioctl(2)), do not measure events happening inside
              guest instances.  This is only meaningful outside the guests; this setting does not change counts gathered inside of a guest.  Currently, this functional‐
              ity is x86 only.

       exclude_callchain_kernel (since Linux 3.7)
              Do not include kernel callchains.

       exclude_callchain_user (since Linux 3.7)
              Do not include user callchains.

       mmap2 (since Linux 3.16)
              Generate  an extended executable mmap record that contains enough additional information to uniquely identify shared mappings.  The mmap flag must also be
              set for this to work.

       comm_exec (since Linux 3.16)
              This is purely a feature-detection flag, it does not change kernel behavior.  If this flag can successfully be  set,  then,  when  comm  is  enabled,  the
              PERF_RECORD_MISC_COMM_EXEC  flag  will  be  set  in  the misc field of a comm record header if the rename event being reported was caused by a call to ex‐
              ecve(2).  This allows tools to distinguish between the various types of process renaming.

       use_clockid (since Linux 4.1)
              This allows selecting which internal Linux clock to use when generating timestamps via the clockid field.  This can make it easier to correlate perf  sam‐
              ple times with timestamps generated by other tools.

       context_switch (since Linux 4.3)
              This  enables  the  generation  of PERF_RECORD_SWITCH records when a context switch occurs.  It also enables the generation of PERF_RECORD_SWITCH_CPU_WIDE
              records when sampling in CPU-wide mode.  This functionality is in addition to existing tracepoint and software events for measuring context switches.  The
              advantage of this method is that it will give full information even with strict perf_event_paranoid settings.

       write_backward (since Linux 4.6)
              This causes the ring buffer to be written from the end to the beginning.  This is to support reading from overwritable ring buffer.

       namespaces (since Linux 4.11)
              This  enables  the  generation of PERF_RECORD_NAMESPACES records when a task enters a new namespace.  Each namespace has a combination of device and inode
              numbers.

       ksymbol (since Linux 5.0)
              This enables the generation of PERF_RECORD_KSYMBOL records when new kernel symbols are registered or unregistered.  This is analyzing dynamic kernel func‐
              tions like eBPF.

       bpf_event (since Linux 5.0)
              This enables the generation of PERF_RECORD_BPF_EVENT records when an eBPF program is loaded or unloaded.

       auxevent (since Linux 5.4)
              This allows normal (non-AUX) events to generate data for AUX events if the hardware supports it.

       cgroup (since Linux 5.7)
              This enables the generation of PERF_RECORD_CGROUP records when a new cgroup is created (and activated).

       text_poke (since Linux 5.8)
              This enables the generation of PERF_RECORD_TEXT_POKE records when there's a change to the kernel text (i.e., self-modifying code).

       wakeup_events, wakeup_watermark
              This  union  sets  how  many samples (wakeup_events) or bytes (wakeup_watermark) happen before an overflow notification happens.  Which one is used is se‐
              lected by the watermark bit flag.

              wakeup_events counts only PERF_RECORD_SAMPLE record types.  To receive overflow notification for all PERF_RECORD types choose watermark and set wakeup_wa‐
              termark to 1.

              Prior to Linux 3.0, setting wakeup_events to 0 resulted in no overflow notifications; more recent kernels treat 0 the same as 1.

       bp_type (since Linux 2.6.33)
              This chooses the breakpoint type.  It is one of:

              HW_BREAKPOINT_EMPTY
                     No breakpoint.

              HW_BREAKPOINT_R
                     Count when we read the memory location.

              HW_BREAKPOINT_W
                     Count when we write the memory location.

              HW_BREAKPOINT_RW
                     Count when we read or write the memory location.

              HW_BREAKPOINT_X
                     Count when we execute code at the memory location.

              The values can be combined via a bitwise or, but the combination of HW_BREAKPOINT_R or HW_BREAKPOINT_W with HW_BREAKPOINT_X is not allowed.

       bp_addr (since Linux 2.6.33)
              This  is  the  address of the breakpoint.  For execution breakpoints, this is the memory address of the instruction of interest; for read and write break‐
              points, it is the memory address of the memory location of interest.

       config1 (since Linux 2.6.39)
              config1 is used for setting events that need an extra register or otherwise do not fit in the regular config field.  Raw OFFCORE_EVENTS  on  Nehalem/West‐
              mere/SandyBridge use this field on Linux 3.3 and later kernels.

       bp_len (since Linux 2.6.33)
              bp_len  is  the  length of the breakpoint being measured if type is PERF_TYPE_BREAKPOINT.  Options are HW_BREAKPOINT_LEN_1, HW_BREAKPOINT_LEN_2, HW_BREAK‐
              POINT_LEN_4, and HW_BREAKPOINT_LEN_8.  For an execution breakpoint, set this to sizeof(long).

       config2 (since Linux 2.6.39)
              config2 is a further extension of the config1 field.

       branch_sample_type (since Linux 3.4)
              If PERF_SAMPLE_BRANCH_STACK is enabled, then this specifies what branches to include in the branch record.

              The first part of the value is the privilege level, which is a combination of one of the values listed below.  If the user does not  set  privilege  level
              explicitly, the kernel will use the event's privilege level.  Event and branch privilege levels do not have to match.

              PERF_SAMPLE_BRANCH_USER
                     Branch target is in user space.

              PERF_SAMPLE_BRANCH_KERNEL
                     Branch target is in kernel space.

              PERF_SAMPLE_BRANCH_HV
                     Branch target is in hypervisor.

              PERF_SAMPLE_BRANCH_PLM_ALL
                     A convenience value that is the three preceding values ORed together.

              In addition to the privilege value, at least one or more of the following bits must be set.

              PERF_SAMPLE_BRANCH_ANY
                     Any branch type.

              PERF_SAMPLE_BRANCH_ANY_CALL
                     Any call branch (includes direct calls, indirect calls, and far jumps).

              PERF_SAMPLE_BRANCH_IND_CALL
                     Indirect calls.

              PERF_SAMPLE_BRANCH_CALL (since Linux 4.4)
                     Direct calls.

              PERF_SAMPLE_BRANCH_ANY_RETURN
                     Any return branch.

              PERF_SAMPLE_BRANCH_IND_JUMP (since Linux 4.2)
                     Indirect jumps.

              PERF_SAMPLE_BRANCH_COND (since Linux 3.16)
                     Conditional branches.

              PERF_SAMPLE_BRANCH_ABORT_TX (since Linux 3.11)
                     Transactional memory aborts.

              PERF_SAMPLE_BRANCH_IN_TX (since Linux 3.11)
                     Branch in transactional memory transaction.

              PERF_SAMPLE_BRANCH_NO_TX (since Linux 3.11)
                     Branch not in transactional memory transaction.  PERF_SAMPLE_BRANCH_CALL_STACK (since Linux 4.1) Branch is part of a hardware-generated call stack.
                     This requires hardware support, currently only found on Intel x86 Haswell or newer.

       sample_regs_user (since Linux 3.7)
              This bit mask defines the set of user CPU registers to dump on samples.  The layout of the register mask is architecture-specific and is described in  the
              kernel header file arch/ARCH/include/uapi/asm/perf_regs.h.

       sample_stack_user (since Linux 3.7)
              This defines the size of the user stack to dump if PERF_SAMPLE_STACK_USER is specified.

       clockid (since Linux 4.1)
              If  use_clockid  is set, then this field selects which internal Linux timer to use for timestamps.  The available timers are defined in linux/time.h, with
              CLOCK_MONOTONIC, CLOCK_MONOTONIC_RAW, CLOCK_REALTIME, CLOCK_BOOTTIME, and CLOCK_TAI currently supported.

       aux_watermark (since Linux 4.1)
              This specifies how much data is required to trigger a PERF_RECORD_AUX sample.

       sample_max_stack (since Linux 4.8)
              When sample_type includes PERF_SAMPLE_CALLCHAIN, this field specifies how many stack frames to report when generating the callchain.

   Reading results
       Once a perf_event_open() file descriptor has been opened, the values of the events can be read from the file descriptor.  The values that are there are specified
       by the read_format field in the attr structure at open time.

       If you attempt to read into a buffer that is not big enough to hold the data, the error ENOSPC results.

       Here is the layout of the data returned by a read:

       * If PERF_FORMAT_GROUP was specified to allow reading all events in a group at once:

             struct read_format {
                 u64 nr;            /* The number of events */
                 u64 time_enabled;  /* if PERF_FORMAT_TOTAL_TIME_ENABLED */
                 u64 time_running;  /* if PERF_FORMAT_TOTAL_TIME_RUNNING */
                 struct {
                     u64 value;     /* The value of the event */
                     u64 id;        /* if PERF_FORMAT_ID */
                 } values[nr];
             };

       * If PERF_FORMAT_GROUP was not specified:

             struct read_format {
                 u64 value;         /* The value of the event */
                 u64 time_enabled;  /* if PERF_FORMAT_TOTAL_TIME_ENABLED */
                 u64 time_running;  /* if PERF_FORMAT_TOTAL_TIME_RUNNING */
                 u64 id;            /* if PERF_FORMAT_ID */
             };

       The values read are as follows:

       nr     The number of events in this file descriptor.  Available only if PERF_FORMAT_GROUP was specified.

       time_enabled, time_running
              Total  time  the event was enabled and running.  Normally these values are the same.  Multiplexing happens if the number of events is more than the number
              of available PMU counter slots.  In that case the events run only part of the time and the time_enabled and time running values can be used  to  scale  an
              estimated value for the count.

       value  An unsigned 64-bit value containing the counter result.

       id     A globally unique value for this particular event; only present if PERF_FORMAT_ID was specified in read_format.

   MMAP layout
       When  using  perf_event_open() in sampled mode, asynchronous events (like counter overflow or PROT_EXEC mmap tracking) are logged into a ring-buffer.  This ring-
       buffer is created and accessed through mmap(2).

       The mmap size should be 1+2^n pages, where the first page is a metadata page (struct perf_event_mmap_page) that contains various  bits  of  information  such  as
       where the ring-buffer head is.

       Before kernel 2.6.39, there is a bug that means you must allocate an mmap ring buffer when sampling even if you do not plan to access it.

       The structure of the first metadata mmap page is as follows:

           struct perf_event_mmap_page {
               __u32 version;        /* version number of this structure */
               __u32 compat_version; /* lowest version this is compat with */
               __u32 lock;           /* seqlock for synchronization */
               __u32 index;          /* hardware counter identifier */
               __s64 offset;         /* add to hardware counter value */
               __u64 time_enabled;   /* time event active */
               __u64 time_running;   /* time event on CPU */
               union {
                   __u64   capabilities;
                   struct {
                       __u64 cap_usr_time / cap_usr_rdpmc / cap_bit0 : 1,
                             cap_bit0_is_deprecated : 1,
                             cap_user_rdpmc         : 1,
                             cap_user_time          : 1,
                             cap_user_time_zero     : 1,
                   };
               };
               __u16 pmc_width;
               __u16 time_shift;
               __u32 time_mult;
               __u64 time_offset;
               __u64 __reserved[120];   /* Pad to 1 k */
               __u64 data_head;         /* head in the data section */
               __u64 data_tail;         /* user-space written tail */
               __u64 data_offset;       /* where the buffer starts */
               __u64 data_size;         /* data buffer size */
               __u64 aux_head;
               __u64 aux_tail;
               __u64 aux_offset;
               __u64 aux_size;

           }

       The following list describes the fields in the perf_event_mmap_page structure in more detail:

       version
              Version number of this structure.

       compat_version
              The lowest version this is compatible with.

       lock   A seqlock for synchronization.

       index  A unique hardware counter identifier.

       offset When using rdpmc for reads this offset value must be added to the one returned by rdpmc to get the current total event count.

       time_enabled
              Time the event was active.

       time_running
              Time the event was running.

       cap_usr_time / cap_usr_rdpmc / cap_bit0 (since Linux 3.4)
              There  was  a  bug in the definition of cap_usr_time and cap_usr_rdpmc from Linux 3.4 until Linux 3.11.  Both bits were defined to point to the same loca‐
              tion, so it was impossible to know if cap_usr_time or cap_usr_rdpmc were actually set.

              Starting with Linux 3.12, these are renamed to cap_bit0 and you should use the cap_user_time and cap_user_rdpmc fields instead.

       cap_bit0_is_deprecated (since Linux 3.12)
              If set, this bit indicates that the kernel supports the properly separated cap_user_time and cap_user_rdpmc bits.

              If not-set, it indicates an older kernel where cap_usr_time and cap_usr_rdpmc map to the same bit and thus both features should be used with caution.

       cap_user_rdpmc (since Linux 3.12)
              If the hardware supports user-space read of performance counters without syscall (this is the "rdpmc" instruction on x86), then the following code can  be
              used to do a read:

                  u32 seq, time_mult, time_shift, idx, width;
                  u64 count, enabled, running;
                  u64 cyc, time_offset;

                  do {
                      seq = pc->lock;
                      barrier();
                      enabled = pc->time_enabled;
                      running = pc->time_running;

                      if (pc->cap_usr_time && enabled != running) {
                          cyc = rdtsc();
                          time_offset = pc->time_offset;
                          time_mult   = pc->time_mult;
                          time_shift  = pc->time_shift;
                      }

                      idx = pc->index;
                      count = pc->offset;

                      if (pc->cap_usr_rdpmc && idx) {
                          width = pc->pmc_width;
                          count += rdpmc(idx - 1);
                      }

                      barrier();
                  } while (pc->lock != seq);

       cap_user_time (since Linux 3.12)
              This bit indicates the hardware has a constant, nonstop timestamp counter (TSC on x86).

       cap_user_time_zero (since Linux 3.12)
              Indicates the presence of time_zero which allows mapping timestamp values to the hardware clock.

       pmc_width
              If  cap_usr_rdpmc, this field provides the bit-width of the value read using the rdpmc or equivalent instruction.  This can be used to sign extend the re‐
              sult like:

                  pmc <<= 64 - pmc_width;
                  pmc >>= 64 - pmc_width; // signed shift right
                  count += pmc;

       time_shift, time_mult, time_offset

              If cap_usr_time, these fields can be used to compute the time delta since time_enabled (in nanoseconds) using rdtsc or similar.

                  u64 quot, rem;
                  u64 delta;

                  quot  = cyc >> time_shift;
                  rem   = cyc & (((u64)1 << time_shift) - 1);
                  delta = time_offset + quot * time_mult +
                          ((rem * time_mult) >> time_shift);

              Where time_offset, time_mult, time_shift, and cyc are read in the seqcount loop described above.  This delta can then be added  to  enabled  and  possible
              running (if idx), improving the scaling:

                  enabled += delta;
                  if (idx)
                      running += delta;
                  quot  = count / running;
                  rem   = count % running;
                  count = quot * enabled + (rem * enabled) / running;

       time_zero (since Linux 3.12)

              If  cap_usr_time_zero  is  set, then the hardware clock (the TSC timestamp counter on x86) can be calculated from the time_zero, time_mult, and time_shift
              values:

                  time = timestamp - time_zero;
                  quot = time / time_mult;
                  rem  = time % time_mult;
                  cyc  = (quot << time_shift) + (rem << time_shift) / time_mult;

              And vice versa:

                  quot = cyc >> time_shift;
                  rem  = cyc & (((u64)1 << time_shift) - 1);
                  timestamp = time_zero + quot * time_mult +
                              ((rem * time_mult) >> time_shift);

       data_head
              This points to the head of the data section.  The value continuously increases, it does not wrap.  The value needs to be manually wrapped by the  size  of
              the mmap buffer before accessing the samples.

              On SMP-capable platforms, after reading the data_head value, user space should issue an rmb().

       data_tail
              When  the  mapping  is  PROT_WRITE,  the data_tail value should be written by user space to reflect the last read data.  In this case, the kernel will not
              overwrite unread data.

       data_offset (since Linux 4.1)
              Contains the offset of the location in the mmap buffer where perf sample data begins.

       data_size (since Linux 4.1)
              Contains the size of the perf sample region within the mmap buffer.

       aux_head, aux_tail, aux_offset, aux_size (since Linux 4.1)
              The AUX region allows mmap(2)-ing a separate sample buffer for high-bandwidth data streams (separate from the main perf sample buffer).  An example  of  a
              high-bandwidth stream is instruction tracing support, as is found in newer Intel processors.

              To set up an AUX area, first aux_offset needs to be set with an offset greater than data_offset+data_size and aux_size needs to be set to the desired buf‐
              fer size.  The desired offset and size must be page aligned, and the size must be a power of two.  These values are then passed to mmap in  order  to  map
              the  AUX  buffer.   Pages  in  the  AUX  buffer  are  included  as  part  of the RLIMIT_MEMLOCK resource limit (see setrlimit(2)), and also as part of the
              perf_event_mlock_kb allowance.

              By default, the AUX buffer will be truncated if it will not fit in the available space in the ring buffer.  If the AUX buffer is mapped  as  a  read  only
              buffer,  then  it  will operate in ring buffer mode where old data will be overwritten by new.  In overwrite mode, it might not be possible to infer where
              the new data began, and it is the consumer's job to disable measurement while reading to avoid possible data races.

              The aux_head and aux_tail ring buffer pointers have the same behavior and ordering rules as the previous described data_head and data_tail.

       The following 2^n ring-buffer pages have the layout described below.

       If perf_event_attr.sample_id_all is set, then all event types will have the sample_type selected fields related to where/when  (identity)  an  event  took  place
       (TID, TIME, ID, CPU, STREAM_ID) described in PERF_RECORD_SAMPLE below, it will be stashed just after the perf_event_header and the fields already present for the
       existing fields, that is, at the end of the payload.  This allows a newer perf.data file to be supported by older perf tools, with the new optional fields  being
       ignored.

       The mmap values start with a header:

           struct perf_event_header {
               __u32   type;
               __u16   misc;
               __u16   size;
           };

       Below, we describe the perf_event_header fields in more detail.  For ease of reading, the fields with shorter descriptions are presented first.

       size   This indicates the size of the record.

       misc   The misc field contains additional information about the sample.

              The  CPU mode can be determined from this value by masking with PERF_RECORD_MISC_CPUMODE_MASK and looking for one of the following (note these are not bit
              masks, only one can be set at a time):

              PERF_RECORD_MISC_CPUMODE_UNKNOWN
                     Unknown CPU mode.

              PERF_RECORD_MISC_KERNEL
                     Sample happened in the kernel.

              PERF_RECORD_MISC_USER
                     Sample happened in user code.

              PERF_RECORD_MISC_HYPERVISOR
                     Sample happened in the hypervisor.

              PERF_RECORD_MISC_GUEST_KERNEL (since Linux 2.6.35)
                     Sample happened in the guest kernel.

              PERF_RECORD_MISC_GUEST_USER  (since Linux 2.6.35)
                     Sample happened in guest user code.

              Since the following three statuses are generated by different record types, they alias to the same bit:

              PERF_RECORD_MISC_MMAP_DATA (since Linux 3.10)
                     This is set when the mapping is not executable; otherwise the mapping is executable.

              PERF_RECORD_MISC_COMM_EXEC (since Linux 3.16)
                     This is set for a PERF_RECORD_COMM record on kernels more recent than Linux 3.16 if a process name change was caused by an execve(2) system call.

              PERF_RECORD_MISC_SWITCH_OUT (since Linux 4.3)
                     When a PERF_RECORD_SWITCH or PERF_RECORD_SWITCH_CPU_WIDE record is generated, this bit indicates that the context switch is away from  the  current
                     process (instead of into the current process).

              In addition, the following bits can be set:

              PERF_RECORD_MISC_EXACT_IP
                     This indicates that the content of PERF_SAMPLE_IP points to the actual instruction that triggered the event.  See also perf_event_attr.precise_ip.

              PERF_RECORD_MISC_EXT_RESERVED (since Linux 2.6.35)
                     This indicates there is extended data available (currently not used).

              PERF_RECORD_MISC_PROC_MAP_PARSE_TIMEOUT
                     This  bit  is not set by the kernel.  It is reserved for the user-space perf utility to indicate that /proc/i[pid]/maps parsing was taking too long
                     and was stopped, and thus the mmap records may be truncated.

       type   The type value is one of the below.  The values in the corresponding record (that follows the header) depend on the type selected as shown.

              PERF_RECORD_MMAP
                  The MMAP events record the PROT_EXEC mappings so that we can correlate user-space IPs to code.  They have the following structure:

                      struct {
                          struct perf_event_header header;
                          u32    pid, tid;
                          u64    addr;
                          u64    len;
                          u64    pgoff;
                          char   filename[];
                      };

                  pid    is the process ID.

                  tid    is the thread ID.

                  addr   is the address of the allocated memory.  len is the length of the allocated memory.  pgoff is the page offset of the allocated  memory.   file‐
                         name is a string describing the backing of the allocated memory.

              PERF_RECORD_LOST
                  This record indicates when events are lost.

                      struct {
                          struct perf_event_header header;
                          u64    id;
                          u64    lost;
                          struct sample_id sample_id;
                      };

                  id     is the unique event ID for the samples that were lost.

                  lost   is the number of events that were lost.

              PERF_RECORD_COMM
                  This record indicates a change in the process name.

                      struct {
                          struct perf_event_header header;
                          u32    pid;
                          u32    tid;
                          char   comm[];
                          struct sample_id sample_id;
                      };

                  pid    is the process ID.

                  tid    is the thread ID.

                  comm   is a string containing the new name of the process.

              PERF_RECORD_EXIT
                  This record indicates a process exit event.

                      struct {
                          struct perf_event_header header;
                          u32    pid, ppid;
                          u32    tid, ptid;
                          u64    time;
                          struct sample_id sample_id;
                      };

              PERF_RECORD_THROTTLE, PERF_RECORD_UNTHROTTLE
                  This record indicates a throttle/unthrottle event.

                      struct {
                          struct perf_event_header header;
                          u64    time;
                          u64    id;
                          u64    stream_id;
                          struct sample_id sample_id;
                      };

              PERF_RECORD_FORK
                  This record indicates a fork event.

                      struct {
                          struct perf_event_header header;
                          u32    pid, ppid;
                          u32    tid, ptid;
                          u64    time;
                          struct sample_id sample_id;
                      };

              PERF_RECORD_READ
                  This record indicates a read event.

                      struct {
                          struct perf_event_header header;
                          u32    pid, tid;
                          struct read_format values;
                          struct sample_id sample_id;
                      };

              PERF_RECORD_SAMPLE
                  This record indicates a sample.

                      struct {
                          struct perf_event_header header;
                          u64    sample_id;   /* if PERF_SAMPLE_IDENTIFIER */
                          u64    ip;          /* if PERF_SAMPLE_IP */
                          u32    pid, tid;    /* if PERF_SAMPLE_TID */
                          u64    time;        /* if PERF_SAMPLE_TIME */
                          u64    addr;        /* if PERF_SAMPLE_ADDR */
                          u64    id;          /* if PERF_SAMPLE_ID */
                          u64    stream_id;   /* if PERF_SAMPLE_STREAM_ID */
                          u32    cpu, res;    /* if PERF_SAMPLE_CPU */
                          u64    period;      /* if PERF_SAMPLE_PERIOD */
                          struct read_format v;
                                              /* if PERF_SAMPLE_READ */
                          u64    nr;          /* if PERF_SAMPLE_CALLCHAIN */
                          u64    ips[nr];     /* if PERF_SAMPLE_CALLCHAIN */
                          u32    size;        /* if PERF_SAMPLE_RAW */
                          char   data[size];  /* if PERF_SAMPLE_RAW */
                          u64    bnr;         /* if PERF_SAMPLE_BRANCH_STACK */
                          struct perf_branch_entry lbr[bnr];
                                              /* if PERF_SAMPLE_BRANCH_STACK */
                          u64    abi;         /* if PERF_SAMPLE_REGS_USER */
                          u64    regs[weight(mask)];
                                              /* if PERF_SAMPLE_REGS_USER */
                          u64    size;        /* if PERF_SAMPLE_STACK_USER */
                          char   data[size];  /* if PERF_SAMPLE_STACK_USER */
                          u64    dyn_size;    /* if PERF_SAMPLE_STACK_USER &&
                                                 size != 0 */
                          u64    weight;      /* if PERF_SAMPLE_WEIGHT */
                          u64    data_src;    /* if PERF_SAMPLE_DATA_SRC */
                          u64    transaction; /* if PERF_SAMPLE_TRANSACTION */
                          u64    abi;         /* if PERF_SAMPLE_REGS_INTR */
                          u64    regs[weight(mask)];
                                              /* if PERF_SAMPLE_REGS_INTR */
                          u64    phys_addr;   /* if PERF_SAMPLE_PHYS_ADDR */
                          u64    cgroup;      /* if PERF_SAMPLE_CGROUP */
                      };

                  sample_id
                      If  PERF_SAMPLE_IDENTIFIER  is enabled, a 64-bit unique ID is included.  This is a duplication of the PERF_SAMPLE_ID id value, but included at the
                      beginning of the sample so parsers can easily obtain the value.

                  ip  If PERF_SAMPLE_IP is enabled, then a 64-bit instruction pointer value is included.

                  pid, tid
                      If PERF_SAMPLE_TID is enabled, then a 32-bit process ID and 32-bit thread ID are included.

                  time
                      If PERF_SAMPLE_TIME is enabled, then a 64-bit timestamp is included.  This is obtained via local_clock() which is a hardware timestamp  if  avail‐
                      able and the jiffies value if not.

                  addr
                      If  PERF_SAMPLE_ADDR  is  enabled, then a 64-bit address is included.  This is usually the address of a tracepoint, breakpoint, or software event;
                      otherwise the value is 0.

                  id  If PERF_SAMPLE_ID is enabled, a 64-bit unique ID is included.  If the event is a member of an event group, the group leader ID is returned.   This
                      ID is the same as the one returned by PERF_FORMAT_ID.

                  stream_id
                      If PERF_SAMPLE_STREAM_ID is enabled, a 64-bit unique ID is included.  Unlike PERF_SAMPLE_ID the actual ID is returned, not the group leader.  This
                      ID is the same as the one returned by PERF_FORMAT_ID.

                  cpu, res
                      If PERF_SAMPLE_CPU is enabled, this is a 32-bit value indicating which CPU was being used, in addition to a reserved (unused) 32-bit value.

                  period
                      If PERF_SAMPLE_PERIOD is enabled, a 64-bit value indicating the current sampling period is written.

                  v   If PERF_SAMPLE_READ is enabled, a structure of type read_format is included which has values for all events in the event group.   The  values  in‐
                      cluded depend on the read_format value used at perf_event_open() time.

                  nr, ips[nr]
                      If  PERF_SAMPLE_CALLCHAIN is enabled, then a 64-bit number is included which indicates how many following 64-bit instruction pointers will follow.
                      This is the current callchain.

                  size, data[size]
                      If PERF_SAMPLE_RAW is enabled, then a 32-bit value indicating size is included followed by an array of 8-bit values of length  size.   The  values
                      are padded with 0 to have 64-bit alignment.

                      This  RAW  record  data is opaque with respect to the ABI.  The ABI doesn't make any promises with respect to the stability of its content, it may
                      vary depending on event, hardware, and kernel version.

                  bnr, lbr[bnr]
                      If PERF_SAMPLE_BRANCH_STACK is enabled, then a 64-bit value indicating the number of records is included, followed by bnr perf_branch_entry struc‐
                      tures which each include the fields:

                      from   This indicates the source instruction (may not be a branch).

                      to     The branch target.

                      mispred
                             The branch target was mispredicted.

                      predicted
                             The branch target was predicted.

                      in_tx (since Linux 3.11)
                             The branch was in a transactional memory transaction.

                      abort (since Linux 3.11)
                             The branch was in an aborted transactional memory transaction.

                      cycles (since Linux 4.3)
                             This reports the number of cycles elapsed since the previous branch stack update.

                      The entries are from most to least recent, so the first entry has the most recent branch.

                      Support for mispred, predicted, and cycles is optional; if not supported, those values will be 0.

                      The type of branches recorded is specified by the branch_sample_type field.

                  abi, regs[weight(mask)]
                      If PERF_SAMPLE_REGS_USER is enabled, then the user CPU registers are recorded.

                      The abi field is one of PERF_SAMPLE_REGS_ABI_NONE, PERF_SAMPLE_REGS_ABI_32, or PERF_SAMPLE_REGS_ABI_64.

                      The  regs  field  is  an array of the CPU registers that were specified by the sample_regs_user attr field.  The number of values is the number of
                      bits set in the sample_regs_user bit mask.

                  size, data[size], dyn_size
                      If PERF_SAMPLE_STACK_USER is enabled, then the user stack is recorded.  This can be used to generate stack backtraces.  size is the size requested
                      by  the  user  in  sample_stack_user  or  else  the maximum record size.  data is the stack data (a raw dump of the memory pointed to by the stack
                      pointer at the time of sampling).  dyn_size is the amount of data actually dumped (can be less than size).  Note that dyn_size is omitted if  size
                      is 0.

                  weight
                      If  PERF_SAMPLE_WEIGHT  is enabled, then a 64-bit value provided by the hardware is recorded that indicates how costly the event was.  This allows
                      expensive events to stand out more clearly in profiles.

                  data_src
                      If PERF_SAMPLE_DATA_SRC is enabled, then a 64-bit value is recorded that is made up of the following fields:

                      mem_op
                          Type of opcode, a bitwise combination of:

                          PERF_MEM_OP_NA          Not available
                          PERF_MEM_OP_LOAD        Load instruction
                          PERF_MEM_OP_STORE       Store instruction
                          PERF_MEM_OP_PFETCH      Prefetch
                          PERF_MEM_OP_EXEC        Executable code

                      mem_lvl
                          Memory hierarchy level hit or miss, a bitwise combination of the following, shifted left by PERF_MEM_LVL_SHIFT:

                          PERF_MEM_LVL_NA         Not available
                          PERF_MEM_LVL_HIT        Hit
                          PERF_MEM_LVL_MISS       Miss
                          PERF_MEM_LVL_L1         Level 1 cache
                          PERF_MEM_LVL_LFB        Line fill buffer
                          PERF_MEM_LVL_L2         Level 2 cache
                          PERF_MEM_LVL_L3         Level 3 cache
                          PERF_MEM_LVL_LOC_RAM    Local DRAM
                          PERF_MEM_LVL_REM_RAM1   Remote DRAM 1 hop
                          PERF_MEM_LVL_REM_RAM2   Remote DRAM 2 hops
                          PERF_MEM_LVL_REM_CCE1   Remote cache 1 hop
                          PERF_MEM_LVL_REM_CCE2   Remote cache 2 hops
                          PERF_MEM_LVL_IO         I/O memory
                          PERF_MEM_LVL_UNC        Uncached memory

                      mem_snoop
                          Snoop mode, a bitwise combination of the following, shifted left by PERF_MEM_SNOOP_SHIFT:

                          PERF_MEM_SNOOP_NA       Not available
                          PERF_MEM_SNOOP_NONE     No snoop
                          PERF_MEM_SNOOP_HIT      Snoop hit
                          PERF_MEM_SNOOP_MISS     Snoop miss
                          PERF_MEM_SNOOP_HITM     Snoop hit modified

                      mem_lock
                          Lock instruction, a bitwise combination of the following, shifted left by PERF_MEM_LOCK_SHIFT:

                          PERF_MEM_LOCK_NA        Not available
                          PERF_MEM_LOCK_LOCKED    Locked transaction

                      mem_dtlb
                          TLB access hit or miss, a bitwise combination of the following, shifted left by PERF_MEM_TLB_SHIFT:

                          PERF_MEM_TLB_NA         Not available
                          PERF_MEM_TLB_HIT        Hit
                          PERF_MEM_TLB_MISS       Miss
                          PERF_MEM_TLB_L1         Level 1 TLB
                          PERF_MEM_TLB_L2         Level 2 TLB
                          PERF_MEM_TLB_WK         Hardware walker
                          PERF_MEM_TLB_OS         OS fault handler

                  transaction
                      If the PERF_SAMPLE_TRANSACTION flag is set, then a 64-bit field is recorded describing the sources of any transactional memory aborts.

                      The field is a bitwise combination of the following values:

                      PERF_TXN_ELISION
                             Abort from an elision type transaction (Intel-CPU-specific).

                      PERF_TXN_TRANSACTION
                             Abort from a generic transaction.

                      PERF_TXN_SYNC
                             Synchronous abort (related to the reported instruction).

                      PERF_TXN_ASYNC
                             Asynchronous abort (not related to the reported instruction).

                      PERF_TXN_RETRY
                             Retryable abort (retrying the transaction may have succeeded).

                      PERF_TXN_CONFLICT
                             Abort due to memory conflicts with other threads.

                      PERF_TXN_CAPACITY_WRITE
                             Abort due to write capacity overflow.

                      PERF_TXN_CAPACITY_READ
                             Abort due to read capacity overflow.

                      In addition, a user-specified abort code can be obtained from the high 32 bits of the field by shifting right by PERF_TXN_ABORT_SHIFT and  masking
                      with the value PERF_TXN_ABORT_MASK.

                  abi, regs[weight(mask)]
                      If PERF_SAMPLE_REGS_INTR is enabled, then the user CPU registers are recorded.

                      The abi field is one of PERF_SAMPLE_REGS_ABI_NONE, PERF_SAMPLE_REGS_ABI_32, or PERF_SAMPLE_REGS_ABI_64.

                      The  regs  field  is  an array of the CPU registers that were specified by the sample_regs_intr attr field.  The number of values is the number of
                      bits set in the sample_regs_intr bit mask.

                  phys_addr
                      If the PERF_SAMPLE_PHYS_ADDR flag is set, then the 64-bit physical address is recorded.

                  cgroup
                      If the PERF_SAMPLE_CGROUP flag is set, then the 64-bit cgroup ID (for the perf_event subsystem) is recorded.  To get the pathname of  the  cgroup,
                      the ID should match to one in a PERF_RECORD_CGROUP .

              PERF_RECORD_MMAP2
                  This  record  includes  extended  information  on  mmap(2) calls returning executable mappings.  The format is similar to that of the PERF_RECORD_MMAP
                  record, but includes extra values that allow uniquely identifying shared mappings.

                      struct {
                          struct perf_event_header header;
                          u32    pid;
                          u32    tid;
                          u64    addr;
                          u64    len;
                          u64    pgoff;
                          u32    maj;
                          u32    min;
                          u64    ino;
                          u64    ino_generation;
                          u32    prot;
                          u32    flags;
                          char   filename[];
                          struct sample_id sample_id;
                      };

                  pid    is the process ID.

                  tid    is the thread ID.

                  addr   is the address of the allocated memory.

                  len    is the length of the allocated memory.

                  pgoff  is the page offset of the allocated memory.

                  maj    is the major ID of the underlying device.

                  min    is the minor ID of the underlying device.

                  ino    is the inode number.

                  ino_generation
                         is the inode generation.

                  prot   is the protection information.

                  flags  is the flags information.

                  filename
                         is a string describing the backing of the allocated memory.

              PERF_RECORD_AUX (since Linux 4.1)
                  This record reports that new data is available in the separate AUX buffer region.

                      struct {
                          struct perf_event_header header;
                          u64    aux_offset;
                          u64    aux_size;
                          u64    flags;
                          struct sample_id sample_id;
                      };

                  aux_offset
                         offset in the AUX mmap region where the new data begins.

                  aux_size
                         size of the data made available.

                  flags  describes the AUX update.

                         PERF_AUX_FLAG_TRUNCATED
                                if set, then the data returned was truncated to fit the available buffer size.

                         PERF_AUX_FLAG_OVERWRITE
                                if set, then the data returned has overwritten previous data.

              PERF_RECORD_ITRACE_START (since Linux 4.1)
                  This record indicates which process has initiated an instruction trace event, allowing tools to properly correlate the instruction  addresses  in  the
                  AUX buffer with the proper executable.

                      struct {
                          struct perf_event_header header;
                          u32    pid;
                          u32    tid;
                      };

                  pid    process ID of the thread starting an instruction trace.

                  tid    thread ID of the thread starting an instruction trace.

              PERF_RECORD_LOST_SAMPLES (since Linux 4.2)
                  When using hardware sampling (such as Intel PEBS) this record indicates some number of samples that may have been lost.

                      struct {
                          struct perf_event_header header;
                          u64    lost;
                          struct sample_id sample_id;
                      };

                  lost   the number of potentially lost samples.

              PERF_RECORD_SWITCH (since Linux 4.3)
                  This  record indicates a context switch has happened.  The PERF_RECORD_MISC_SWITCH_OUT bit in the misc field indicates whether it was a context switch
                  into or away from the current process.

                      struct {
                          struct perf_event_header header;
                          struct sample_id sample_id;
                      };

              PERF_RECORD_SWITCH_CPU_WIDE (since Linux 4.3)
                  As with PERF_RECORD_SWITCH this record indicates a context switch has happened, but it only occurs when sampling in CPU-wide mode and  provides  addi‐
                  tional  information  on  the process being switched to/from.  The PERF_RECORD_MISC_SWITCH_OUT bit in the misc field indicates whether it was a context
                  switch into or away from the current process.

                      struct {
                          struct perf_event_header header;
                          u32 next_prev_pid;
                          u32 next_prev_tid;
                          struct sample_id sample_id;
                      };

                  next_prev_pid
                         The process ID of the previous (if switching in) or next (if switching out) process on the CPU.

                  next_prev_tid
                         The thread ID of the previous (if switching in) or next (if switching out) thread on the CPU.

              PERF_RECORD_NAMESPACES (since Linux 4.11)
                  This record includes various namespace information of a process.

                      struct {
                          struct perf_event_header header;
                          u32    pid;
                          u32    tid;
                          u64    nr_namespaces;
                          struct { u64 dev, inode } [nr_namespaces];
                          struct sample_id sample_id;
                      };

                  pid    is the process ID

                  tid    is the thread ID

                  nr_namespace
                         is the number of namespaces in this record

                  Each namespace has dev and inode fields and is recorded in the fixed position like below:

                  NET_NS_INDEX=0
                         Network namespace

                  UTS_NS_INDEX=1
                         UTS namespace

                  IPC_NS_INDEX=2
                         IPC namespace

                  PID_NS_INDEX=3
                         PID namespace

                  USER_NS_INDEX=4
                         User namespace

                  MNT_NS_INDEX=5
                         Mount namespace

                  CGROUP_NS_INDEX=6
                         Cgroup namespace

              PERF_RECORD_KSYMBOL (since Linux 5.0)
                  This record indicates kernel symbol register/unregister events.

                      struct {
                          struct perf_event_header header;
                          u64    addr;
                          u32    len;
                          u16    ksym_type;
                          u16    flags;
                          char   name[];
                          struct sample_id sample_id;
                      };

                  addr   is the address of the kernel symbol.

                  len    is the length of the kernel symbol.

                  ksym_type
                         is the type of the kernel symbol.  Currently the following types are available:

                         PERF_RECORD_KSYMBOL_TYPE_BPF
                                The kernel symbol is a BPF function.

                  flags  If the PERF_RECORD_KSYMBOL_FLAGS_UNREGISTER is set, then this event is for unregistering the kernel symbol.

              PERF_RECORD_BPF_EVENT (since Linux 5.0)
                  This record indicates BPF program is loaded or unloaded.

                      struct {
                          struct perf_event_header header;
                          u16 type;
                          u16 flags;
                          u32 id;
                          u8 tag[BPF_TAG_SIZE];
                          struct sample_id sample_id;
                      };

                  type   is one of the following values:

                         PERF_BPF_EVENT_PROG_LOAD
                                A BPF program is loaded

                         PERF_BPF_EVENT_PROG_UNLOAD
                                A BPF program is unloaded

                  id     is the ID of the BPF program.

                  tag    is the tag of the BPF program.  Currently, BPF_TAG_SIZE is defined as 8.

              PERF_RECORD_CGROUP (since Linux 5.7)
                  This record indicates a new cgroup is created and activated.

                      struct {
                          struct perf_event_header header;
                          u64    id;
                          char   path[];
                          struct sample_id sample_id;
                      };

                  id     is the cgroup identifier.  This can be also retrieved by name_to_handle_at(2) on the cgroup path (as a file handle).

                  path   is the path of the cgroup from the root.

              PERF_RECORD_TEXT_POKE (since Linux 5.8)
                  This record indicates a change in the kernel text.  This includes addition and removal of the text and the corresponding length is zero in this case.

                      struct {
                          struct perf_event_header header;
                          u64    addr;
                          u16    old_len;
                          u16    new_len;
                          u8     bytes[];
                          struct sample_id sample_id;
                      };

                  addr   is the address of the change

                  old_len
                         is the old length

                  new_len
                         is the new length

                  bytes  contains old bytes immediately followed by new bytes.

   Overflow handling
       Events can be set to notify when a threshold is crossed, indicating an overflow.  Overflow conditions can be captured by monitoring  the  event  file  descriptor
       with  poll(2),  select(2), or epoll(7).  Alternatively, the overflow events can be captured via sa signal handler, by enabling I/O signaling on the file descrip‐
       tor; see the discussion of the F_SETOWN and F_SETSIG operations in fcntl(2).

       Overflows are generated only by sampling events (sample_period must have a nonzero value).

       There are two ways to generate overflow notifications.

       The first is to set a wakeup_events or wakeup_watermark value that will trigger if a certain number of samples or bytes have been written to the mmap  ring  buf‐
       fer.  In this case, POLL_IN is indicated.

       The  other way is by use of the PERF_EVENT_IOC_REFRESH ioctl.  This ioctl adds to a counter that decrements each time the event overflows.  When nonzero, POLL_IN
       is indicated, but once the counter reaches 0 POLL_HUP is indicated and the underlying event is disabled.

       Refreshing an event group leader refreshes all siblings and refreshing with a parameter of 0 currently enables infinite refreshes;  these  behaviors  are  unsup‐
       ported and should not be relied on.

       Starting with Linux 3.18, POLL_HUP is indicated if the event being monitored is attached to a different process and that process exits.

   rdpmc instruction
       Starting with Linux 3.4 on x86, you can use the rdpmc instruction to get low-latency reads without having to enter the kernel.  Note that using rdpmc is not nec‐
       essarily faster than other methods for reading event values.

       Support for this can be detected with the cap_usr_rdpmc field in the mmap page; documentation on how to calculate event values can be found in that section.

       Originally, when rdpmc support was enabled, any process (not just ones with an active perf event) could use the rdpmc instruction to access the counters.  Start‐
       ing  with  Linux  4.0,  rdpmc support is only allowed if an event is currently enabled in a process's context.  To restore the old behavior, write the value 2 to
       /sys/devices/cpu/rdpmc.

   perf_event ioctl calls
       Various ioctls act on perf_event_open() file descriptors:

       PERF_EVENT_IOC_ENABLE
              This enables the individual event or event group specified by the file descriptor argument.

              If the PERF_IOC_FLAG_GROUP bit is set in the ioctl argument, then all events in a group are enabled, even if the event specified is not the  group  leader
              (but see BUGS).

       PERF_EVENT_IOC_DISABLE
              This disables the individual counter or event group specified by the file descriptor argument.

              Enabling or disabling the leader of a group enables or disables the entire group; that is, while the group leader is disabled, none of the counters in the
              group will count.  Enabling or disabling a member of a group other than the leader affects only that counter; disabling a non-leader  stops  that  counter
              from counting but doesn't affect any other counter.

              If  the PERF_IOC_FLAG_GROUP bit is set in the ioctl argument, then all events in a group are disabled, even if the event specified is not the group leader
              (but see BUGS).

       PERF_EVENT_IOC_REFRESH
              Non-inherited overflow counters can use this to enable a counter for a number of overflows specified by the argument, after which it is disabled.   Subse‐
              quent  calls  of this ioctl add the argument value to the current count.  An overflow notification with POLL_IN set will happen on each overflow until the
              count reaches 0; when that happens a notification with POLL_HUP set is sent and the event is disabled.  Using an argument of 0 is considered undefined be‐
              havior.

       PERF_EVENT_IOC_RESET
              Reset  the event count specified by the file descriptor argument to zero.  This resets only the counts; there is no way to reset the multiplexing time_en‐
              abled or time_running values.

              If the PERF_IOC_FLAG_GROUP bit is set in the ioctl argument, then all events in a group are reset, even if the event specified is  not  the  group  leader
              (but see BUGS).

       PERF_EVENT_IOC_PERIOD
              This updates the overflow period for the event.

              Since Linux 3.7 (on ARM) and Linux 3.14 (all other architectures), the new period takes effect immediately.  On older kernels, the new period did not take
              effect until after the next overflow.

              The argument is a pointer to a 64-bit value containing the desired new period.

              Prior to Linux 2.6.36, this ioctl always failed due to a bug in the kernel.

       PERF_EVENT_IOC_SET_OUTPUT
              This tells the kernel to report event notifications to the specified file descriptor rather than the default one.  The file descriptors must all be on the
              same CPU.

              The argument specifies the desired file descriptor, or -1 if output should be ignored.

       PERF_EVENT_IOC_SET_FILTER (since Linux 2.6.33)
              This adds an ftrace filter to this event.

              The argument is a pointer to the desired ftrace filter.

       PERF_EVENT_IOC_ID (since Linux 3.12)
              This returns the event ID value for the given event file descriptor.

              The argument is a pointer to a 64-bit unsigned integer to hold the result.

       PERF_EVENT_IOC_SET_BPF (since Linux 4.1)
              This  allows  attaching  a  Berkeley  Packet  Filter  (BPF)  program  to  an  existing kprobe tracepoint event.  You need CAP_PERFMON (since Linux 5.8) or
              CAP_SYS_ADMIN privileges to use this ioctl.

              The argument is a BPF program file descriptor that was created by a previous bpf(2) system call.

       PERF_EVENT_IOC_PAUSE_OUTPUT (since Linux 4.7)
              This allows pausing and resuming the event's ring-buffer.  A paused ring-buffer does not prevent generation of samples, but  simply  discards  them.   The
              discarded  samples  are  considered lost, and cause a PERF_RECORD_LOST sample to be generated when possible.  An overflow signal may still be triggered by
              the discarded sample even though the ring-buffer remains empty.

              The argument is an unsigned 32-bit integer.  A nonzero value pauses the ring-buffer, while a zero value resumes the ring-buffer.

       PERF_EVENT_MODIFY_ATTRIBUTES (since Linux 4.17)
              This allows modifying an existing event without the overhead of closing and reopening a new event.   Currently  this  is  supported  only  for  breakpoint
              events.

              The argument is a pointer to a perf_event_attr structure containing the updated event settings.

       PERF_EVENT_IOC_QUERY_BPF (since Linux 4.16)
              This  allows  querying which Berkeley Packet Filter (BPF) programs are attached to an existing kprobe tracepoint.  You can only attach one BPF program per
              event, but you can have multiple events attached to a tracepoint.  Querying this value on one tracepoint event returns the ID of all BPF programs  in  all
              events attached to the tracepoint.  You need CAP_PERFMON (since Linux 5.8) or CAP_SYS_ADMIN privileges to use this ioctl.

              The argument is a pointer to a structure
                  struct perf_event_query_bpf {
                      __u32    ids_len;
                      __u32    prog_cnt;
                      __u32    ids[0];
                  };

              The  ids_len  field  indicates the number of ids that can fit in the provided ids array.  The prog_cnt value is filled in by the kernel with the number of
              attached BPF programs.  The ids array is filled with the ID of each attached BPF program.  If there are more programs than will fit in the array, then the
              kernel will return ENOSPC and ids_len will indicate the number of program IDs that were successfully copied.

   Using prctl(2)
       A  process  can enable or disable all currently open event groups using the prctl(2) PR_TASK_PERF_EVENTS_ENABLE and PR_TASK_PERF_EVENTS_DISABLE operations.  This
       applies only to events created locally by the calling process.  This does not apply to events created by other processes attached to the calling process  or  in‐
       herited events from a parent process.  Only group leaders are enabled and disabled, not any other members of the groups.

   perf_event related configuration files
       Files in /proc/sys/kernel/

           /proc/sys/kernel/perf_event_paranoid
                  The perf_event_paranoid file can be set to restrict access to the performance counters.

                  2   allow only user-space measurements (default since Linux 4.6).
                  1   allow both kernel and user measurements (default before Linux 4.6).
                  0   allow access to CPU-specific data but not raw tracepoint samples.
                  -1  no restrictions.

                  The existence of the perf_event_paranoid file is the official method for determining if a kernel supports perf_event_open().

           /proc/sys/kernel/perf_event_max_sample_rate
                  This sets the maximum sample rate.  Setting this too high can allow users to sample at a rate that impacts overall machine performance and potentially
                  lock up the machine.  The default value is 100000 (samples per second).

           /proc/sys/kernel/perf_event_max_stack
                  This file sets the maximum depth of stack frame entries reported when generating a call trace.

           /proc/sys/kernel/perf_event_mlock_kb
                  Maximum number of pages an unprivileged user can mlock(2).  The default is 516 (kB).

       Files in /sys/bus/event_source/devices/

           Since Linux 2.6.34, the kernel supports having multiple PMUs available for monitoring.  Information  on  how  to  program  these  PMUs  can  be  found  under
           /sys/bus/event_source/devices/.  Each subdirectory corresponds to a different PMU.

           /sys/bus/event_source/devices/*/type (since Linux 2.6.38)
                  This contains an integer that can be used in the type field of perf_event_attr to indicate that you wish to use this PMU.

           /sys/bus/event_source/devices/cpu/rdpmc (since Linux 3.4)
                  If  this  file is 1, then direct user-space access to the performance counter registers is allowed via the rdpmc instruction.  This can be disabled by
                  echoing 0 to the file.

                  As of Linux 4.0 the behavior has changed, so that 1 now means only allow access to processes with active perf events, with 2 indicating the old allow-
                  anyone-access behavior.

           /sys/bus/event_source/devices/*/format/ (since Linux 3.4)
                  This  subdirectory  contains  information  on  the  architecture-specific  subfields  available  for  programming  the  various  config  fields in the
                  perf_event_attr struct.

                  The content of each file is the name of the config field, followed by a colon, followed by a series of integer bit ranges separated  by  commas.   For
                  example,  the  file  event  may  contain  the  value config1:1,6-10,44 which indicates that event is an attribute that occupies bits 1,6–10, and 44 of
                  perf_event_attr::config1.

           /sys/bus/event_source/devices/*/events/ (since Linux 3.4)
                  This subdirectory contains files with predefined events.  The contents are strings describing the event settings expressed  in  terms  of  the  fields
                  found  in the previously mentioned ./format/ directory.  These are not necessarily complete lists of all events supported by a PMU, but usually a sub‐
                  set of events deemed useful or interesting.

                  The content of each file is a list of attribute names separated by commas.  Each entry has an optional value (either hex or decimal).  If no value  is
                  specified, then it is assumed to be a single-bit field with a value of 1.  An example entry may look like this: event=0x2,inv,ldlat=3.

           /sys/bus/event_source/devices/*/uevent
                  This file is the standard kernel device interface for injecting hotplug events.

           /sys/bus/event_source/devices/*/cpumask (since Linux 3.7)
                  The  cpumask  file contains a comma-separated list of integers that indicate a representative CPU number for each socket (package) on the motherboard.
                  This is needed when setting up uncore or northbridge events, as those PMUs present socket-wide events.

RETURN VALUE
       On success, perf_event_open() returns the new file descriptor.  On error, -1 is returned and errno is set to indicate the error.

ERRORS
       The errors returned by perf_event_open() can be inconsistent, and may vary across processor architectures and performance monitoring units.

       E2BIG  Returned if the perf_event_attr size value is too small (smaller than PERF_ATTR_SIZE_VER0), too big (larger than the page size), or larger than the kernel
              supports  and  the  extra  bytes  are not zero.  When E2BIG is returned, the perf_event_attr size field is overwritten by the kernel to be the size of the
              structure it was expecting.

       EACCES Returned when the requested event requires CAP_PERFMON (since Linux 5.8) or CAP_SYS_ADMIN permissions (or a more permissive perf_event paranoid  setting).
              Some  common cases where an unprivileged process may encounter this error: attaching to a process owned by a different user; monitoring all processes on a
              given CPU (i.e., specifying the pid argument as -1); and not setting exclude_kernel when the paranoid setting requires it.

       EBADF  Returned if the group_fd file descriptor is not valid, or, if PERF_FLAG_PID_CGROUP is set, the cgroup file descriptor in pid is not valid.

       EBUSY (since Linux 4.1)
              Returned if another event already has exclusive access to the PMU.

       EFAULT Returned if the attr pointer points at an invalid memory address.

       EINTR  Returned when trying to mix perf and ftrace handling for a uprobe.

       EINVAL Returned if the specified event is invalid.  There are many possible reasons for this.  A not-exhaustive list: sample_freq is higher than the maximum set‐
              ting;  the  cpu  to monitor does not exist; read_format is out of range; sample_type is out of range; the flags value is out of range; exclusive or pinned
              set and the event is not a group leader; the event config values are out of range or set reserved bits; the generic event selected is  not  supported;  or
              there is not enough room to add the selected event.

       EMFILE Each  opened event uses one file descriptor.  If a large number of events are opened, the per-process limit on the number of open file descriptors will be
              reached, and no more events can be created.

       ENODEV Returned when the event involves a feature not supported by the current CPU.

       ENOENT Returned if the type setting is not valid.  This error is also returned for some unsupported generic events.

       ENOSPC Prior to Linux 3.3, if there was not enough room for the event, ENOSPC was returned.  In Linux 3.3, this was changed to EINVAL.  ENOSPC is still  returned
              if you try to add more breakpoint events than supported by the hardware.

       ENOSYS Returned if PERF_SAMPLE_STACK_USER is set in sample_type and it is not supported by hardware.

       EOPNOTSUPP
              Returned if an event requiring a specific hardware feature is requested but there is no hardware support.  This includes requesting low-skid events if not
              supported, branch tracing if it is not available, sampling if no PMU interrupt is available, and branch stacks for software events.

       EOVERFLOW (since Linux 4.8)
              Returned if PERF_SAMPLE_CALLCHAIN is requested and sample_max_stack is larger than the maximum specified in /proc/sys/kernel/perf_event_max_stack.

       EPERM  Returned on many (but not all) architectures when an unsupported exclude_hv, exclude_idle, exclude_user, or exclude_kernel setting is specified.

              It can also happen, as with EACCES, when the requested event requires CAP_PERFMON (since Linux 5.8) or CAP_SYS_ADMIN permissions  (or  a  more  permissive
              perf_event paranoid setting).  This includes setting a breakpoint on a kernel address, and (since Linux 3.13) setting a kernel function-trace tracepoint.

       ESRCH  Returned if attempting to attach to a process that does not exist.

VERSION
       perf_event_open() was introduced in Linux 2.6.31 but was called perf_counter_open().  It was renamed in Linux 2.6.32.

CONFORMING TO
       This perf_event_open() system call Linux-specific and should not be used in programs intended to be portable.

NOTES
       The official way of knowing if perf_event_open() support is enabled is checking for the existence of the file /proc/sys/kernel/perf_event_paranoid.

       CAP_PERFMON  capability  (since Linux 5.8) provides secure approach to performance monitoring and observability operations in a system according to the principal
       of least privilege (POSIX IEEE 1003.1e).  Accessing system performance monitoring and observability operations using CAP_PERFMON rather than the much more power‐
       ful  CAP_SYS_ADMIN excludes chances to misuse credentials and makes operations more secure.  CAP_SYS_ADMIN usage for secure system performance monitoring and ob‐
       servability is discouraged in favor of the CAP_PERFMON capability.

BUGS
       The F_SETOWN_EX option to fcntl(2) is needed to properly get overflow signals in threads.  This was introduced in Linux 2.6.32.

       Prior to Linux 2.6.33 (at least for x86), the kernel did not check if events could be scheduled together until read time.  The same happens on all known  kernels
       if  the  NMI  watchdog is enabled.  This means to see if a given set of events works you have to perf_event_open(), start, then read before you know for sure you
       can get valid measurements.

       Prior to Linux 2.6.34, event constraints were not enforced by the kernel.  In that case, some events would silently return "0" if the kernel scheduled them in an
       improper counter slot.

       Prior to Linux 2.6.34, there was a bug when multiplexing where the wrong results could be returned.

       Kernels from Linux 2.6.35 to Linux 2.6.39 can quickly crash the kernel if "inherit" is enabled and many threads are started.

       Prior to Linux 2.6.35, PERF_FORMAT_GROUP did not work with attached processes.

       There is a bug in the kernel code between Linux 2.6.36 and Linux 3.0 that ignores the "watermark" field and acts as if a wakeup_event was chosen if the union has
       a nonzero value in it.

       From Linux 2.6.31 to Linux 3.4, the PERF_IOC_FLAG_GROUP ioctl argument was broken and would repeatedly operate on  the  event  specified  rather  than  iterating
       across all sibling events in a group.

       From  Linux  3.4  to  Linux  3.11,  the  mmap cap_usr_rdpmc and cap_usr_time bits mapped to the same location.  Code should migrate to the new cap_user_rdpmc and
       cap_user_time fields instead.

       Always double-check your results!  Various generalized events have had wrong values.  For example, retired branches measured the wrong thing on AMD machines  un‐
       til Linux 2.6.35.

EXAMPLES
       The following is a short example that measures the total instruction count of a call to printf(3).

       #include <stdlib.h>
       #include <stdio.h>
       #include <unistd.h>
       #include <string.h>
       #include <sys/ioctl.h>
       #include <linux/perf_event.h>
       #include <asm/unistd.h>

       static long
       perf_event_open(struct perf_event_attr *hw_event, pid_t pid,
                       int cpu, int group_fd, unsigned long flags)
       {
           int ret;

           ret = syscall(__NR_perf_event_open, hw_event, pid, cpu,
                          group_fd, flags);
           return ret;
       }

       int
       main(int argc, char *argv[])
       {
           struct perf_event_attr pe;
           long long count;
           int fd;

           memset(&pe, 0, sizeof(pe));
           pe.type = PERF_TYPE_HARDWARE;
           pe.size = sizeof(pe);
           pe.config = PERF_COUNT_HW_INSTRUCTIONS;
           pe.disabled = 1;
           pe.exclude_kernel = 1;
           pe.exclude_hv = 1;

           fd = perf_event_open(&pe, 0, -1, -1, 0);
           if (fd == -1) {
              fprintf(stderr, "Error opening leader %llx\n", pe.config);
              exit(EXIT_FAILURE);
           }

           ioctl(fd, PERF_EVENT_IOC_RESET, 0);
           ioctl(fd, PERF_EVENT_IOC_ENABLE, 0);

           printf("Measuring instruction count for this printf\n");

           ioctl(fd, PERF_EVENT_IOC_DISABLE, 0);
           read(fd, &count, sizeof(count));

           printf("Used %lld instructions\n", count);

           close(fd);
       }

SEE ALSO
       perf(1), fcntl(2), mmap(2), open(2), prctl(2), read(2)

       Documentation/admin-guide/perf-security.rst in the kernel source tree

Linux                                                                          2021-08-27                                                             PERF_EVENT_OPEN(2)