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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>
       #include <linux/hw_breakpoint.h>

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

       Note: There is no glibc wrapper for this system call; see NOTES.

DESCRIPTION
       Given  a	 list of parameters, perf_event_open() returns a file descrip‐
       tor, 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 mea‐
       suring performance information.	Each file  descriptor  corresponds  to
       one  event  that	 is measured; 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 continue 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 argument pid allows events to be attached to processes  in  various
       ways.   If  pid is 0, measurements happen on the current thread, if pid
       is greater than 0, the process indicated by pid is measured, and if pid
       is -1, all processes are counted.

       The  cpu	 argument allows measurements to be specific to a CPU.	If cpu
       is greater than or equal to 0, measurements are restricted to the spec‐
       ified CPU; if cpu is -1, the events are measured on all CPUs.

       Note that the combination of pid == -1 and cpu == -1 is not valid.

       A  pid  > 0 and cpu == -1 setting measures per-process and follows that
       process to whatever CPU the process  gets  scheduled  to.   Per-process
       events can be created by any user.

       A  pid == -1 and cpu >= 0 setting is per-CPU and measures all processes
       on the specified CPU.  Per-CPU events need the CAP_SYS_ADMIN capability
       or a /proc/sys/kernel/perf_event_paranoid value of less than 1.

       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
       fd  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), etc., 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 fol‐
       lowing values:

       PERF_FLAG_FD_NO_GROUP
	      This flag allows creating an event as part of an event group but
	      having no group leader.  It is unclear why this is useful.

       PERF_FLAG_FD_OUTPUT
	      This  flag  re-routes  the  output  from	an  event to the group
	      leader.

       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 grain 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 mon‐
	      itor   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 monitoring is
	      available only for system-wide events and may therefore  require
	      extra permissions.

       The  perf_event_attr structure provides detailed configuration informa‐
       tion 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 */
			 __reserved_1	: 41;

	       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 config1;	   /* extension of config */
	       };

	       union {
		   __u64 bp_len;	   /* breakpoint length */
		   __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 */
	       __u32   __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  pro‐
		     vided  by	the  kernel  (even  if	no hardware support is
		     available).

	      PERF_TYPE_TRACEPOINT
		     This indicates a tracepoint provided by the kernel trace‐
		     point infrastructure.

	      PERF_TYPE_HW_CACHE
		     This  indicates  a hardware cache event.  This has a spe‐
		     cial 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 instruction address.

	      dynamic PMU
		     Since  Linux 2.6.39, perf_event_open() can support multi‐
		     ple 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 filesys‐
		     tem:  there  is  a	 subdirectory  per  PMU instance under
		     /sys/bus/event_source/devices.   In  each	 sub-directory
		     there is a type file whose content is an integer that can
		     be	  used	 in   the   type   field.     For    instance,
		     /sys/bus/event_source/devices/cpu/type contains the value
		     for the core CPU PMU, which is usually 4.

       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  compila‐
	      tion.

	      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  addition  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_ATR_SIZE_VER3 is 96 cor‐
	      responding  to  the  addition  of	 sample_regs_user   and	  sam‐
	      ple_stack_user in Linux 3.7.

       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.

	      The most significant bit (bit 63) of config  signifies  CPU-spe‐
	      cific  (raw) counter configuration data; if the most significant
	      bit is unset, the next 7 bits are an event type and the rest  of
	      the bits are the event identifier.

	      There  are  various ways to set the config field that are depen‐
	      dent 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  gen‐
	      eralized 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 mes‐
			  sages; 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 con‐
			  junction  with  the	PERF_COUNT_HW_CACHE_REFERENCES
			  event to calculate cache miss rates.

		   PERF_COUNT_HW_BRANCH_INSTRUCTIONS
			  Retired branch instructions.	Prior to Linux 2.6.34,
			  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 nega‐
			  tively impact performance.

	      If type is PERF_TYPE_TRACEPOINT, then we	are  measuring	kernel
	      tracepoints.   The  value	 to use in config can be obtained from
	      under debugfs tracing/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:

		      (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 3.0)
			     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  architec‐
	      tural  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.

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

	      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 supports 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 hard‐
		     ware 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 only available if the underlying  hardware
		     supports this feature.

       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  dis‐
	      abled  or	 enabled.  If disabled, the event can later be enabled
	      by ioctl(2), prctl(2), or enable_on_exec.

       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 chil‐
	      dren).

	      Inherit does not work for	 some  combinations  of	 read_formats,
	      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	 coun‐
	      ters  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 subse‐
	      quently 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.

       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 idle.

       mmap   The mmap bit enables recording of exec mmap events.

       comm   The  comm	 bit enables tracking of process command name as modi‐
	      fied by the exec(2) and prctl(PR_SET_NAME) system calls.	Unfor‐
	      tunately	for  tools,  there is no way to distinguish one system
	      call versus the other.

       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 exec(2).

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

       watermark
	      If  set,	have  a	 sampling  interrupt  happen when we cross the
	      wakeup_watermark boundary.  Otherwise  interrupts	 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 limited with how small
	      this can be.

	      The values of this 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
		     PERF_RECORD_MISC_EXACT_IP.

       mmap_data (Since Linux 2.6.36)
	      The  counterpart	of  the mmap field, but enables including data
	      mmap events in the ring-buffer.

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

       exclude_host (Since Linux 3.2)
	      Do not measure time spent in VM host.

       exclude_guest (Since Linux 3.2)
	      Do not measure time spent in VM guest.

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

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

       wakeup_events, wakeup_watermark
	      This union  sets	how  many  samples  (wakeup_events)  or	 bytes
	      (wakeup_watermark)  happen  before  an  overflow signal happens.
	      Which one is used is selected by the watermark bitflag.

	      wakeup_events only counts PERF_RECORD_SAMPLE record  types.   To
	      receive  a  signal  for  every  incoming	PERF_RECORD  type  set
	      wakeup_watermark to 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)
	      bp_addr address of the breakpoint.   For	execution  breakpoints
	      this  is	the memory address of the instruction of interest; for
	      read and write breakpoints it is the memory address of the  mem‐
	      ory 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  OFF‐
	      CORE_EVENTS  on  Nehalem/Westmere/SandyBridge  use this field on
	      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_BREAKPOINT_LEN_4,  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 following values.  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.

	      PERF_SAMPLE_BRANCH_ANY_RETURN
		     Any return branch.

	      PERF_SAMPLE_BRANCH_IND_CALL
		     Indirect calls.

	      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.

       sample_regs_user (Since Linux 3.7)
	      This bitmask defines the set of user CPU registers  to  dump  on
	      samples.	 The  layout of the register mask is architecture spe‐
	      cific	and	described     in     the     kernel	header
	      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_SAM‐
	      PLE_STACK_USER is specified.

   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 ENOSPC is returned

       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.  Only available if
	      PERF_FORMAT_GROUP was specified.

       time_enabled, time_running
	      Total time the event was enabled and  running.   Normally	 these
	      are the same.  If more events are started than available counter
	      slots on the PMU, then multiplexing happens and events run  only
	      part  of	the time.  In that case the time_enabled and time run‐
	      ning 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 there 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 infor‐
       mation such as where the ring-buffer head is.

       Before kernel 2.6.39, there is a bug that means	you  must  allocate  a
       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;
		   __u64   cap_usr_time	 : 1,
			   cap_usr_rdpmc : 1,
	       };
	       __u16   pmc_width;
	       __u16   time_shift;
	       __u32   time_mult;
	       __u64   time_offset;
	       __u64   __reserved[120];	  /* Pad to 1k */
	       __u64   data_head;	  /* head in the data section */
	       __u64   data_tail;	  /* user-space written tail */
	   }

       The following looks at 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 Add this to hardware counter value??

       time_enabled
	      Time the event was active.

       time_running
	      Time the event was running.

       cap_usr_time
	      User time capability.

       cap_usr_rdpmc
	      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;
		  s64 pmc = 0;

		  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;
			  pmc = rdpmc(idx - 1);
		      }

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

       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 result 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 simi‐
	      lar.

		  u64 quot, rem;
		  u64 delta;
		  quot = (cyc >> time_shift);
		  rem = cyc & ((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;

       data_head
	      This points to the head of the data section.  The value continu‐
	      ously  increases, it does not wrap.  The value needs to be manu‐
	      ally 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 over-write unread data.

       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.   That way a newer
       perf.data file will be supported by older perf tools,  with  these  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 follow‐
	      ing  (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
		     Sample happened in the guest kernel.

	      PERF_RECORD_MISC_GUEST_USER
		     Sample happened in guest user code.

	      In addition, one of the following bits can be set:

	      PERF_RECORD_MISC_MMAP_DATA
		     This is set when the mapping is not executable; otherwise
		     the mapping is executable.

	      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
		     This  indicates  there  is	 extended data available (cur‐
		     rently not used).

       type   The type value is one of the below.  The values  in  the	corre‐
	      sponding	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[];
		      };

	      PERF_RECORD_LOST
		  This record indicates when events are lost.

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

		  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, tid;
			  char comm[];
		      };

	      PERF_RECORD_EXIT
		  This record indicates a process exit event.

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

	      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;
		      };

	      PERF_RECORD_FORK
		  This record indicates a fork event.

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

	      PERF_RECORD_READ
		  This record indicates a read event.

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

	      PERF_RECORD_SAMPLE
		  This record indicates a sample.

		      struct {
			  struct perf_event_header header;
			  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 */
			  u64	weight;	    /* if PERF_SAMPLE_WEIGHT */
			  u64	data_src;   /* if PERF_SAMPLE_DATA_SRC */
		      };

		  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 available 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	 indi‐
		      cating 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 included 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  num‐
		      ber  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 indi‐
		      cating 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, fol‐
		      lowed  by	 bnr  perf_branch_entry	 structures 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 transac‐
			     tion.

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

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

		      Support for mispred and predicted is  optional;  if  not
		      supported, both 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  sam‐
		      ple_regs_user bitmask.

		  size, data[size], dyn_size
		      If  PERF_SAMPLE_STACK_USER  is  enabled, then record the
		      user stack to enable  backtracing.   size	 is  the  size
		      requested	 by  the  user	in stack_user_size or else the
		      maximum record size.  data is the stack data.   dyn_size
		      is  the amount of data actually dumped (can be less than
		      size).

		  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 combi‐
			  nation of:

			  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:

			  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:

			  PERF_MEM_LOCK_NA	  Not available
			  PERF_MEM_LOCK_LOCKED	  Locked transaction

		      mem_dtlb
			  TLB access hit or miss, a bitwise combination of:

			  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

   Signal overflow
       Events can be set to deliver a signal when a threshold is crossed.  The
       signal handler is set up using the  poll(2),  select(2),	 epoll(2)  and
       fcntl(2), system calls.

       To  generate signals, sampling must be enabled (sample_period must have
       a non-zero value).

       There are two ways to generate signals.

       The first is to set a wakeup_events or wakeup_watermark value that will
       generate	 a  signal  if	a certain number of samples or bytes have been
       written to the mmap ring buffer.	 In this case a signal of type POLL_IN
       is sent.

       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 non-zero, a POLL_IN signal is sent on overflow, but once the value
       reaches 0, a signal is sent of type POLL_HUP and the  underlying	 event
       is disabled.

       Note:  on  newer kernels (definitely noticed with 3.2) a signal is pro‐
       vided for every overflow, even if wakeup_events is not set.

   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 necessarily 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.

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

       PERF_EVENT_IOC_ENABLE
	      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 speci‐
	      fied is not the group leader (but see BUGS).

       PERF_EVENT_IOC_DISABLE
	      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 dis‐
	      abling 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 spec‐
	      ified 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.  Subsequent calls of this ioctl add the argument
	      value to the current count.  A signal with POLL_IN set will hap‐
	      pen on each overflow until the count reaches 0; when  that  hap‐
	      pens  a  signal  with POLL_HUP set is sent and the event is dis‐
	      abled.  Using an argument of 0 is considered undefined behavior.

       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_enabled 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 speci‐
	      fied is not the group leader (but see BUGS).

       PERF_EVENT_IOC_PERIOD
	      IOC_PERIOD is the command to update  the	period;	 it  does  not
	      update the current period but instead defers until next.

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

       PERF_EVENT_IOC_SET_OUTPUT
	      This tells the kernel to report event notifications to the spec‐
	      ified  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.

   Using prctl
       A  process can enable or disable all the event groups that are attached
       to   it	 using	  the	 prctl(2)    PR_TASK_PERF_EVENTS_ENABLE	   and
       PR_TASK_PERF_EVENTS_DISABLE  operations.	  This applies to all counters
       on the current process, whether created by this process or by  another,
       and does not affect any counters that this process has created on other
       processes.  It enables or disables only	the  group  leaders,  not  any
       other members in 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   only allow user-space measurements.

		  1   allow both kernel and user measurements (default).

		  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 offi‐
		  cial	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_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 avail‐
	   able 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 you wish to use this PMU.

	   /sys/bus/event_source/devices/*/rdpmc (Since Linux 3.4)
		  If this file is 1, then direct user-space access to the per‐
		  formance counter registers is allowed via the rdpmc instruc‐
		  tion.	 This can be disabled by echoing 0 to the file.

	   /sys/bus/event_source/devices/*/format/ (Since Linux 3.4)
		  This sub-directory contains information on the architecture-
		  specific sub-fields 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 sub-directory contains files with  pre-defined  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 subset of events deemed useful or interesting.

		  The  content of each file is a list of attribute names sepa‐
		  rated by commas.  Each entry has an optional	value  (either
		  hex  or  decimal).   If  no  value  is  specified than 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
       perf_event_open() returns the new file descriptor, or -1	 if  an	 error
       occurred (in which case, errno is set appropriately).

ERRORS
       EINVAL Returned if the specified event is not available.

       ENOSPC Prior  to Linux 3.3, if there was not enough room for the event,
	      ENOSPC was returned.  Linus did not  like	 this,	and  this  was
	      changed  to EINVAL.  ENOSPC is still returned if you try to read
	      results into too small of a buffer.

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
       Glibc does not provide a wrapper for this system call;  call  it	 using
       syscall(2).  See the example below.

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

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  ker‐
       nel.  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.

       In older Linux 2.6 versions, refreshing an event group leader refreshed
       all siblings, and refreshing with a parameter  of  0  enabled  infinite
       refresh.	 This behavior is unsupported and should not be relied on.

       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 non-zero 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.

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

EXAMPLE
       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>

       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(struct perf_event_attr));
	   pe.type = PERF_TYPE_HARDWARE;
	   pe.size = sizeof(struct perf_event_attr);
	   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(long long));

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

	   close(fd);
       }

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

COLOPHON
       This page is part of release 3.54 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

Linux				  2013-09-13		    PERF_EVENT_OPEN(2)
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