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. 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. The value is formed by ORing together zero or more of
the following values, although PERF_SAMPLE_BRANCH_ANY covers all
branch types.
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_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_PLM_ALL
User, kernel, and hv
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, i.e., 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.
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 instruc-
tion 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 time-
stamp 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 trace-
point, 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 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
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 depend-
ing 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 structures which
each include the fields:
from indicating the source instruction (may not be
a branch)
to the branch target
mispred
the branch target was mispredicted
predicted
the branch target was predicted.
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 sup-
ported, both values will be 0.
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 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), and
PERF_MEM_OP_EXEC (executable code).
mem_lvl
memory hierarchy level hit or miss, a bitwise
combination of PERF_MEM_LVL_NA (not avail-
able), 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), and
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), and
PERF_MEM_SNOOP_HITM (snoop hit modified).
mem_lock
lock instruction, a bitwise combination of
PERF_MEM_LOCK_NA (not available) and
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), and PERF_MEM_TLB_OS (OS
fault handler).
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).
size This indicates the size of the record.
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 - (default) allow both kernel and user measurements
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 3.53 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 2013-07-16 PERF_EVENT_OPEN(2)
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