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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 descriptor, for use in
subsequent system calls (read(2), mmap(2), prctl(2), fcntl(2), etc.).
A call to perf_event_open() creates a file descriptor that allows measuring performance
information. Each file descriptor corresponds to one event that is 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 specified 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 following 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 monitor is called test, then a file
descriptor opened on /dev/cgroup/test (assuming cgroupfs is mounted on /dev/cgroup)
must be passed as the pid parameter. cgroup monitoring is available only for
system-wide events and may therefore require extra permissions.
The perf_event_attr structure provides detailed configuration information for the event
being created.
struct perf_event_attr {
__u32 type; /* Type of event */
__u32 size; /* Size of attribute structure */
__u64 config; /* Type-specific configuration */
union {
__u64 sample_period; /* Period of sampling */
__u64 sample_freq; /* Frequency of sampling */
};
__u64 sample_type; /* Specifies values included in sample */
__u64 read_format; /* Specifies values returned in read */
__u64 disabled : 1, /* off by default */
inherit : 1, /* children inherit it */
pinned : 1, /* must always be on PMU */
exclusive : 1, /* only group on PMU */
exclude_user : 1, /* don't count user */
exclude_kernel : 1, /* don't count kernel */
exclude_hv : 1, /* don't count hypervisor */
exclude_idle : 1, /* don't count when idle */
mmap : 1, /* include mmap data */
comm : 1, /* include comm data */
freq : 1, /* use freq, not period */
inherit_stat : 1, /* per task counts */
enable_on_exec : 1, /* next exec enables */
task : 1, /* trace fork/exit */
watermark : 1, /* wakeup_watermark */
precise_ip : 2, /* skid constraint */
mmap_data : 1, /* non-exec mmap data */
sample_id_all : 1, /* sample_type all events */
exclude_host : 1, /* don't count in host */
exclude_guest : 1, /* don't count in guest */
exclude_callchain_kernel : 1,
/* exclude kernel callchains */
exclude_callchain_user : 1,
/* exclude user callchains */
__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 provided by the kernel
(even if no hardware support is available).
PERF_TYPE_TRACEPOINT
This indicates a tracepoint provided by the kernel tracepoint
infrastructure.
PERF_TYPE_HW_CACHE
This indicates a hardware cache event. This has a special encoding,
described in the config field definition.
PERF_TYPE_RAW
This indicates a "raw" implementation-specific event in the config field.
PERF_TYPE_BREAKPOINT (Since Linux 2.6.33)
This indicates a hardware breakpoint as provided by the CPU. Breakpoints
can be read/write accesses to an address as well as execution of an
instruction address.
dynamic PMU
Since Linux 2.6.39, perf_event_open() can support multiple PMUs. To enable
this, a value exported by the kernel can be used in the type field to
indicate which PMU to use. The value to use can be found in the sysfs
filesystem: there is a subdirectory per PMU instance under
/sys/bus/event_source/devices. In each 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 compilation.
The related define PERF_ATTR_SIZE_VER0 is set to 64; this was the size of the first
published struct. PERF_ATTR_SIZE_VER1 is 72, corresponding to the 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 corresponding
to the addition of sample_regs_user and sample_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-specific (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 dependent on the value of
the previously described type field. What follows are various possible settings
for config separated out by type.
If type is PERF_TYPE_HARDWARE, we are measuring one of the generalized hardware CPU
events. Not all of these are available on all platforms. Set config to one of the
following:
PERF_COUNT_HW_CPU_CYCLES
Total cycles. Be wary of what happens during CPU frequency scaling.
PERF_COUNT_HW_INSTRUCTIONS
Retired instructions. Be careful, these can be affected by various
issues, most notably hardware interrupt counts.
PERF_COUNT_HW_CACHE_REFERENCES
Cache accesses. Usually this indicates Last Level Cache accesses but
this may vary depending on your CPU. This may include prefetches and
coherency messages; again this depends on the design of your CPU.
PERF_COUNT_HW_CACHE_MISSES
Cache misses. Usually this indicates Last Level Cache misses; this is
intended to be used in conjunction with the
PERF_COUNT_HW_CACHE_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 negatively 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 architectural manuals to the raw hex
value perf_event_open() expects in this field.
If type is PERF_TYPE_BREAKPOINT, then leave config set to zero. Its parameters are
set in other places.
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 hardware to highlight expensive events
in a profile.
PERF_SAMPLE_DATA_SRC (Since Linux 3.10)
Records the data source: where in the memory hierarchy the data associated
with the sampled instruction came from. This is 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 disabled 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
children).
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 counters and only to group leaders. If a
pinned counter cannot be put onto the CPU (e.g., because there are not enough
hardware counters or because of a conflict with some other event), then the counter
goes into an 'error' state, where reads return end-of-file (i.e., read(2) returns
0) until the counter is subsequently enabled or disabled.
exclusive
The exclusive bit specifies that when this counter's group is on the CPU, it should
be the only group using the CPU's counters. In the future this may allow
monitoring programs to support PMU features that need to run alone so that they do
not disrupt other hardware counters.
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 modified by the exec(2)
and prctl(PR_SET_NAME) system calls. Unfortunately 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 sample_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 memory location of interest.
config1 (Since Linux 2.6.39)
config1 is used for setting events that need an extra register or otherwise do not
fit in the regular config field. Raw OFFCORE_EVENTS on
Nehalem/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 specific 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_SAMPLE_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 running values can be used to scale an estimated value for the count.
value An unsigned 64-bit value containing the counter result.
id A globally unique value for this particular event, only 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 information 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 similar.
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 continuously increases, it
does not wrap. The value needs to be manually wrapped by the size of the mmap
buffer before accessing the samples.
On SMP-capable platforms, after reading the data_head value, user space should
issue an rmb().
data_tail;
When the mapping is PROT_WRITE, the data_tail value should be written by user space
to reflect the last read data. In this case the kernel will not 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 following (note these are
not bit masks, only one can be set at a time):
PERF_RECORD_MISC_CPUMODE_UNKNOWN
Unknown CPU mode.
PERF_RECORD_MISC_KERNEL
Sample happened in the kernel.
PERF_RECORD_MISC_USER
Sample happened in user code.
PERF_RECORD_MISC_HYPERVISOR
Sample happened in the hypervisor.
PERF_RECORD_MISC_GUEST_KERNEL
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 (currently not used).
type The type value is one of the below. The values in the corresponding record (that
follows the header) depend on the type selected as shown.
PERF_RECORD_MMAP
The MMAP events record the PROT_EXEC mappings so that we can correlate user-
space IPs to code. They have the following structure:
struct {
struct perf_event_header header;
u32 pid, tid;
u64 addr;
u64 len;
u64 pgoff;
char filename[];
};
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 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
depending on event, hardware, and kernel version.
bnr, lbr[bnr]
If PERF_SAMPLE_BRANCH_STACK is enabled, then a 64-bit value indicating the
number of records is included, followed by bnr perf_branch_entry 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 transaction.
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 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
PERF_MEM_OP_EXEC Executable code
mem_lvl
Memory hierarchy level hit or miss, a bitwise combination 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 provided 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 specified 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 disabling a member of a group other than the leader affects
only that counter; disabling a non-leader stops that counter from counting but
doesn't affect any other counter.
If the PERF_IOC_FLAG_GROUP bit is set in the ioctl argument, then all events in a
group are disabled, even if the event specified is not the group leader (but see
BUGS).
PERF_EVENT_IOC_REFRESH
Non-inherited overflow counters can use this to enable a counter for a number of
overflows specified by the argument, after which it is disabled. Subsequent calls
of this ioctl add the argument value to the current count. A signal with POLL_IN
set will happen on each overflow until the count reaches 0; when that happens a
signal with POLL_HUP set is sent and the event is disabled. 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 specified 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 specified file
descriptor rather than the default one. The file descriptors must all be on the
same CPU.
The argument specifies the desired file descriptor, or -1 if output should be
ignored.
PERF_EVENT_IOC_SET_FILTER (Since Linux 2.6.33)
This adds an ftrace filter to this event.
The argument is a pointer to the desired ftrace filter.
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 official method for
determining if a kernel supports perf_event_open().
/proc/sys/kernel/perf_event_max_sample_rate
This sets the maximum sample rate. Setting this too high can allow users to
sample at a rate that impacts overall machine performance and potentially lock
up the machine. The default value is 100000 (samples per second).
/proc/sys/kernel/perf_event_mlock_kb
Maximum number of pages an unprivileged user can mlock (2) . The default is
516 (kB).
Files in /sys/bus/event_source/devices/
Since Linux 2.6.34 the kernel supports having multiple PMUs available for monitoring.
Information on how to program these PMUs can be found under
/sys/bus/event_source/devices/. Each subdirectory corresponds to a different PMU.
/sys/bus/event_source/devices/*/type (Since Linux 2.6.38)
This contains an integer that can be used in the type field of perf_event_attr
to indicate 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 performance counter
registers is allowed via the rdpmc instruction. 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 separated 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/kernel/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 kernel. In that case,
some events would silently return "0" if the kernel scheduled them in an improper counter
slot.
Prior to Linux 2.6.34 there was a bug when multiplexing where the wrong results could be
returned.
Kernels from Linux 2.6.35 to Linux 2.6.39 can quickly crash the kernel if "inherit" is
enabled and many threads are started.
Prior to Linux 2.6.35, PERF_FORMAT_GROUP did not work with attached processes.
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/.