perf counters: update docs

thus be used to profile the code that runs on that CPU.

The Linux Performance Counter subsystem provides an abstraction of these

hardware capabilities. It provides per task and per CPU counters, and

it provides event capabilities on top of those.

hardware capabilities. It provides per task and per CPU counters, counter

groups, and it provides event capabilities on top of those.

Performance counters are accessed via special file descriptors.

There's one file descriptor per virtual counter used.

The special file descriptor is opened via the perf_counter_open()

system call:

 int

 perf_counter_open(u32 hw_event_type,

                   u32 hw_event_period,

                   u32 record_type,

                   pid_t pid,

                   int cpu);

   int sys_perf_counter_open(struct perf_counter_hw_event *hw_event_uptr,

			     pid_t pid, int cpu, int group_fd);

The syscall returns the new fd. The fd can be used via the normal

VFS system calls: read() can be used to read the counter, fcntl()

Multiple counters can be kept open at a time, and the counters

can be poll()ed.

When creating a new counter fd, 'hw_event_type' is one of:

When creating a new counter fd, 'perf_counter_hw_event' is:

/*

 * Hardware event to monitor via a performance monitoring counter:

 */

struct perf_counter_hw_event {

	s64			type;

	u64			irq_period;

	u32			record_type;

	u32			disabled     :  1, /* off by default */

				nmi	     :  1, /* NMI sampling   */

				raw	     :  1, /* raw event type */

				__reserved_1 : 29;

	u64			__reserved_2;

};

/*

 * Generalized performance counter event types, used by the hw_event.type

 * parameter of the sys_perf_counter_open() syscall:

 */

enum hw_event_types {

	PERF_COUNT_CYCLES,

	PERF_COUNT_INSTRUCTIONS,

	PERF_COUNT_CACHE_REFERENCES,

	PERF_COUNT_CACHE_MISSES,

	PERF_COUNT_BRANCH_INSTRUCTIONS,

	PERF_COUNT_BRANCH_MISSES,

	/*

	 * Common hardware events, generalized by the kernel:

	 */

	PERF_COUNT_CYCLES		=  0,

	PERF_COUNT_INSTRUCTIONS		=  1,

	PERF_COUNT_CACHE_REFERENCES	=  2,

	PERF_COUNT_CACHE_MISSES		=  3,

	PERF_COUNT_BRANCH_INSTRUCTIONS	=  4,

	PERF_COUNT_BRANCH_MISSES	=  5,

	/*

	 * Special "software" counters provided by the kernel, even if

	 * the hardware does not support performance counters. These

	 * counters measure various physical and sw events of the

	 * kernel (and allow the profiling of them as well):

	 */

	PERF_COUNT_CPU_CLOCK		= -1,

	PERF_COUNT_TASK_CLOCK		= -2,

	/*

	 * Future software events:

	 */

	/* PERF_COUNT_PAGE_FAULTS	= -3,

	   PERF_COUNT_CONTEXT_SWITCHES	= -4, */

};

These are standardized types of events that work uniformly on all CPUs

not able to count branch-misses, then the system call will return

-EINVAL.

[ Note: more hw_event_types are supported as well, but they are CPU

More hw_event_types are supported as well, but they are CPU

specific and are enumerated via /sys on a per CPU basis. Raw hw event

  types can be passed in as negative numbers. For example, to count

  "External bus cycles while bus lock signal asserted" events on Intel

  Core CPUs, pass in a -0x4064 event type value. ]

The parameter 'hw_event_period' is the number of events before waking up

a read() that is blocked on a counter fd. Zero value means a non-blocking

counter.

types can be passed in under hw_event.type if hw_event.raw is 1.

For example, to count "External bus cycles while bus lock signal asserted"

events on Intel Core CPUs, pass in a 0x4064 event type value and set

hw_event.raw to 1.

'record_type' is the type of data that a read() will provide for the

counter, and it can be one of:

  enum perf_record_type {

	PERF_RECORD_SIMPLE,

	PERF_RECORD_IRQ,

/*

 * IRQ-notification data record type:

 */

enum perf_counter_record_type {

	PERF_RECORD_SIMPLE		=  0,

	PERF_RECORD_IRQ			=  1,

	PERF_RECORD_GROUP		=  2,

};

a "simple" counter is one that counts hardware events and allows

the 8-byte counter value, plus the Instruction Pointer address of the

interrupted context.

The parameter 'hw_event_period' is the number of events before waking up

a read() that is blocked on a counter fd. Zero value means a non-blocking

counter.

The 'pid' parameter allows the counter to be specific to a task:

 pid == 0: if the pid parameter is zero, the counter is attached to the

 cpu >= 0: the counter is restricted to a specific CPU

 cpu == -1: the counter counts on all CPUs

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

(Note: the combination of 'pid == -1' and 'cpu == -1' is not valid.)

A 'pid > 0' and 'cpu == -1' counter is a per task counter that counts

events of that task and 'follows' that task to whatever CPU the task

A 'pid == -1' and 'cpu == x' counter is a per CPU counter that counts

all events on CPU-x. Per CPU counters need CAP_SYS_ADMIN privilege.

Group counters are created by passing in a group_fd of another counter.

Groups are scheduled at once and can be used with PERF_RECORD_GROUP

to record multi-dimensional timestamps.