perf tools: Update some code references in design.txt

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_event_open()

The special file descriptor is opened via the sys_perf_event_open()

system call:

   int sys_perf_event_open(struct perf_event_attr *hw_event_uptr,

If 'raw_type' is 0, then the 'type' field says what kind of counter

this is, with the following encoding:

enum perf_event_types {

enum perf_type_id {

	PERF_TYPE_HARDWARE		= 0,

	PERF_TYPE_SOFTWARE		= 1,

	PERF_TYPE_TRACEPOINT		= 2,

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

 * parameter of the sys_perf_event_open() syscall:

 */

enum hw_event_ids {

enum perf_hw_id {

	/*

	 * Common hardware events, generalized by the kernel:

	 */

 * physical and sw events of the kernel (and allow the profiling of them as

 * well):

 */

enum sw_event_ids {

enum perf_sw_ids {

	PERF_COUNT_SW_CPU_CLOCK		= 0,

	PERF_COUNT_SW_TASK_CLOCK	= 1,

	PERF_COUNT_SW_PAGE_FAULTS	= 2,

The 'comm' bit allows tracking of process comm data on process creation.

This too is recorded in the ring-buffer (see below).

The 'pid' parameter to the perf_event_open() system call allows the

The 'pid' parameter to the sys_perf_event_open() system call allows the

counter to be specific to a task:

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

The 'group_fd' parameter allows counter "groups" to be set up.  A

counter group has one counter which is the group "leader".  The leader

is created first, with group_fd = -1 in the perf_event_open call

is created first, with group_fd = -1 in the sys_perf_event_open call

that creates it.  The rest of the group members are created

subsequently, with group_fd giving the fd of the group leader.

(A single counter on its own is created with group_fd = -1 and is