Commit 6a694a60 authored Feb 05, 2015 by Shaohua Li Committed by Ingo Molnar Feb 18, 2015

perf: Update userspace page info for software event



For hardware events, the userspace page of the event gets updated in
context switches, so if we read the timestamp in the page, we get
fresh info.

For software events, this is missing currently. This patch makes the
behavior consistent.

With this patch, we can implement clock_gettime(THREAD_CPUTIME) with
PERF_COUNT_SW_DUMMY in userspace as suggested by Andy and Peter. Code
like this:

  if (pc->cap_user_time) {
	do {
		seq = pc->lock;
		barrier();

		running = pc->time_running;
		cyc = rdtsc();
		time_mult = pc->time_mult;
		time_shift = pc->time_shift;
		time_offset = pc->time_offset;

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

	quot = (cyc >> time_shift);
	rem = cyc & ((1 << time_shift) - 1);
	delta = time_offset + quot * time_mult +
		((rem * time_mult) >> time_shift);

	running += delta;
	return running;
  }

I tried it on a busy system, the userspace page updating doesn't
have noticeable overhead.

Signed-off-by: Shaohua Li <shli@fb.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/aa2dd2e4f1e9f2225758be5ba00f14d6909a8ce1.1423180257.git.shli@fb.com


[ Improved the changelog. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent 72f669c0

kernel/events/core.c

+3 −0

Original line number	Diff line number	Diff line
		@@ -6123,6 +6123,7 @@ static int perf_swevent_add(struct perf_event *event, int flags)
		}

		hlist_add_head_rcu(&event->hlist_entry, head);
		perf_event_update_userpage(event);

		return 0;
		}
		@@ -6592,6 +6593,7 @@ static int cpu_clock_event_add(struct perf_event *event, int flags)
		{
		if (flags & PERF_EF_START)
		cpu_clock_event_start(event, flags);
		perf_event_update_userpage(event);

		return 0;
		}
		@@ -6666,6 +6668,7 @@ static int task_clock_event_add(struct perf_event *event, int flags)
		{
		if (flags & PERF_EF_START)
		task_clock_event_start(event, flags);
		perf_event_update_userpage(event);

		return 0;
		}

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