[RFC 2/2] perf: update userspace page info for software event

[Shaohua Li]

For hardware event, the userspace page of the event gets updated in
context switch, so if we read time in the page, we get updated info. For
software event, this is missed currently. This patch makes the behavior
consistency.

With this patch, we can implement clock_gettime(THREAD_CPUTIME) with
PERF_COUNT_SW_DUMMY in userspace as suggested by Andy and Peter. Code
likes 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 in a busy system, the userspace page updating hasn't noticeable
overhead.

Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Signed-off-by: Shaohua Li <shli@xxxxxx>
---
 kernel/events/core.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 4edde3e..4221240 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -5950,6 +5950,7 @@ 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;
 }
@@ -6419,6 +6420,7 @@ 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;
 }
@@ -6493,6 +6495,7 @@ 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;
 }
-- 
1.8.1

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