Commit f7925653 authored by Song Liu's avatar Song Liu Committed by Peter Zijlstra
Browse files

perf/core: fix userpage->time_enabled of inactive events 



Users of rdpmc rely on the mmapped user page to calculate accurate
time_enabled. Currently, userpage->time_enabled is only updated when the
event is added to the pmu. As a result, inactive event (due to counter
multiplexing) does not have accurate userpage->time_enabled. This can
be reproduced with something like:

   /* open 20 task perf_event "cycles", to create multiplexing */

   fd = perf_event_open();  /* open task perf_event "cycles" */
   userpage = mmap(fd);     /* use mmap and rdmpc */

   while (true) {
     time_enabled_mmap = xxx; /* use logic in perf_event_mmap_page */
     time_enabled_read = read(fd).time_enabled;
     if (time_enabled_mmap > time_enabled_read)
         BUG();
   }

Fix this by updating userpage for inactive events in merge_sched_in.

Suggested-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Reported-and-tested-by: default avatarLucian Grijincu <lucian@fb.com>
Signed-off-by: default avatarSong Liu <songliubraving@fb.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20210929194313.2398474-1-songliubraving@fb.com
parent ecc2123e

include/linux/perf_event.h

+3 −1
Original line number Diff line number Diff line
@@ -683,7 +683,9 @@ struct perf_event { 
	/*

	 * timestamp shadows the actual context timing but it can

	 * be safely used in NMI interrupt context. It reflects the

	 * context time as it was when the event was last scheduled in.

	 * context time as it was when the event was last scheduled in,

	 * or when ctx_sched_in failed to schedule the event because we

	 * run out of PMC.

	 *

	 * ctx_time already accounts for ctx->timestamp. Therefore to

	 * compute ctx_time for a sample, simply add perf_clock().

kernel/events/core.c

+30 −4
Original line number Diff line number Diff line
@@ -3707,6 +3707,29 @@ static noinline int visit_groups_merge(struct perf_cpu_context *cpuctx, 
	return 0;

}



static inline bool event_update_userpage(struct perf_event *event)

{

	if (likely(!atomic_read(&event->mmap_count)))

		return false;



	perf_event_update_time(event);

	perf_set_shadow_time(event, event->ctx);

	perf_event_update_userpage(event);



	return true;

}



static inline void group_update_userpage(struct perf_event *group_event)

{

	struct perf_event *event;



	if (!event_update_userpage(group_event))

		return;



	for_each_sibling_event(event, group_event)

		event_update_userpage(event);

}



static int merge_sched_in(struct perf_event *event, void *data)

{

	struct perf_event_context *ctx = event->ctx;
@@ -3725,14 +3748,15 @@ static int merge_sched_in(struct perf_event *event, void *data) 
	}



	if (event->state == PERF_EVENT_STATE_INACTIVE) {

		*can_add_hw = 0;

		if (event->attr.pinned) {

			perf_cgroup_event_disable(event, ctx);

			perf_event_set_state(event, PERF_EVENT_STATE_ERROR);

		}



		*can_add_hw = 0;

		} else {

			ctx->rotate_necessary = 1;

			perf_mux_hrtimer_restart(cpuctx);

			group_update_userpage(event);

		}

	}



	return 0;
@@ -6324,6 +6348,8 @@ static int perf_mmap(struct file *file, struct vm_area_struct *vma) 


		ring_buffer_attach(event, rb);



		perf_event_update_time(event);

		perf_set_shadow_time(event, event->ctx);

		perf_event_init_userpage(event);

		perf_event_update_userpage(event);

	} else {