Re: [PATCH] perf/core: fix userpage->time_enabled of inactive events

[Peter Zijlstra]

On Tue, Sep 21, 2021 at 06:17:15PM -0700, Song Liu wrote:
> 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();
>    }

*groan*, yes I fear you're right.

> @@ -3807,6 +3816,23 @@ ctx_sched_in(struct perf_event_context *ctx,
>  	/* Then walk through the lower prio flexible groups */
>  	if (is_active & EVENT_FLEXIBLE)
>  		ctx_flexible_sched_in(ctx, cpuctx);
> +
> +	/*
> +	 * Update userpage for inactive events. This is needed for accurate
> +	 * time_enabled.
> +	 */
> +	if (unlikely(ctx->rotate_necessary)) {
> +		struct perf_event *event;
> +
> +		perf_event_groups_for_each(event, &ctx->pinned_groups) {
> +			if (event->state == PERF_EVENT_STATE_INACTIVE)
> +				perf_event_update_inactive_userpage(event, ctx);
> +		}

That's a straight up error, if a pinned event doesn't get scheduled,
it's game over.

> +		perf_event_groups_for_each(event, &ctx->flexible_groups) {
> +			if (event->state == PERF_EVENT_STATE_INACTIVE)
> +				perf_event_update_inactive_userpage(event, ctx);
> +		}
> +	}
>  }

That's terrible though, and also wrong I think.

It's wrong because:

 - you should only do this for (is_active & EVENT_TIME)
 - you should only consider the events visit_groups_merge() would have
 - you miss updating group-siling events

(I also think it's possible to try harder to avoid the work)

Now, looking at visit_groups_merge() we only terminate the iteration
when func returns !0, which is merge_sched_in(), and that *never*
returns !0.

So we already iterate all the right events. So I'm thinking we can do
something like the below, hmm?


diff --git a/kernel/events/core.c b/kernel/events/core.c
index 0c000cb01eeb..4d1e962c2ebe 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -3707,6 +3712,28 @@ static noinline int visit_groups_merge(struct perf_cpu_context *cpuctx,
 	return 0;
 }
 
+static inline bool event_update_userpage(struct perf_event *event)
+{
+	if (!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 +3752,18 @@ 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;
-		ctx->rotate_necessary = 1;
-		perf_mux_hrtimer_restart(cpuctx);
+		} else {
+			ctx->rotate_necessary = 1;
+			perf_mux_hrtimer_restart(cpuctx);
+
+			group_update_userpage(event);
+		}
 	}
 
 	return 0;