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

[Song Liu <songliubraving@fb.com>]

Hi Peter, 

> On Sep 21, 2021, at 6:17 PM, Song Liu <songliubraving@fb.com> 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();
>   }
> 
> Fix this by updating userpage for inactive events in ctx_sched_in.
> 
> Reported-and-tested-by: Lucian Grijincu <lucian@fb.com>
> Signed-off-by: Song Liu <songliubraving@fb.com>

Could you please share your thoughts on this? It works well in our 
tests, but we would like to know your opinion before shipping it to
production. 

Thanks,
Song


> ---
> include/linux/perf_event.h |  4 +++-
> kernel/events/core.c       | 26 ++++++++++++++++++++++++++
> 2 files changed, 29 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index 2d510ad750edc..4aa52f7a48c16 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -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().
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 1cb1f9b8392e2..549ce22df7bc7 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -3766,6 +3766,15 @@ ctx_flexible_sched_in(struct perf_event_context *ctx,
> 			   merge_sched_in, &can_add_hw);
> }
> 
> +static inline void
> +perf_event_update_inactive_userpage(struct perf_event *event,
> +				    struct perf_event_context *ctx)
> +{
> +	perf_event_update_time(event);
> +	perf_set_shadow_time(event, ctx);
> +	perf_event_update_userpage(event);
> +}
> +
> static void
> ctx_sched_in(struct perf_event_context *ctx,
> 	     struct perf_cpu_context *cpuctx,
> @@ -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);
> +		}
> +		perf_event_groups_for_each(event, &ctx->flexible_groups) {
> +			if (event->state == PERF_EVENT_STATE_INACTIVE)
> +				perf_event_update_inactive_userpage(event, ctx);
> +		}
> +	}
> }
> 
> static void cpu_ctx_sched_in(struct perf_cpu_context *cpuctx,
> -- 
> 2.30.2
>