Re: [PATCH v3] perf/core: Set event shadow time for inactive events too

[Namhyung Kim <namhyung@kernel.org>]

On Thu, Dec 9, 2021 at 3:35 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Thu, Dec 09, 2021 at 12:26:32PM +0100, Peter Zijlstra wrote:
> > On Sun, Dec 05, 2021 at 02:48:43PM -0800, Namhyung Kim wrote:
> >
> > > Actually 18446744069443110306 is 0xffffffff01b345a2 so it seems to
> > > have a negative enabled time.  In fact, bperf keeps values returned by
> > > bpf_perf_event_read_value() which calls perf_event_read_local(), and
> > > accumulates delta between two calls.  When event->shadow_ctx_time is
> > > not set, it'd return invalid enabled time which is bigger than normal.
> >
> > *that*, how does it happen that shadow_time isn't set? It should be last
> > set when the event switches to INACTIVE, no? At which point the logic in
> > perf_event_read_local() should make @enabled move forward while @running
> > stays put.
> >
> > Let me go rummage around a bit... either I'm missing something obvious
> > or something's smelly.
>
> How's this then?

Still the same :(

Maybe because the event is enabled from the beginning.
Then it might miss set_state/update_time at all.

>
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index 7b7525e9155f..82baa8bdfc86 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -686,9 +686,7 @@ 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,
> -        * or when ctx_sched_in failed to schedule the event because we
> -        * run out of PMC.
> +        * context time as it was when the event was last scheduled in.
>          *
>          * 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 28aaeacdaea1..20637b7f420c 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -640,6 +640,9 @@ __perf_update_times(struct perf_event *event, u64 now, u64 *enabled, u64 *runnin
>                 *running += delta;
>  }
>
> +static void perf_set_shadow_time(struct perf_event *event,
> +                                struct perf_event_context *ctx);
> +
>  static void perf_event_update_time(struct perf_event *event)
>  {
>         u64 now = perf_event_time(event);
> @@ -647,6 +650,7 @@ static void perf_event_update_time(struct perf_event *event)
>         __perf_update_times(event, now, &event->total_time_enabled,
>                                         &event->total_time_running);
>         event->tstamp = now;
> +       perf_set_shadow_time(event, event->ctx);

I like this.

>  }
>
>  static void perf_event_update_sibling_time(struct perf_event *leader)
> @@ -2552,8 +2556,6 @@ event_sched_in(struct perf_event *event,
>
>         perf_pmu_disable(event->pmu);
>
> -       perf_set_shadow_time(event, ctx);
> -
>         perf_log_itrace_start(event);
>
>         if (event->pmu->add(event, PERF_EF_START)) {
> @@ -3707,29 +3709,6 @@ 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;
> @@ -3748,15 +3727,14 @@ 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);
> -               } else {
> -                       ctx->rotate_necessary = 1;
> -                       perf_mux_hrtimer_restart(cpuctx);
> -                       group_update_userpage(event);
>                 }
> +
> +               *can_add_hw = 0;
> +               ctx->rotate_necessary = 1;
> +               perf_mux_hrtimer_restart(cpuctx);

Not sure about this.  We might not want to rotate them
if a pinned event failed...?

Thanks,
Namhyung


>         }
>
>         return 0;
> @@ -6349,7 +6327,6 @@ 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 {