linux-kernel - Re: [PATCH v2] events/core: fix acoount failure for event's total_enable

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Message-ID: <Z74p1QaMdNKQRNeu@gmail.com>
Date: Tue, 25 Feb 2025 21:36:37 +0100
From: Ingo Molnar <mingo@...nel.org>
To: Yeo Reum Yun <YeoReum.Yun@....com>
Cc: Peter Zijlstra <peterz@...radead.org>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"acme@...nel.org" <acme@...nel.org>,
	"namhyung@...nel.org" <namhyung@...nel.org>,
	Mark Rutland <Mark.Rutland@....com>,
	"alexander.shishkin@...ux.intel.com" <alexander.shishkin@...ux.intel.com>,
	"jolsa@...nel.org" <jolsa@...nel.org>,
	"irogers@...gle.com" <irogers@...gle.com>,
	"adrian.hunter@...el.com" <adrian.hunter@...el.com>,
	"kan.liang@...ux.intel.com" <kan.liang@...ux.intel.com>,
	"linux-perf-users@...r.kernel.org" <linux-perf-users@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] events/core: fix acoount failure for event's
 total_enable_time


* Yeo Reum Yun <YeoReum.Yun@....com> wrote:

> Hi Peter,
> 
> Sorry to late answer. I've missed your last repsonse in this thread,
> and waiting for in new thread:
> https://lore.kernel.org/all/20250110163643.GB4213@noisy.programming.kicks-ass.net/
> 
> > > This patch doesn't work when the event is child event.
> > > In case of parent's event, when you see the list_del_event(),
> > > the total_enable_time is updated properly by changing state with
> > > PERF_EVENT_STATE_OFF.
> > >
> > > However, child event's total_enable_time is added before list_del_event.
> > > So, the missing total_enable_time isn't added to parents event and the
> > > error print happens.
> > >
> > > So, I think it wouldn't be possible to update time with set_state.
> > > instead I think it should update total_enable_time before
> > > child's total_enable_time is added to parents' child_total_enable_time
> > >
> > > like
> > >
> > > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > > index 065f9188b44a..d27717c44924 100644
> > > --- a/kernel/events/core.c
> > > +++ b/kernel/events/core.c
> > > @@ -13337,6 +13337,7 @@ static void sync_child_event(struct perf_event *child_event)
> > >         }
> > >
> > >         child_val = perf_event_count(child_event, false);
> > > +       perf_event_update_time(child_event);
> > >
> > >         /*
> > >          * Add back the child's count to the parent's count:
> >
> > Well, that again violates the rule that we update time on state change.
> > 
> > AFAICT there is no issue with simply moving the perf_event_set_state()
> > up a few lines in __perf_remove_from_context().
> >
> > Notably event_sched_out() will already put us in OFF state; and nothing
> > after that cares about further states AFAICT.
> >
> > So isn't the below the simpler solution?
> >
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> > @@ -2438,14 +2438,13 @@ __perf_remove_from_context(struct perf_e
> >                state = PERF_EVENT_STATE_DEAD;
> >        }
> >       event_sched_out(event, ctx);
> > +       perf_event_set_state(event, min(event->state, state));
> >       if (flags & DETACH_GROUP)
> >                perf_group_detach(event);
> >       if (flags & DETACH_CHILD)
> >                perf_child_detach(event);
> >        list_del_event(event, ctx);
> >
> > -       perf_event_set_state(event, min(event->state, state));
> > -
> >        if (!pmu_ctx->nr_events) {
> >                pmu_ctx->rotate_necessary = 0;
> 
> Agree, for DETACH_EXIT case, below code in list_del_event()
> doesn't need to be considered because
> the all of event related to event ctx would be freed:
> 
>      /*
>       * If event was in error state, then keep it
>       * that way, otherwise bogus counts will be
>       * returned on read(). The only way to get out
>       * of error state is by explicit re-enabling
>       * of the event
>       */
>       if (event->state > PERF_EVENT_STATE_OFF) {
>           perf_cgroup_event_disable(event, ctx);
>           perf_event_set_state(event, PERF_EVENT_STATE_OFF);
>       }
> 
> With your suggestion, Could I send the v4 for this?

Yes, please send a -v4 version.

Thanks,

	Ingo