From: Namhyung Kim <namhyung@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>,
Arnaldo Carvalho de Melo <acme@kernel.org>,
Jiri Olsa <jolsa@redhat.com>, Mark Rutland <mark.rutland@arm.com>,
Alexander Shishkin <alexander.shishkin@linux.intel.com>,
LKML <linux-kernel@vger.kernel.org>,
Stephane Eranian <eranian@google.com>,
Andi Kleen <ak@linux.intel.com>, Ian Rogers <irogers@google.com>,
Song Liu <songliubraving@fb.com>
Subject: Re: [PATCH v3] perf/core: Set event shadow time for inactive events too
Date: Thu, 9 Dec 2021 13:35:11 -0800 [thread overview]
Message-ID: <CAM9d7ciJTJB1rumzmxGeJrAdeE9R4eXhtJRUQGj9y6DBN-ovig@mail.gmail.com> (raw)
In-Reply-To: <YbHn6JaaOo3b5GLO@hirez.programming.kicks-ass.net>
On Thu, Dec 9, 2021 at 3:26 AM Peter Zijlstra <peterz@infradead.org> 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?
As you can see, perf_event_set_state() doesn't set the shadow time.
It's called from event_sched_in() which might result in ACTIVE or
INACTIVE. But the problem is that there's a case that event_sched_in
was not called at all - when group_can_go_on() returns false.
> At which point the logic in
> perf_event_read_local() should make @enabled move forward while @running
> stays put.
It's not about updating event->total_time_enabled, it only
afftects the returned value of @enabled.
I'd say the time calculation is broken so it'd break @running
as well. But this case can only happen on INACTIVE -
otherwise it'd call event_sched_in() and update the shadow
time properly, so no issue there. And then we can see
the broken value of enabled time only.
>
> Let me go rummage around a bit... either I'm missing something obvious
> or something's smelly.
Thank you for doing that!
Thanks,
Namhyung
next prev parent reply other threads:[~2021-12-09 21:35 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-12-05 22:48 [PATCH v3] perf/core: Set event shadow time for inactive events too Namhyung Kim
2021-12-06 23:11 ` Song Liu
2021-12-08 23:22 ` Peter Zijlstra
2021-12-09 5:52 ` Namhyung Kim
2021-12-09 8:21 ` Peter Zijlstra
2021-12-09 11:26 ` Peter Zijlstra
2021-12-09 11:34 ` Peter Zijlstra
2021-12-09 21:51 ` Namhyung Kim
2021-12-10 10:19 ` Peter Zijlstra
2021-12-10 18:59 ` Namhyung Kim
2021-12-20 9:39 ` Peter Zijlstra
2021-12-09 21:35 ` Namhyung Kim [this message]
2021-12-10 10:33 ` Peter Zijlstra
2021-12-10 23:19 ` Namhyung Kim
2021-12-17 16:35 ` Peter Zijlstra
2021-12-18 9:09 ` Song Liu
2021-12-20 9:30 ` Peter Zijlstra
2021-12-20 9:54 ` Peter Zijlstra
2021-12-21 12:39 ` Peter Zijlstra
2021-12-20 12:19 ` Peter Zijlstra
2021-12-21 5:54 ` Namhyung Kim
2021-12-21 7:23 ` Song Liu
2021-12-21 11:21 ` Peter Zijlstra
2022-01-18 11:17 ` [tip: perf/urgent] perf: Fix perf_event_read_local() time tip-bot2 for Peter Zijlstra
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