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: Fri, 10 Dec 2021 10:59:49 -0800 [thread overview]
Message-ID: <CAM9d7cgugnvrv5CJPe_EP_M8pp8h+GsCCW3-RDmyrd+JDGYJrQ@mail.gmail.com> (raw)
In-Reply-To: <20211210101950.GR16608@worktop.programming.kicks-ass.net>
On Fri, Dec 10, 2021 at 2:20 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Thu, Dec 09, 2021 at 01:51:42PM -0800, Namhyung Kim wrote:
> > 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 :(
>
> You're doing that bpf-cgroup crud, right? Where exactly do you hook into
> to do the counter reads?
That's true but it doesn't use cgroup events actually. They are plain cpu
events and BPF is called from a separate 'cgroup-switches' event to
read out the counters.
>
> > Maybe because the event is enabled from the beginning.
> > Then it might miss set_state/update_time at all.
>
> Even then, it's set to INACTIVE and any state change thereafter needs to
> go through perf_event_set_state() and update the relevant timestamps.
Right, but the problem happens when you read the event *before*
any state change.
Thanks,
Namhyung
next prev parent reply other threads:[~2021-12-10 19:00 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 [this message]
2021-12-20 9:39 ` Peter Zijlstra
2021-12-09 21:35 ` Namhyung Kim
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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