linux-kernel - Re: [PATCH] perf: Fix 0 count issue of cpu-clock

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Message-ID: <CAP-5=fVCo=9f0_2pt=4mXZ4Zc0Ah4DPEVBH9dUTe+rOs5vnpkg@mail.gmail.com>
Date: Mon, 17 Nov 2025 09:04:03 -0800
From: Ian Rogers <irogers@...gle.com>
To: Dapeng Mi <dapeng1.mi@...ux.intel.com>, Ingo Molnar <mingo@...hat.com>, 
	Peter Zijlstra <peterz@...radead.org>
Cc: Arnaldo Carvalho de Melo <acme@...nel.org>, Namhyung Kim <namhyung@...nel.org>, 
	Adrian Hunter <adrian.hunter@...el.com>, 
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Andi Kleen <ak@...ux.intel.com>, 
	Eranian Stephane <eranian@...gle.com>, linux-kernel@...r.kernel.org, 
	linux-perf-users@...r.kernel.org, Dapeng Mi <dapeng1.mi@...el.com>, 
	Zide Chen <zide.chen@...el.com>, Falcon Thomas <thomas.falcon@...el.com>, 
	Xudong Hao <xudong.hao@...el.com>
Subject: Re: [PATCH] perf: Fix 0 count issue of cpu-clock

On Wed, Nov 12, 2025 at 8:42 AM Ian Rogers <irogers@...gle.com> wrote:
>
> On Wed, Nov 12, 2025 at 12:07 AM Dapeng Mi <dapeng1.mi@...ux.intel.com> wrote:
> >
> > Currently cpu-clock event always returns 0 count, e.g.,
> >
> > perf stat -e cpu-clock -- sleep 1
> >
> >  Performance counter stats for 'sleep 1':
> >                  0      cpu-clock                        #    0.000 CPUs utilized
> >        1.002308394 seconds time elapsed
> >
> > The root cause is the commit 'bc4394e5e79c ("perf: Fix the throttle
> >  error of some clock events")' adds PERF_EF_UPDATE flag check before
> > calling cpu_clock_event_update() to update the count, however the
> > PERF_EF_UPDATE flag is never set when the cpu-clock event is stopped in
> > counting mode (pmu->dev() -> cpu_clock_event_del() ->
> > cpu_clock_event_stop()). This leads to the cpu-clock event count is
> > never updated.
> >
> > To fix this issue, force to set PERF_EF_UPDATE flag for cpu-clock event
> > just like what task-clock does. Besides, or flags with PERF_EF_UPDATE
> > for task-clock although currently the flags argument would always be 0.
> >
> > Fixes: bc4394e5e79c ("perf: Fix the throttle error of some clock events")
> > Signed-off-by: Dapeng Mi <dapeng1.mi@...ux.intel.com>
>
> Thanks Dapeng! This is a fairly major regression and I'm surprised my
> kernel picked it up so quickly. For those interested the relevant part
> of the breaking change is requiring PERF_EF_UPDATE:
>
> ```
> @@ -11829,7 +11834,8 @@ static void cpu_clock_event_start(struct
> perf_event *event, int flags)
>  static void cpu_clock_event_stop(struct perf_event *event, int flags)
>  {
>         perf_swevent_cancel_hrtimer(event);
> -       cpu_clock_event_update(event);
> +       if (flags & PERF_EF_UPDATE)
> +               cpu_clock_event_update(event);
>  }
>  ```
>
> Reviewed-by: Ian Rogers <irogers@...gle.com>
>
> Hopefully a maintainer can pick this up quickly. Thanks,
> Ian

Hi Peter and Ingo,

any update on picking this up? Having timer based software events
(task-clock, cpu-clock) broken is fairly major regression especially
in environments like hypervisors where PMU access is blocked.

Thanks,
Ian

> > ---
> >
> > With this change, both cpu-clock and task-clock can do counting and
> > samping correctly.
> >
> > 1. perf stat -e cpu-clock,task-clock -- true
> >
> >  Performance counter stats for 'true':
> >            240,636      cpu-clock                        #    0.358 CPUs utilized
> >            243,319      task-clock                       #    0.362 CPUs utilized
> >
> > 2. perf record -e cpu-clock -c 10000 -Iax,bx -- sleep 1
> > [ perf record: Woken up 2 times to write data ]
> > [ perf record: Captured and wrote 0.028 MB perf.data (36 samples) ]
> >
> > 3. perf record -e task-clock -c 10000 -Iax,bx -- sleep 1
> > [ perf record: Woken up 1 times to write data ]
> > [ perf record: Captured and wrote 0.029 MB perf.data (41 samples) ]
> >
> >  kernel/events/core.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > index f6a08c73f783..77d3af5959c1 100644
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> > @@ -11964,7 +11964,7 @@ static int cpu_clock_event_add(struct perf_event *event, int flags)
> >
> >  static void cpu_clock_event_del(struct perf_event *event, int flags)
> >  {
> > -       cpu_clock_event_stop(event, flags);
> > +       cpu_clock_event_stop(event, flags | PERF_EF_UPDATE);
> >  }
> >
> >  static void cpu_clock_event_read(struct perf_event *event)
> > @@ -12043,7 +12043,7 @@ static int task_clock_event_add(struct perf_event *event, int flags)
> >
> >  static void task_clock_event_del(struct perf_event *event, int flags)
> >  {
> > -       task_clock_event_stop(event, PERF_EF_UPDATE);
> > +       task_clock_event_stop(event, flags | PERF_EF_UPDATE);
> >  }
> >
> >  static void task_clock_event_read(struct perf_event *event)
> >
> > base-commit: 2093d8cf80fa5552d1025a78a8f3a10bf3b6466e
> > prerequisite-patch-id: a15bcd62a8dcd219d17489eef88b66ea5488a2a0
> > prerequisite-patch-id: 2a0eefce67b21d1f30c272fd8115b0dc1aca3897
> > --
> > 2.34.1
> >