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

[Ian Rogers]

On Wed, Nov 12, 2025 at 12:07 AM Dapeng Mi <dapeng1.mi@xxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx>

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@xxxxxxxxxx>

Hopefully a maintainer can pick this up quickly. 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
>