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

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Message-ID: <aRvPP18TCNKKCw3r@google.com>
Date: Mon, 17 Nov 2025 17:43:27 -0800
From: Namhyung Kim <namhyung@...nel.org>
To: Dapeng Mi <dapeng1.mi@...ux.intel.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Ian Rogers <irogers@...gle.com>,
	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

Hello,

On Wed, Nov 12, 2025 at 04:05:26PM +0800, Dapeng Mi 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>

Acked-by: Namhyung Kim <namhyung@...nel.org>

Thanks,
Namhyung

> ---
> 
> 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
>