Re: [PATCH] perf/core: Update cgroup time when rotating multiplexed events

[sashiko-bot@kernel.org]

> From: Antoine Huret <antoine.huret@bull.com>
> 
> Since commit bd2756811766 ("perf: Rewrite core context handling"),
> perf_rotate_context() only updates the context time before scheduling
> out the flexible events. It no longer updates the cgroup time, which
> ctx_sched_out() used to do.
> 
> Cgroup events take their times from the per-cpu cgroup clock, so
> between two cgroup switches the rotations see a frozen clock. The
> events rotated out lose the running time they accumulated, and the next
> update, usually a read, credits the whole interval to the events that
> happen to be on the PMU at that moment.
> 
> As a result, the time_running of multiplexed cgroup events is wrong,
> and so are their scaled counts, as long as no other cgroup runs on the
> CPU, e.g. a busy job alone on its CPUs. Counting cycles over one second
> of a busy loop in a cgroup gives from half to several times the max
> frequency of the CPU.
> 
> Update the cgroup time before rotating the CPU context, as was done
> before the rewrite.
> 
> Fixes: bd2756811766 ("perf: Rewrite core context handling")
> Closes: https://lore.kernel.org/r/PR3PR07MB69223331EED5BEBE150375B7849C2@PR3PR07MB6922.eurprd07.prod.outlook.com/
> Cc: stable@vger.kernel.org
> Assisted-by: Claude:claude-opus-5-5
> Signed-off-by: Antoine Huret <antoine.huret@bull.com>

Sashiko has reviewed this patch and found no issues. It looks great!

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Sashiko AI review · https://sashiko.dev/#/patchset/20261015-perf-cgroup-rotation-time-v1-1-93e177dfee5d@bull.com?part=1