Since commit bd27568117 ("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: bd27568117 ("perf: Rewrite core context handling")
Closes: https://lore.kernel.org/r/PR3PR07MB69223331EED5BEBE150375B7849C2@PR3PR07MB6922.eurprd07.prod.outlook.com/
Assisted-by: Claude:claude-opus-5-5
Signed-off-by: Antoine Huret <antoine.huret@bull.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20261015-perf-cgroup-rotation-time-v1-1-93e177dfee5d@bull.com
e4c7c401c6