6.18-stable review patch. If anyone has any objections, please let me know. ------------------ From: Antoine Huret commit e4c7c401c6b9174c0c893084fdaa6a84bd2520d7 upstream. 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/ Assisted-by: Claude:claude-opus-5-5 Signed-off-by: Antoine Huret Signed-off-by: Peter Zijlstra (Intel) Cc: stable@vger.kernel.org Link: https://patch.msgid.link/20261015-perf-cgroup-rotation-time-v1-1-93e177dfee5d@bull.com Signed-off-by: Greg Kroah-Hartman --- kernel/events/core.c | 1 + 1 file changed, 1 insertion(+) --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -4496,6 +4496,7 @@ static bool perf_rotate_context(struct p if (cpu_event) { update_context_time(&cpuctx->ctx); + update_cgrp_time_from_cpuctx(cpuctx, false); __pmu_ctx_sched_out(cpu_epc, EVENT_FLEXIBLE); rotate_ctx(&cpuctx->ctx, cpu_event); __pmu_ctx_sched_in(cpu_epc, EVENT_FLEXIBLE);