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perf: Optimize perf_pmu_migrate_context()

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Thomas reported that offlining CPUs spends a lot of time in
synchronize_rcu() as called from perf_pmu_migrate_context() even though
he's not actually using uncore events.

Turns out, the thing is unconditionally waiting for RCU, even if there's
no actual events to migrate.

Fixes: 0cda4c0231 ("perf: Introduce perf_pmu_migrate_context()")
Reported-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Link: https://lkml.kernel.org/r/20230403090858.GT4253@hirez.programming.kicks-ass.net
This commit is contained in:
Peter Zijlstra 2023-04-03 11:08:58 +02:00
parent 197b6b60ae
commit b168098912
1 changed files with 7 additions and 5 deletions
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12
kernel/events/core.c
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@ -12893,12 +12893,14 @@ void perf_pmu_migrate_context(struct pmu *pmu, int src_cpu, int dst_cpu)
__perf_pmu_remove(src_ctx, src_cpu, pmu, &src_ctx->pinned_groups, &events);
__perf_pmu_remove(src_ctx, src_cpu, pmu, &src_ctx->flexible_groups, &events);
/*
* Wait for the events to quiesce before re-instating them.
*/
synchronize_rcu();
if (!list_empty(&events)) {
/*
* Wait for the events to quiesce before re-instating them.
*/
synchronize_rcu();
__perf_pmu_install(dst_ctx, dst_cpu, pmu, &events);
__perf_pmu_install(dst_ctx, dst_cpu, pmu, &events);
}
mutex_unlock(&dst_ctx->mutex);
mutex_unlock(&src_ctx->mutex);