Messages in this thread Patch in this message | | | Date | Mon, 3 Apr 2023 11:08:58 +0200 | From | Peter Zijlstra <> | Subject | [PATCH] 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: 0cda4c023132 ("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> --- kernel/events/core.c | 12 +++++++----- 1 file changed, 7 insertions(+), 5 deletions(-)
diff --git a/kernel/events/core.c b/kernel/events/core.c index fb3e436bcd4a..115320faf1db 100644 --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -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);
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