perf: Optimize perf_pmu_migrate_context()

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>
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

diff --git a/kernel/events/core.c b/kernel/events/core.c
index fb3e436bcd4ac7e8a04da3e16123f3d8b0d066a9..115320faf1db4c1e280c0f40d18c1d6c0e9eb076 100644 (file)
--- 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);