[tip: perf/urgent] perf: Optimize perf_pmu_migrate_context()

[tip-bot2 for Peter Zijlstra]

The following commit has been merged into the perf/urgent branch of tip:

Commit-ID:     b168098912926236bbeebaf7795eb7aab76d2b45
Gitweb:        https://git.kernel.org/tip/b168098912926236bbeebaf7795eb7aab76d2b45
Author:        Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate:    Mon, 03 Apr 2023 11:08:58 +02:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Wed, 05 Apr 2023 09:58:46 +02:00

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@xxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Tested-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Reviewed-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Reviewed-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Link: https://lkml.kernel.org/r/20230403090858.GT4253@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
---
 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 fb3e436..115320f 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);