From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
[ Upstream commit b168098912926236bbeebaf7795eb7aab76d2b45 ]
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
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
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 fad170b475921..4b3205f6bed5e 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -12875,12 +12875,14 @@ 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);
--
2.39.2
