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Date:   Mon, 3 Apr 2023 11:08:58 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Thomas Gleixner <tglx@linutronix.de>,
        Ravi Bangoria <ravi.bangoria@amd.com>,
        Ingo Molnar <mingo@kernel.org>
Cc:     linux-kernel@vger.kernel.org,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Jiri Olsa <jolsa@kernel.org>
Subject: [PATCH] perf: Optimize perf_pmu_migrate_context()
Message-ID: <20230403090858.GT4253@hirez.programming.kicks-ass.net>
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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);