From: Peter Zijlstra peterz@infradead.org
[ 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@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.... Signed-off-by: Sasha Levin sashal@kernel.org --- 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);