[PATCH] perf/core: Avoid context switch overheads

[çç]

From: "leilei.lin" <leilei.lin@xxxxxxxxxxxxxxx>

A performance issue caused by less strickly check in task
sched when these tasks were once attached by per-task perf_event.

A task will alloc task->perf_event_ctxp[ctxn] when it was called
by perf_event_open, and task->perf_event_ctxp[ctxn] would not
ever be freed to NULL.

__perf_event_task_sched_in()
	if (task->perf_event_ctxp[ctxn]) //  here is always true
		perf_event_context_sched_in() // operate pmu

50% at most performance overhead was observed under some extreme
test case. Therefor, add a more strick check as to ctx->nr_events,
when ctx->nr_events == 0, it's no need to continue.

Signed-off-by: leilei.lin <leilei.lin@xxxxxxxxxxxxxxx>
---
 kernel/events/core.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 426c2ff..f071013 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -3179,6 +3179,9 @@ static void perf_event_context_sched_in(struct perf_event_context *ctx,
 	if (cpuctx->task_ctx == ctx)
 		return;
 
+	if (!cpuctx->task_ctx && !ctx->nr_events)
+		return;
+
 	perf_ctx_lock(cpuctx, ctx);
 	perf_pmu_disable(ctx->pmu);
 	/*
-- 
2.8.4.31.g9ed660f