perf/core: Avoid context switch overheads

From: "leilei.lin" <[email protected]>

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. Therefore, add a more strick check as to ctx->nr_events, when ctx->nr_events == 0, it's no need to continue.

---
 kernel/events/core.c | 9 +++++++++
 1 file changed, 9 insertions(+)

index 426c2ff..3d86695 100644
@@ -3180,6 +3180,13 @@ static void perf_event_context_sched_in(struct perf_event_context *ctx,
 		return;
 
 	perf_ctx_lock(cpuctx, ctx);
+	/*
+	 * We must check ctx->nr_events while holding ctx->lock, such
+	 * that we serialize against perf_install_in_context().
+	 */
+	if (!ctx->nr_events)
+		goto unlock;
+
 	perf_pmu_disable(ctx->pmu);
 	/*
 	 * We want to keep the following priority order:

@@ -3193,6 +3200,8 @@ static void perf_event_context_sched_in(struct perf_event_context *ctx,
 		cpu_ctx_sched_out(cpuctx, EVENT_FLEXIBLE);
 	perf_event_sched_in(cpuctx, ctx, task);
 	perf_pmu_enable(ctx->pmu);
+
+unlock:
 	perf_ctx_unlock(cpuctx, ctx);
 }
 

2.8.4.31.g9ed660f