perf: Provide better condition for event rotation

Try to avoid useless rotation and PMU disables.

[ Could be improved by keeping a nr_runnable count to better account
  for the < PERF_STAT_INACTIVE counters ]

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Arnaldo Carvalho de Melo <acme@infradead.org>
Cc: paulus@samba.org
Cc: eranian@google.com
Cc: robert.richter@amd.com
Cc: fweisbec@gmail.com
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/perf_event.c

@@ -1524,12 +1524,15 @@ static void perf_ctx_adjust_freq(struct perf_event_context *ctx)
 		 */
 		if (interrupts == MAX_INTERRUPTS) {
 			perf_log_throttle(event, 1);
+			perf_disable();
 			event->pmu->unthrottle(event);
+			perf_enable();
 		}
 
 		if (!event->attr.freq || !event->attr.sample_freq)
 			continue;
 
+		perf_disable();
 		event->pmu->read(event);
 		now = atomic64_read(&event->count);
 		delta = now - hwc->freq_count_stamp;
@@ -1537,6 +1540,7 @@ static void perf_ctx_adjust_freq(struct perf_event_context *ctx)
 
 		if (delta > 0)
 			perf_adjust_period(event, TICK_NSEC, delta);
+		perf_enable();
 	}
 	raw_spin_unlock(&ctx->lock);
 }
@@ -1546,9 +1550,6 @@ static void perf_ctx_adjust_freq(struct perf_event_context *ctx)
  */
 static void rotate_ctx(struct perf_event_context *ctx)
 {
-	if (!ctx->nr_events)
-		return;
-
 	raw_spin_lock(&ctx->lock);
 
 	/* Rotate the first entry last of non-pinned groups */
@@ -1561,19 +1562,28 @@ void perf_event_task_tick(struct task_struct *curr)
 {
 	struct perf_cpu_context *cpuctx;
 	struct perf_event_context *ctx;
+	int rotate = 0;
 
 	if (!atomic_read(&nr_events))
 		return;
 
 	cpuctx = &__get_cpu_var(perf_cpu_context);
-	ctx = curr->perf_event_ctxp;
+	if (cpuctx->ctx.nr_events &&
+	    cpuctx->ctx.nr_events != cpuctx->ctx.nr_active)
+		rotate = 1;
 
-	perf_disable();
+	ctx = curr->perf_event_ctxp;
+	if (ctx && ctx->nr_events && ctx->nr_events != ctx->nr_active)
+		rotate = 1;
 
 	perf_ctx_adjust_freq(&cpuctx->ctx);
 	if (ctx)
 		perf_ctx_adjust_freq(ctx);
 
+	if (!rotate)
+		return;
+
+	perf_disable();
 	cpu_ctx_sched_out(cpuctx, EVENT_FLEXIBLE);
 	if (ctx)
 		task_ctx_sched_out(ctx, EVENT_FLEXIBLE);
@@ -1585,7 +1595,6 @@ void perf_event_task_tick(struct task_struct *curr)
 	cpu_ctx_sched_in(cpuctx, EVENT_FLEXIBLE);
 	if (ctx)
 		task_ctx_sched_in(curr, EVENT_FLEXIBLE);
-
 	perf_enable();
 }