perf/core: Fix perf_event_read()

perf_event_read() has a number of issues regarding the timekeeping bits.

 - The IPI didn't update group times when it found INACTIVE

 - The direct call would not re-check ->state after taking ctx->lock
   which can result in ->count and timestamps getting out of sync.

And we can make use of the ordering introduced for perf_event_stop()
to make it more accurate for ACTIVE.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/events/core.c

@@ -2081,8 +2081,9 @@ event_sched_in(struct perf_event *event,
 
 	WRITE_ONCE(event->oncpu, smp_processor_id());
 	/*
-	 * Order event::oncpu write to happen before the ACTIVE state
-	 * is visible.
+	 * Order event::oncpu write to happen before the ACTIVE state is
+	 * visible. This allows perf_event_{stop,read}() to observe the correct
+	 * ->oncpu if it sees ACTIVE.
 	 */
 	smp_wmb();
 	WRITE_ONCE(event->state, PERF_EVENT_STATE_ACTIVE);
@@ -3638,12 +3639,16 @@ static void __perf_event_read(void *info)
 		return;
 
 	raw_spin_lock(&ctx->lock);
-	if (ctx->is_active) {
+	if (ctx->is_active & EVENT_TIME) {
 		update_context_time(ctx);
 		update_cgrp_time_from_event(event);
 	}
 
-	update_event_times(event);
+	if (!data->group)
+		update_event_times(event);
+	else
+		update_group_times(event);
+
 	if (event->state != PERF_EVENT_STATE_ACTIVE)
 		goto unlock;
 
@@ -3658,7 +3663,6 @@ static void __perf_event_read(void *info)
 	pmu->read(event);
 
 	list_for_each_entry(sub, &event->sibling_list, group_entry) {
-		update_event_times(sub);
 		if (sub->state == PERF_EVENT_STATE_ACTIVE) {
 			/*
 			 * Use sibling's PMU rather than @event's since
@@ -3748,23 +3752,35 @@ out:
 
 static int perf_event_read(struct perf_event *event, bool group)
 {
+	enum perf_event_state state = READ_ONCE(event->state);
 	int event_cpu, ret = 0;
 
 	/*
 	 * If event is enabled and currently active on a CPU, update the
 	 * value in the event structure:
 	 */
-	if (event->state == PERF_EVENT_STATE_ACTIVE) {
-		struct perf_read_data data = {
-			.event = event,
-			.group = group,
-			.ret = 0,
-		};
+again:
+	if (state == PERF_EVENT_STATE_ACTIVE) {
+		struct perf_read_data data;
+
+		/*
+		 * Orders the ->state and ->oncpu loads such that if we see
+		 * ACTIVE we must also see the right ->oncpu.
+		 *
+		 * Matches the smp_wmb() from event_sched_in().
+		 */
+		smp_rmb();
 
 		event_cpu = READ_ONCE(event->oncpu);
 		if ((unsigned)event_cpu >= nr_cpu_ids)
 			return 0;
 
+		data = (struct perf_read_data){
+			.event = event,
+			.group = group,
+			.ret = 0,
+		};
+
 		preempt_disable();
 		event_cpu = __perf_event_read_cpu(event, event_cpu);
 
@@ -3781,20 +3797,27 @@ static int perf_event_read(struct perf_event *event, bool group)
 		(void)smp_call_function_single(event_cpu, __perf_event_read, &data, 1);
 		preempt_enable();
 		ret = data.ret;
-	} else if (event->state == PERF_EVENT_STATE_INACTIVE) {
+
+	} else if (state == PERF_EVENT_STATE_INACTIVE) {
 		struct perf_event_context *ctx = event->ctx;
 		unsigned long flags;
 
 		raw_spin_lock_irqsave(&ctx->lock, flags);
+		state = event->state;
+		if (state != PERF_EVENT_STATE_INACTIVE) {
+			raw_spin_unlock_irqrestore(&ctx->lock, flags);
+			goto again;
+		}
+
 		/*
-		 * may read while context is not active
-		 * (e.g., thread is blocked), in that case
-		 * we cannot update context time
+		 * May read while context is not active (e.g., thread is
+		 * blocked), in that case we cannot update context time
 		 */
-		if (ctx->is_active) {
+		if (ctx->is_active & EVENT_TIME) {
 			update_context_time(ctx);
 			update_cgrp_time_from_event(event);
 		}
+
 		if (group)
 			update_group_times(event);
 		else