sched/fair: Improve the accuracy of sched_stat_wait statistics

When the sched_schedstat changes from 0 to 1, some sched se maybe
already in the runqueue, the se->statistics.wait_start will be 0.
So it will let the (rq_of(cfs_rq)) - se->statistics.wait_start)
wrong. We need to avoid this scenario.
Signed-off-by: jun qian <qianjun.kernel@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Yafang Shao <laoar.shao@gmail.com>
Link: https://lkml.kernel.org/r/20201015064846.19809-1-qianjun.kernel@gmail.com

kernel/sched/fair.c

@@ -906,6 +906,15 @@ update_stats_wait_end(struct cfs_rq *cfs_rq, struct sched_entity *se)
 	if (!schedstat_enabled())
 		return;
 
+	/*
+	 * When the sched_schedstat changes from 0 to 1, some sched se
+	 * maybe already in the runqueue, the se->statistics.wait_start
+	 * will be 0.So it will let the delta wrong. We need to avoid this
+	 * scenario.
+	 */
+	if (unlikely(!schedstat_val(se->statistics.wait_start)))
+		return;
+
 	delta = rq_clock(rq_of(cfs_rq)) - schedstat_val(se->statistics.wait_start);
 
 	if (entity_is_task(se)) {