[tip: sched/core] sched/fair: Improve the accuracy of sched_stat_wait statistics

[tip-bot2 for jun qian]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     b9c88f752268383beff0d56e50d52b8ae62a02f8
Gitweb:        https://git.kernel.org/tip/b9c88f752268383beff0d56e50d52b8ae62a02f8
Author:        jun qian <qianjun.kernel@xxxxxxxxx>
AuthorDate:    Thu, 15 Oct 2020 14:48:46 +08:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Thu, 29 Oct 2020 11:00:28 +01:00

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@xxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Yafang Shao <laoar.shao@xxxxxxxxx>
Link: https://lkml.kernel.org/r/20201015064846.19809-1-qianjun.kernel@xxxxxxxxx
---
 kernel/sched/fair.c |  9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 290f9e3..b9368d1 100644
--- a/kernel/sched/fair.c
+++ b/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)) {