[PATCH 5.4] sched/fair: Fix overutilized update in enqueue_task_fair()

[PATCH 5.4] sched/fair: Fix overutilized update in enqueue_task_fair()

[Quentin Perret]

[ Upstream commit 8e1ac4299a6e8726de42310d9c1379f188140c71 ]

enqueue_task_fair() attempts to skip the overutilized update for new
tasks as their util_avg is not accurate yet. However, the flag we check
to do so is overwritten earlier on in the function, which makes the
condition pretty much a nop.

Fix this by saving the flag early on.

Fixes: 2802bf3cd936 ("sched/fair: Add over-utilization/tipping point indicator")
Reported-by: Rick Yiu <rickyiu@google.com>
Signed-off-by: Quentin Perret <qperret@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Link: https://lkml.kernel.org/r/20201112111201.2081902-1-qperret@google.com
---
 kernel/sched/fair.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index dddaf61378f6..200e12110109 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5228,6 +5228,7 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
 	struct cfs_rq *cfs_rq;
 	struct sched_entity *se = &p->se;
 	int idle_h_nr_running = task_has_idle_policy(p);
+	int task_new = !(flags & ENQUEUE_WAKEUP);
 
 	/*
 	 * The code below (indirectly) updates schedutil which looks at
@@ -5299,7 +5300,7 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
 		 * into account, but that is not straightforward to implement,
 		 * and the following generally works well enough in practice.
 		 */
-		if (flags & ENQUEUE_WAKEUP)
+		if (!task_new)
 			update_overutilized_status(rq);
 
 	}
-- 
2.29.2.454.gaff20da3a2-goog

Re: [PATCH 5.4] sched/fair: Fix overutilized update in enqueue_task_fair()

[Greg KH]

On Mon, Nov 23, 2020 at 10:18:27AM +0000, Quentin Perret wrote:
> [ Upstream commit 8e1ac4299a6e8726de42310d9c1379f188140c71 ]
> 
> enqueue_task_fair() attempts to skip the overutilized update for new
> tasks as their util_avg is not accurate yet. However, the flag we check
> to do so is overwritten earlier on in the function, which makes the
> condition pretty much a nop.
> 
> Fix this by saving the flag early on.

Now queued up, thanks.

greg k-h