linux-kernel - Re: 5.6-rc3: WARNING: CPU: 48 PID: 17435 at kernel/sched/fair.c:380 enqueue_task

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Date:   Fri, 28 Feb 2020 17:35:45 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Christian Borntraeger <borntraeger@...ibm.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: 5.6-rc3: WARNING: CPU: 48 PID: 17435 at kernel/sched/fair.c:380
 enqueue_task_fair+0x328/0x440

Le vendredi 28 févr. 2020 à 16:42:27 (+0100), Christian Borntraeger a écrit :
> 
> 
> On 28.02.20 16:37, Vincent Guittot wrote:
> > On Fri, 28 Feb 2020 at 16:08, Christian Borntraeger
> > <borntraeger@...ibm.com> wrote:
> >>
> >> Also happened with 5.4:
> >> Seems that I just happen to have an interesting test workload/system size interaction
> >> on a newly installed system that triggers this.
> > 
> > you will probably go back to 5.1 which is the version where we put
> > back the deletion of unused cfs_rq from the list which can trigger the
> > warning:
> > commit 039ae8bcf7a5 : (Fix O(nr_cgroups) in the load balancing path)
> > 
> > AFAICT, we haven't changed this since
> 
> So you do know what is the problem? If not is there any debug option or
> patch that I could apply to give you more information?

No I don't know what is happening. Your test probably goes through an unexpected path

Would it be difficult for me to reproduce your test env ?

There is an optimization in the code which could generate problem if assumption is not
true. Could you try the patch below ?

---
 kernel/sched/fair.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 3c8a379c357e..beb773c23e7d 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4035,8 +4035,8 @@ enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
 		__enqueue_entity(cfs_rq, se);
 	se->on_rq = 1;
 
+	list_add_leaf_cfs_rq(cfs_rq);
 	if (cfs_rq->nr_running == 1) {
-		list_add_leaf_cfs_rq(cfs_rq);
 		check_enqueue_throttle(cfs_rq);
 	}
 }
-- 
2.17.1



>