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Subject: Re: 5.6-rc3: WARNING: CPU: 48 PID: 17435 at kernel/sched/fair.c:380
 enqueue_task_fair+0x328/0x440
To:     Vincent Guittot <vincent.guittot@linaro.org>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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From:   Christian Borntraeger <borntraeger@de.ibm.com>
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Date:   Mon, 2 Mar 2020 12:16:26 +0100
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On 28.02.20 17:35, Vincent Guittot wrote:
> 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@de.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 ?

Not sure. Its a 32CPU (SMT2 -> 64) host. I have about 10 KVM guests running doing different
things.

> 
> 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);
>  	}
>  }

Now running for 3 hours. I have not seen the issue yet. I can tell tomorrow if this fixes 
the issue.