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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
From:   Christian Borntraeger <borntraeger@de.ibm.com>
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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Date:   Mon, 2 Mar 2020 19:17:26 +0100
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On 02.03.20 12:16, Christian Borntraeger wrote:
> 
> 
> 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.


Still running fine. I can tell for sure tomorrow, but I have the impression that this makes the
WARN_ON go away.