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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>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>
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:   Thu, 5 Mar 2020 14:18:27 +0100
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On 05.03.20 13:48, Christian Borntraeger wrote:
> 
> 
> On 05.03.20 13:33, Vincent Guittot wrote:
>> Le jeudi 05 mars 2020 à 13:12:39 (+0100), Dietmar Eggemann a écrit :
>>> On 05/03/2020 12:28, Christian Borntraeger wrote:
>>>>
>>>> On 05.03.20 10:30, Vincent Guittot wrote:
>>>>> Le mercredi 04 mars 2020 à 20:59:33 (+0100), Christian Borntraeger a écrit :
>>>>>>
>>>>>> On 04.03.20 20:38, Christian Borntraeger wrote:
>>>>>>>
>>>>>>>
>>>>>>> On 04.03.20 20:19, Dietmar Eggemann wrote:
>>>
>>> [...]
>>>
>>>> It seems to speed up the issue when I do a compile job in parallel on the host:
>>>>
>>>> Do you also need the sysfs tree?
>>>
>>> [   87.932552] CPU23 path=/machine.slice/machine-test.slice/machine-qemu\x2d18\x2dtest10. on_list=1 nr_running=1 throttled=0 p=[CPU 2/KVM 2662]
>>> [   87.932559] CPU23 path=/machine.slice/machine-test.slice/machine-qemu\x2d18\x2dtest10. on_list=0 nr_running=3 throttled=0 p=[CPU 2/KVM 2662]
>>> [   87.932562] CPU23 path=/machine.slice/machine-test.slice on_list=1 nr_running=1 throttled=1 p=[CPU 2/KVM 2662]
>>> [   87.932564] CPU23 path=/machine.slice on_list=1 nr_running=0 throttled=0 p=[CPU 2/KVM 2662]
>>> [   87.932566] CPU23 path=/ on_list=1 nr_running=1 throttled=0 p=[CPU 2/KVM 2662]
>>> [   87.951872] CPU23 path=/ on_list=1 nr_running=2 throttled=0 p=[ksoftirqd/23 126]
>>> [   87.987528] CPU23 path=/user.slice on_list=1 nr_running=2 throttled=0 p=[as 6737]
>>> [   87.987533] CPU23 path=/ on_list=1 nr_running=1 throttled=0 p=[as 6737]
>>>
>>> Arrh, looks like 'char path[64]' is too small to hold 'machine.slice/machine-test.slice/machine-qemu\x2d18\x2dtest10.scope/vcpuX' !
>>>                                                                                                                     ^  
>>> But I guess that the 'on_list=0' for 'machine-qemu\x2d18\x2dtest10.scope' could be the missing hint?
>>
>> yes the if (cfs_bandwidth_used()) at the end of enqueue_task_fair is not enough
>> to ensure that all cfs will be added back. It will "work" for the 1st enqueue
>> because the throttled cfs will be added and will reset tmp_alone_branch but not
>> for the next one
>>
>> Compare to the previous proposed fix, we can optimize it a bit with:
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 9ccde775e02e..3b19e508641d 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -4035,10 +4035,16 @@ enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
>>                 __enqueue_entity(cfs_rq, se);
>>         se->on_rq = 1;
>>
>> -       if (cfs_rq->nr_running == 1) {
>> +       /*
>> +        * When bandwidth control is enabled, cfs might have been removed because of
>> +        * a parent been throttled but cfs->nr_running > 1. Try to add it
>> +        * unconditionnally.
>> +        */
>> +       if (cfs_rq->nr_running == 1 || cfs_bandwidth_used())
> 
> This needs a forward declaration for cfs_bandwidth_used, but with that it compiles fine 
> and its seems to work fine so far. Will keep it running for while.

So I am no longer able to reproduce this issue in the last 30 minutes. As I have been
able to reproduce the issue pretty quickly in the latest trials (more guests, more
gcc threads) it looks like that this patch fixes the issue. I will keep it running
for a day or so, but I think I can already say.


Tested-by: Christian Borntraeger <borntraeger@de.ibm.com>