Re: 5.6-rc3: WARNING: CPU: 48 PID: 17435 at kernel/sched/fair.c:380 enqueue_task_fair+0x328/0x440

[Christian Borntraeger <borntraeger@de.ibm.com>]

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.

>                 list_add_leaf_cfs_rq(cfs_rq);
> +
> +       if (cfs_rq->nr_running == 1)
>                 check_enqueue_throttle(cfs_rq);
> -       }
>  }
> 
>  static void __clear_buddies_last(struct sched_entity *se)