Re: [PATCH] mm: memcg: Fix memcg reclaim soft lockup

[xunlei <xlpang@linux.alibaba.com>]

On 2020/8/26 下午8:07, Michal Hocko wrote:
> On Wed 26-08-20 20:00:47, xunlei wrote:
>> On 2020/8/26 下午7:00, Michal Hocko wrote:
>>> On Wed 26-08-20 18:41:18, xunlei wrote:
>>>> On 2020/8/26 下午4:11, Michal Hocko wrote:
>>>>> On Wed 26-08-20 15:27:02, Xunlei Pang wrote:
>>>>>> We've met softlockup with "CONFIG_PREEMPT_NONE=y", when
>>>>>> the target memcg doesn't have any reclaimable memory.
>>>>>
>>>>> Do you have any scenario when this happens or is this some sort of a
>>>>> test case?
>>>>
>>>> It can happen on tiny guest scenarios.
>>>
>>> OK, you made me more curious. If this is a tiny guest and this is a hard
>>> limit reclaim path then we should trigger an oom killer which should
>>> kill the offender and that in turn bail out from the try_charge lopp
>>> (see should_force_charge). So how come this repeats enough in your setup
>>> that it causes soft lockups?
>>>
>>
>> should_force_charge() is false, the current trapped in endless loop is
>> not the oom victim.
> 
> How is that possible? If the oom killer kills a task and that doesn't
> resolve the oom situation then it would go after another one until all
> tasks are killed. Or is your task living outside of the memcg it tries
> to charge?
> 

All tasks are in memcgs. Looks like the first oom victim is not finished
(unable to schedule), later mem_cgroup_oom()->...->oom_evaluate_task()
will set oc->chosen to -1 and abort.