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

[xunlei]

On 2020/8/26 下午8:48, Michal Hocko wrote:
> On Wed 26-08-20 20:21:39, xunlei wrote:
>> 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.
> 
> This shouldn't be possible for too long because oom_reaper would
> make it invisible to the oom killer so it should proceed. Also
> mem_cgroup_out_of_memory takes a mutex and that is an implicit
> scheduling point already.
> 
> Which kernel version is this?
> 

I reproduced it on "5.9.0-rc2".

oom_reaper also can't get scheduled because of 1-cpu, and the mutex
uses might_sleep() which is noop in case of "CONFIG_PREEMPT_VOLUNTARY is
not set" I mentioned in the commit log.

> And just for the clarification. I am not against the additional
> cond_resched. That sounds like a good thing in general because we do
> want to have a predictable scheduling during reclaim which is
> independent on reclaimability as much as possible. But I would like to
> drill down to why you are seeing the lockup because those shouldn't
> really happen.
>