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

[Xunlei Pang <xlpang@linux.alibaba.com>]

On 2020/8/26 下午7:45, 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?
>>
> 
>     oom_status = mem_cgroup_oom(mem_over_limit, gfp_mask,
>                get_order(nr_pages * PAGE_SIZE));
>     switch (oom_status) {
>     case OOM_SUCCESS:
>         nr_retries = MAX_RECLAIM_RETRIES;

Actually we can add "cond_resched()" here, but I think it's better to
have one at the memcg reclaim path to avoid other unexpected issues.

>         goto retry;
> 
> It retries here endlessly, because oom reaper has no cpu to schedule.
>