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Reply-To: xlpang@linux.alibaba.com
Subject: Re: [PATCH] mm: memcg: Fix memcg reclaim soft lockup
To:     Michal Hocko <mhocko@suse.com>
Cc:     Johannes Weiner <hannes@cmpxchg.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
References: <1598426822-93737-1-git-send-email-xlpang@linux.alibaba.com>
 <20200826081102.GM22869@dhcp22.suse.cz>
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 <20200826110015.GO22869@dhcp22.suse.cz>
From:   xunlei <xlpang@linux.alibaba.com>
Message-ID: <5b22890d-190f-be1d-3be8-995765dbb957@linux.alibaba.com>
Date:   Wed, 26 Aug 2020 19:45:06 +0800
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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;
        goto retry;

It retries here endlessly, because oom reaper has no cpu to schedule.