linux-kernel - Re: [RFC PATCH 2/2] memcg: do not report racy no-eligible OOM tasks

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Date:   Fri, 7 Dec 2018 21:43:07 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Michal Hocko <mhocko@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>
Cc:     linux-mm@...ck.org, David Rientjes <rientjes@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 2/2] memcg: do not report racy no-eligible OOM tasks

On 2018/11/07 19:08, Michal Hocko wrote:
> On Wed 07-11-18 18:45:27, Tetsuo Handa wrote:
>> On 2018/11/06 21:42, Michal Hocko wrote:
>>> On Tue 06-11-18 18:44:43, Tetsuo Handa wrote:
>>> [...]
>>>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>>>> index 6e1469b..a97648a 100644
>>>> --- a/mm/memcontrol.c
>>>> +++ b/mm/memcontrol.c
>>>> @@ -1382,8 +1382,13 @@ static bool mem_cgroup_out_of_memory(struct mem_cgroup *memcg, gfp_t gfp_mask,
>>>>  	};
>>>>  	bool ret;
>>>>  
>>>> -	mutex_lock(&oom_lock);
>>>> -	ret = out_of_memory(&oc);
>>>> +	if (mutex_lock_killable(&oom_lock))
>>>> +		return true;
>>>> +	/*
>>>> +	 * A few threads which were not waiting at mutex_lock_killable() can
>>>> +	 * fail to bail out. Therefore, check again after holding oom_lock.
>>>> +	 */
>>>> +	ret = fatal_signal_pending(current) || out_of_memory(&oc);
>>>>  	mutex_unlock(&oom_lock);
>>>>  	return ret;
>>>>  }
>>>
>>> If we are goging with a memcg specific thingy then I really prefer
>>> tsk_is_oom_victim approach. Or is there any reason why this is not
>>> suitable?
>>>
>>
>> Why need to wait for mark_oom_victim() called after slow printk() messages?
>>
>> If current thread got Ctrl-C and thus current thread can terminate, what is
>> nice with waiting for the OOM killer? If there are several OOM events in
>> multiple memcg domains waiting for completion of printk() messages? I don't
>> see points with waiting for oom_lock, for try_charge() already allows current
>> thread to terminate due to fatal_signal_pending() test.
> 
> mutex_lock_killable would take care of exiting task already. I would
> then still prefer to check for mark_oom_victim because that is not racy
> with the exit path clearing signals. I can update my patch to use
> _killable lock variant if we are really going with the memcg specific
> fix.
> 
> Johaness?
> 

No response for one month. When can we get to an RCU stall problem syzbot reported?