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Subject: Re: [PATCH] mm, oom: Introduce time limit for dump_tasks duration.
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From: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Message-ID: <32c58019-5e2d-b3a1-a6ad-ea374ccd8b60@i-love.sakura.ne.jp>
Date: Fri, 7 Sep 2018 05:58:06 +0900
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To: Michal Hocko <mhocko@kernel.org>
Cc: Dmitry Vyukov <dvyukov@google.com>, Andrew Morton <akpm@linux-foundation.org>, David Rientjes <rientjes@google.com>, syzbot <syzbot+f0fc7f62e88b1de99af3@syzkaller.appspotmail.com>, 'Dmitry Vyukov' via syzkaller-upstream-moderation <syzkaller-upstream-moderation@googlegroups.com>, linux-mm <linux-mm@kvack.org>

On 2018/09/06 23:39, Michal Hocko wrote:
>>>> I know /proc/sys/vm/oom_dump_tasks . Showing some entries while not always
>>>> printing all entries might be helpful.
>>>
>>> Not really. It could be more confusing than helpful. The main purpose of
>>> the listing is to double check the list to understand the oom victim
>>> selection. If you have a partial list you simply cannot do that.
>>
>> It serves as a safeguard for avoiding RCU stall warnings.
>>
>>>
>>> If the iteration takes too long and I can imagine it does with zillions
>>> of tasks then the proper way around it is either release the lock
>>> periodically after N tasks is processed or outright skip the whole thing
>>> if there are too many tasks. The first option is obviously tricky to
>>> prevent from duplicate entries or other artifacts.
>>>
>>
>> Can we add rcu_lock_break() like check_hung_uninterruptible_tasks() does?
> 
> This would be a better variant of your timeout based approach. But it
> can still produce an incomplete task list so it still consumes a lot of
> resources to print a long list of tasks potentially while that list is not
> useful for any evaluation. Maybe that is good enough. I don't know. I
> would generally recommend to disable the whole thing with workloads with
> many tasks though.
> 

The "safeguard" is useful when there are _unexpectedly_ many tasks (like
syzbot in this case). Why not to allow those who want to avoid lockup to
avoid lockup rather than forcing them to disable the whole thing?