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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: <1de89f00-42f7-16e3-e718-02fb9f125db1@i-love.sakura.ne.jp>
Date: Thu, 6 Sep 2018 20:25:19 +0900
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To: Dmitry Vyukov <dvyukov@google.com>
Cc: Michal Hocko <mhocko@kernel.org>, 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 20:07, Dmitry Vyukov wrote:
>> Since printk() is slow, printing one line takes nearly 0.01 second.
>> As a result, syzbot is stalling for 52 seconds trying to dump 5600
> 
> I wonder why there are so many of them?
> We have at most 8 test processes (each having no more than 16 threads
> if that matters).
> No more than 1 instance of syz-executor1 at a time. But we see output
> like the one below. It has lots of instances of syz-executor1 with
> different pid's. So does it print all tasks that ever existed (kernel
> does not store that info, right)? Or it livelocks picking up new and
> new tasks as they are created slower than they are created? Or we have
> tons of zombies?
> 
> ...

I don't think they are zombies. Since tasks which already released ->mm
are not printed, these tasks are still alive.

  [  pid  ]   uid  tgid total_vm      rss pgtables_bytes swapents oom_score_adj name
> [   8037]     0  8037    17618     8738   131072        0             0 syz-executor1

Maybe something signal / fork() / exit() / wait() related regression?