linux-kernel - Re: INFO: rcu detected stall in shmem

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Message-ID: <CACT4Y+ZfVdeB-WNeLCWJvTHNeCUtR3r1R+3Qjv9XjZXPxaV2WA@mail.gmail.com>
Date:   Wed, 10 Oct 2018 14:29:40 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        syzbot <syzbot+77e6b28a7a7106ad0def@...kaller.appspotmail.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Andrew Morton <akpm@...ux-foundation.org>, guro@...com,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        David Rientjes <rientjes@...gle.com>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Yang Shi <yang.s@...baba-inc.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Petr Mladek <pmladek@...e.com>
Subject: Re: INFO: rcu detected stall in shmem_fault

On Wed, Oct 10, 2018 at 2:25 PM, Michal Hocko <mhocko@...nel.org> wrote:
> On Wed 10-10-18 20:48:33, Sergey Senozhatsky wrote:
>> On (10/10/18 13:35), Michal Hocko wrote:
>> > > Just flooding out of memory messages can trigger RCU stall problems.
>> > > For example, a severe skbuff_head_cache or kmalloc-512 leak bug is causing
>> >
>> > [...]
>> >
>> > Quite some of them, indeed! I guess we want to rate limit the output.
>> > What about the following?
>>
>> A bit unrelated, but while we are at it:
>>
>>   I like it when we rate-limit printk-s that lookup the system.
>> But it seems that default rate-limit values are not always good enough,
>> DEFAULT_RATELIMIT_INTERVAL / DEFAULT_RATELIMIT_BURST can still be too
>> verbose. For instance, when we have a very slow IPMI emulated serial
>> console -- e.g. baud rate at 57600. DEFAULT_RATELIMIT_INTERVAL and
>> DEFAULT_RATELIMIT_BURST can add new OOM headers and backtraces faster
>> than we evict them.
>>
>> Does it sound reasonable enough to use larger than default rate-limits
>> for printk-s in OOM print-outs? OOM reports tend to be somewhat large
>> and the reported numbers are not always *very* unique.
>>
>> What do you think?
>
> I do not really care about the current inerval/burst values. This change
> should be done seprately and ideally with some numbers.

I think Sergey meant that this place may need to use
larger-than-default values because it prints lots of output per
instance (whereas the default limit is more tuned for cases that print
just 1 line).

I've found at least 1 place that uses DEFAULT_RATELIMIT_INTERVAL*10:
https://elixir.bootlin.com/linux/latest/source/fs/btrfs/extent-tree.c#L8365
Probably we need something similar here.