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

On Wed, Oct 10, 2018 at 2:25 PM, Michal Hocko <mhocko@kernel.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.