Re: INFO: rcu detected stall in shmem_fault

[Dmitry Vyukov]

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