Re: INFO: rcu_sched detected stalls on CPUs/tasks with `kswapd` and `mem_cgroup_shrink_node`

[Donald Buczek <buczek@molgen.mpg.de>]

On 24.11.2016 11:15, Michal Hocko wrote:

> On Mon 21-11-16 16:35:53, Donald Buczek wrote:
> [...]
>> Hello,
>>
>> thanks a lot for looking into this!
>>
>> Let me add some information from the reporting site:
>>
>> * We've tried the patch from Paul E. McKenney (the one posted Wed, 16 Nov
>> 2016)  and it doesn't shut up the rcu stall warnings.
>>
>> * Log file from a boot with the patch applied ( grep kernel
>> /var/log/messages ) is here :
>> http://owww.molgen.mpg.de/~buczek/321322/2016-11-21_syslog.txt
>>
>> * This system is a backup server and walks over thousands of files sometimes
>> with multiple parallel rsync processes.
>>
>> * No rcu_* warnings on that machine with 4.7.2, but with 4.8.4 , 4.8.6 ,
>> 4.8.8 and now 4.9.0-rc5+Pauls patch
> I assume you haven't tried the Linus 4.8 kernel without any further
> stable patches? Just to be sure we are not talking about some later
> regression which found its way to the stable tree.
>
>> * When the backups are actually happening there might be relevant memory
>> pressure from inode cache and the rsync processes. We saw the oom-killer
>> kick in on another machine with same hardware and similar (a bit higher)
>> workload. This other machine also shows a lot of rcu stall warnings since
>> 4.8.4.
>>
>> * We see "rcu_sched detected stalls" also on some other machines since we
>> switched to 4.8 but not as frequently as on the two backup servers. Usually
>> there's "shrink_node" and "kswapd" on the top of the stack. Often
>> "xfs_reclaim_inodes" variants on top of that.
> I would be interested to see some reclaim tracepoints enabled. Could you
> try that out? At least mm_shrink_slab_{start,end} and
> mm_vmscan_lru_shrink_inactive. This should tell us more about how the
> reclaim behaved.

We'll try that tomorrow!

Donald

-- 
Donald Buczek
buczek@molgen.mpg.de
Tel: +49 30 8413 1433