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

[Donald Buczek]

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.

We've tried v4.8 and got the first rcu stall warnings with this, too. 
First one after about 20 hours uptime.


> > * 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.

http://owww.molgen.mpg.de/~buczek/321322/2016-11-26.dmesg.txt  (80K)
http://owww.molgen.mpg.de/~buczek/321322/2016-11-26.trace.txt (50M)

Traces wrapped, but the last event is covered. all vmscan events were 
enabled


--
Donald Buczek
buczek@xxxxxxxxxxxxx
Tel: +49 30 8413 1433