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

[Michal Hocko]

On Sun 27-11-16 10:19:06, Donald Buczek wrote:
> 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

OK, so one of the stall is reported at
[118077.988410] INFO: rcu_sched detected stalls on CPUs/tasks:
[118077.988416] 	1-...: (181 ticks this GP) idle=6d5/140000000000000/0 softirq=46417663/46417663 fqs=10691 
[118077.988417] 	(detected by 4, t=60002 jiffies, g=11845915, c=11845914, q=46475)
[118077.988421] Task dump for CPU 1:
[118077.988421] kswapd1         R  running task        0    86      2 0x00000008
[118077.988424]  ffff88080ad87c58 ffff88080ad87c58 ffff88080ad87cf8 ffff88100c1e5200
[118077.988426]  0000000000000003 0000000000000000 ffff88080ad87e60 ffff88080ad87d90
[118077.988428]  ffffffff811345f5 ffff88080ad87da0 ffff88100c1e5200 ffff88080ad87dd0
[118077.988430] Call Trace:
[118077.988436]  [<ffffffff811345f5>] ? shrink_node_memcg+0x605/0x870
[118077.988438]  [<ffffffff8113491f>] ? shrink_node+0xbf/0x1c0
[118077.988440]  [<ffffffff81135642>] ? kswapd+0x342/0x6b0

the interesting part of the traces would be around the same time:
        clusterd-989   [009] .... 118023.654491: mm_vmscan_direct_reclaim_end: nr_reclaimed=193
         kswapd1-86    [001] dN.. 118023.987475: mm_vmscan_lru_isolate: isolate_mode=0 classzone=0 order=0 nr_requested=32 nr_scanned=4239830 nr_taken=0 file=1
         kswapd1-86    [001] dN.. 118024.320968: mm_vmscan_lru_isolate: isolate_mode=0 classzone=0 order=0 nr_requested=32 nr_scanned=4239844 nr_taken=0 file=1
         kswapd1-86    [001] dN.. 118024.654375: mm_vmscan_lru_isolate: isolate_mode=0 classzone=0 order=0 nr_requested=32 nr_scanned=4239858 nr_taken=0 file=1
         kswapd1-86    [001] dN.. 118024.987036: mm_vmscan_lru_isolate: isolate_mode=0 classzone=0 order=0 nr_requested=32 nr_scanned=4239872 nr_taken=0 file=1
         kswapd1-86    [001] dN.. 118025.319651: mm_vmscan_lru_isolate: isolate_mode=0 classzone=0 order=0 nr_requested=32 nr_scanned=4239886 nr_taken=0 file=1
         kswapd1-86    [001] dN.. 118025.652248: mm_vmscan_lru_isolate: isolate_mode=0 classzone=0 order=0 nr_requested=32 nr_scanned=4239900 nr_taken=0 file=1
         kswapd1-86    [001] dN.. 118025.984870: mm_vmscan_lru_isolate: isolate_mode=0 classzone=0 order=0 nr_requested=32 nr_scanned=4239914 nr_taken=0 file=1
[...]
         kswapd1-86    [001] dN.. 118084.274403: mm_vmscan_lru_isolate: isolate_mode=0 classzone=0 order=0 nr_requested=32 nr_scanned=4241133 nr_taken=0 file=1

Note the Need resched flag. The IRQ off part is expected because we are
holding the LRU lock which is IRQ safe. That is not a problem because
the lock is only held for SWAP_CLUSTER_MAX pages at maximum. It is also
interesing to see that we have scanned only 1303 pages during that 1
minute. That would be dead slow. None of them were good enough for the
reclaim but that doesn't sound like a problem. The trace simply suggests
that the reclaim was preempted by something else. Otherwise I cannot
imagine such a slow scanning.

Is it possible that something else is hogging the CPU and the RCU just
happens to blame kswapd which is running in the standard user process
context?

-- 
Michal Hocko
SUSE Labs