Re: [regression -next0117] What is kcompactd and why is he eating 100% of my cpu?

[Mel Gorman <mgorman@techsingularity.net>]

On Mon, Jan 28, 2019 at 10:16:27AM +0100, Jan Kara wrote:
> On Sun 27-01-19 16:36:34, valdis.kletnieks@vt.edu wrote:
> > On Sun, 27 Jan 2019 17:00:27 +0100, Pavel Machek said:
> > > > > I've noticed this as well on earlier kernels (next-20181224 to 20190115)
> > > > > Some more info:
> > > > > 1) echo 3 > /proc/sys/vm/drop_caches  unwedges kcompactd in 1-3 seconds.
> > > > This aspect is curious as it indicates that kcompactd could potentially
> > > > be infinite looping but it's not something I've experienced myself. By
> > > > any chance is there a preditable reproduction case for this?
> > >
> > > I seen it exactly once, so not sure how reproducible this is. x86-32
> > > machine, running chromium browser, so yes, there was some swapping
> > > involved.
> > 
> > I don't have a surefire replicator, but my laptop (x86_64, so it's not a 32-bit
> > only issue) triggers it fairly often, up to multiple times a day. Doesn't seem to
> > be just the Chrome browser that triggers it - usually I'm doing other stuff as
> > well, like a compile or similar.  The fact that 'drop_caches' clears it  makes me
> > wonder if we're hitting a corner case where cache data isn't being automatically
> > cleared and clogging something up.
> 
> So my buffer_migrate_page_norefs() is certainly buggy in its current
> incarnation (as a result block device page cache is not migratable at all).
> I've sent Andrew a patch over week ago but so far it got ignored. The patch
> is attached, can you give it a try whether it changes something for you?
> Thanks!
> 

Definetly worth trying and hopefully both the migration and compaction
patches sync up soon. In the event this patch does not help, I would
appreciate the following

1) A trace while kcompactd is pegged at 100%

	trace-cmd record -a -e compaction -e migrate -e kmem:mm_page_alloc -e vmscan:mm_vmscan_kswapd_wake -e vmscan:mm_vmscan_kswapd_sleep sleep 10

   Compress the resulting trace.dat and email it to me. If it's too big
   for a reasonable email, drop "-e kmem:mm_page_alloc" from the command
   line and it should be a more reasonable size. If not, reduce the sleep
   time to gather a shorter inverval.

2) Sample stack traces of kcompact while pegged at 100%

   echo -n > /tmp/kcompactd-stack; for i in `seq 1 100`; do echo sample $i >> /tmp/kcompactd-stack; cat /proc/`pidof kcompactd0`/stack >> /tmp/kcompactd-stack; done; gzip -f /tmp/kcompactd-stack

   And mail me the resulting /tmp/kcompactd-stack.gz

Thanks.

-- 
Mel Gorman
SUSE Labs