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

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Message-ID: <20190128091627.GA27972@quack2.suse.cz>
Date:   Mon, 28 Jan 2019 10:16:27 +0100
From:   Jan Kara <jack@...e.cz>
To:     valdis.kletnieks@...edu
Cc:     Pavel Machek <pavel@....cz>,
        Mel Gorman <mgorman@...hsingularity.net>,
        kernel list <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...l.org>, vbabka@...e.cz,
        aarcange@...hat.com, rientjes@...gle.com, mhocko@...nel.org,
        zi.yan@...rutgers.edu, hannes@...xchg.org, jack@...e.cz
Subject: Re: [regression -next0117] What is kcompactd and why is he eating
 100% of my cpu?

On Sun 27-01-19 16:36:34, valdis.kletnieks@...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!

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR

View attachment "0001-mm-migrate-Make-buffer_migrate_page_norefs-actually-.patch" of type "text/x-patch" (2305 bytes)