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

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Date:   Sun, 27 Jan 2019 16:36:34 -0500
From:   valdis.kletnieks@...edu
To:     Pavel Machek <pavel@....cz>
cc:     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 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.

Any particular diagnostic info you want me to get next time it hits?  (Am
currently on next-20190125, if that matters).