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

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Date:   Tue, 29 Jan 2019 23:29:37 -0500
From:   valdis.kletnieks@...edu
To:     Jan Kara <jack@...e.cz>, 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
Subject: Re: [regression -next0117] What is kcompactd and why is he eating 100% of my cpu?

On Tue, 29 Jan 2019 20:06:39 -0500, valdis.kletnieks@...edu said:
> On Mon, 28 Jan 2019 10:16:27 +0100, Jan Kara said:
>
> > 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!
>
> Been running with the patch for about 24 hours, haven't seen kcompactd
> misbehave.  I even fired up a Chrome with a lot of tabs open, a Firefox, and a
> kernel build, intentionally drove the system into swapping, and kcompactd
> didn't make it into the top 10 on 'top'.
>
> I'm willing to say put a "tested-by:" on that one,  it looks fixed from here.
> If there's any remaining bugs, they're ones I can't seem to trigger...

Spoke too soon.  Sitting here not stressing the laptop at all, plenty of free
memory, and ka-blam.

Will keep my eyes open and do the data gathering Mel Gorban wanted - I discovered
too late that trace-cmd wasn't installed, and things broke free by themselves (probably
not coincidence that I launched a terminal window and then it cleared....)

top - 23:24:03 up  2:19,  1 user,  load average: 2.70, 2.00, 1.55
Tasks: 221 total,   3 running, 218 sleeping,   0 stopped,   0 zombie
%Cpu(s): 15.6 us, 67.3 sy,  0.0 ni,  9.5 id,  0.0 wa,  5.6 hi,  2.0 si,  0.0 st
GiB Mem :      7.6 total,      2.7 free,      3.1 used,      1.8 buff/cache
GiB Swap:      8.0 total,      8.0 free,      0.0 used.      4.1 avail Mem 

  PID  PPID  %MEM  PR  NI S    VIRT    RES    SHR   SWAP   UID  %CPU     TIME+ COMMAND                                 
   27     2   0.0  20   0 R    0.0m   0.0m   0.0m   0.0m     0  78.5   2:11.91 kcompactd0