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Date:   Thu, 9 May 2019 14:13:43 +0300
From:   "Kirill A. Shutemov" <kirill@shutemov.name>
To:     Justin Piszcz <jpiszcz@lucidpixels.com>
Cc:     linux-kernel@vger.kernel.org
Subject: Re: 5.1 kernel: khugepaged stuck at 100%
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On Thu, May 09, 2019 at 05:54:56AM -0400, Justin Piszcz wrote:
> Hello,
> 
> Kernel: 5.1 (self-compiled, no modules)
> Arch: x86_64
> Distro: Debian Testing
> 
> Issue: I was performing a dump of ext3 and ext4 filesystems and then
> restoring them to a separate volume (testing)-- afterwards I noticed that
> khugepaged is stuck at 100% CPU. It is currently still stuck at 100% CPU, is
> there anything I can do to debug what is happening prior to a reboot to work
> around the issue?  I have never seen this behavior prior to 5.1. 
> 
> $ cat /proc/cmdline
> auto BOOT_IMAGE=5.1.0-2 ro root=901 3w-sas.use_msi=1 nohugeiomap
> page_poison=1 pcie_aspm=off pti=on rootfstype=ext4 slub_debug=P
> zram.enabled=1 zram.num_devices=12 zswap.enabled=1 zswap.compressor=lz4
> zswap.zpool=z3fold
> 
> $ 5.1 .config attached: config.txt.gz
> 
> $ attachment: graphic.JPG -> graph of the processes, dark green ->
> khugepaged
> 
> $ top
> 
>   PID USER      PR  NI    VIRT    RES    SHR S  %CPU  %MEM     TIME+ COMMAND
>    77 root      39  19       0      0      0 R 100.0   0.0  23:29.27
> khugepaged
>     1 root      20   0  171800   7832   4948 S   0.0   0.0   1:25.59 systemd
>     2 root      20   0       0      0      0 S   0.0   0.0   0:00.02
> kthreadd
>     3 root       0 -20       0      0      0 I   0.0   0.0   0:00.00 rcu_gp
>     4 root       0 -20       0      0      0 I   0.0   0.0   0:00.00
> rcu_par_gp
>     6 root       0 -20       0      0      0 I   0.0   0.0   0:00.00
> kworker/0+
>     8 root       0 -20       0      0      0 I   0.0   0.0   0:00.00
> mm_percpu+
> 
> Thoughts on debugging / before reboot to clear this up?

Could you check what khugepaged doing?

cat /proc/$(pidof khugepaged)/stack