linux-kernel - Re: [RFC PATCH] mm: silence soft lockups from unlock

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Message-ID: <20200721121752.GK4061@dhcp22.suse.cz>
Date:   Tue, 21 Jul 2020 14:17:52 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Qian Cai <cai@....pw>
Cc:     linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>
Subject: Re: [RFC PATCH] mm: silence soft lockups from unlock_page

On Tue 21-07-20 07:44:07, Qian Cai wrote:
> 
> 
> > On Jul 21, 2020, at 7:25 AM, Michal Hocko <mhocko@...nel.org> wrote:
> > 
> > Are these really important? I believe I can dig that out from the bug
> > report but I didn't really consider that important enough.
> 
> Please dig them out. We have also been running those things on
> “large” powerpc as well and never saw such soft-lockups. Those
> details may give us some clues about the actual problem.

I strongly suspect this is not really relevant but just FYI this is
16Node, 11.9TB with 1536CPUs system.

> Once we
> understand the problem better, we may judge if this “hack” is
> really worth it.

I do not have access to the machine so I can only judge from the boot
log I have in hands. And from that it is pretty clear that 
$ grep BUG tmp/attachment.txt | wc -l
896

$ grep BUG tmp/attachment.txt | grep "\[systemd" | wc -l
860

$ grep do_fault+0x448 tmp/attachment.txt | wc -l
860

that the boot struggles, lockups happen from udev workers and most of
them are stuck at the very same place which is unlock_page. The rest is
a part of the changelog.
-- 
Michal Hocko
SUSE Labs