linux-kernel - Re: 5.8-rc*: kernel BUG at kernel/signal.c:1917

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Date:   Tue, 21 Jul 2020 10:37:53 +0200
From:   peterz@...radead.org
To:     Paul Gortmaker <paul.gortmaker@...driver.com>
Cc:     Oleg Nesterov <oleg@...hat.com>, Jiri Slaby <jirislaby@...nel.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        <christian@...uner.io>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Linux kernel mailing list <linux-kernel@...r.kernel.org>,
        Mel Gorman <mgorman@...e.de>,
        Dave Jones <davej@...emonkey.org.uk>
Subject: Re: 5.8-rc*: kernel BUG at kernel/signal.c:1917

On Tue, Jul 21, 2020 at 12:52:52AM -0400, Paul Gortmaker wrote:
> [Re: 5.8-rc*: kernel BUG at kernel/signal.c:1917] On 20/07/2020 (Mon 16:21) Peter Zijlstra wrote:
> 
> > On Mon, Jul 20, 2020 at 04:02:24PM +0200, Oleg Nesterov wrote:
> > > I have to admit, I do not understand the usage of prev_state in schedule(),
> > > it looks really, really subtle...
> > 
> > Right, so commit dbfb089d360 solved a problem where schedule() re-read
> > prev->state vs prev->on_rq = 0. That is, schedule()'s dequeue and
> > ttwu()'s enqueue disagreed over sched_contributes_to_load. and as a
> > result load-accounting went wobbly.
> > 
> > Now, looking at that commit again, I might've solved the problem twice
> > :-P
> 
> [...]
> 
> > That said, in a crossed email, I just proposed we could simplify all
> > this like so.. but now I need to go ask people to re-validate that
> > loadavg muck again :-/
> 
> After a two hour "quick" sanity test I then gave it a full 7h run (which
> always seemed to break before dbfb089d360) and I didn't see any stuck
> load average with master from today + this change.
> 
> Paul.
> 
> root@...10:/home/paul/git/linux-head#
> [1]+  Done                    nohup tools/testing/selftests/rcutorture/bin/kvm.sh --cpus 24 --duration 420 --configs TREE03 --trust-make > /tmp/kvm.sh.out 2>&1
> root@...10:/home/paul/git/linux-head# cat /proc/version
> Linux version 5.8.0-rc6-00001-g5714ee50bb43-dirty (paul@...10) (gcc (Ubuntu 9.3.0-10ubuntu2) 9.3.0, GNU ld (GNU Binutils for Ubuntu) 2.34) #3 SMP Mon Jul 20 12:30:33 EDT 2020
> root@...10:/home/paul/git/linux-head# uptime
>  00:49:18 up  7:41,  2 users,  load average: 0.01, 0.00, 0.63
> root@...10:/home/paul/git/linux-head# 
> 
> --

Thanks a lot for re-testing that Paul, much appreciated!

Let me go write a Changelog for this then...