linux-kernel - Re: sched/fair: Kernel panics in pick_next

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Message-ID: <12242825054bb861578de6405504875e4d1bb6c2.camel@gmx.de>
Date: Wed, 02 Oct 2024 08:40:44 +0200
From: Mike Galbraith <efault@....de>
To: Vishal Chourasia <vishalc@...ux.ibm.com>
Cc: Peter Zijlstra <peterz@...radead.org>, linux-kernel@...r.kernel.org, 
 Ingo Molnar <mingo@...hat.com>, Vincent Guittot
 <vincent.guittot@...aro.org>, Juri Lelli <juri.lelli@...hat.com>, Dietmar
 Eggemann <dietmar.eggemann@....com>, Steven Rostedt <rostedt@...dmis.org>,
 Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,  Valentin
 Schneider <vschneid@...hat.com>, luis.machado@....com
Subject: Re: sched/fair: Kernel panics in pick_next_entity

On Tue, 2024-10-01 at 18:41 +0200, Mike Galbraith wrote:
>
> When I hit $subject, LTPs cfs_bandwidth01 was running, but there was no
> warning prelude, box went straight to panic.  Trying to reproduce using
> that testcase plus hackbench as efficacy booster produced lots of dying
> box noise, but zero sneaky $subject instances before or after quash.

Hohum, this morning I did hit..

1. WARNING: CPU: 5 PID: 931 at kernel/sched/fair.c:6062 unthrottle_cfs_rq+0x4c3/0x4d0
2. WARNING: CPU: 0 PID: 786 at kernel/sched/fair.c:704 update_entity_lag+0x79/0x90
3. NULL dereference in pick_next_entity()

..instead of brick, workqueue stall etc. Twice.  Not that it matters.
I was only mucking about with it because I was curious whether telling
LB to stop moving sched_delayed tasks about would matter. (nope)

	-Mike