linux-kernel - Re: Crashes with 874bbfe600a6 in 3.18.25

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Message-ID: <1455040955.3604.15.camel@gmail.com>
Date:	Tue, 09 Feb 2016 19:02:35 +0100
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Michal Hocko <mhocko@...nel.org>, Jiri Slaby <jslaby@...e.cz>,
	Thomas Gleixner <tglx@...utronix.de>,
	Petr Mladek <pmladek@...e.com>, Jan Kara <jack@...e.cz>,
	Ben Hutchings <ben@...adent.org.uk>,
	Sasha Levin <sasha.levin@...cle.com>, Shaohua Li <shli@...com>,
	LKML <linux-kernel@...r.kernel.org>,
	stable <stable@...r.kernel.org>,
	Daniel Bilik <daniel.bilik@...system.cz>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: Crashes with 874bbfe600a6 in 3.18.25

On Tue, 2016-02-09 at 18:56 +0100, Mike Galbraith wrote:
> On Tue, 2016-02-09 at 12:54 -0500, Tejun Heo wrote:
> > Hello, Mike.
> > 
> > On Tue, Feb 09, 2016 at 06:04:04PM +0100, Mike Galbraith wrote:
> > > workqueue: schedule WORK_CPU_UNBOUND work on wq_unbound_cpumask
> > > CPUs
> > > 
> > > WORK_CPU_UNBOUND work items queued to a bound workqueue always
> > > run
> > > locally.  This is a good thing normally, but not when the user
> > > has
> > > asked us to keep unbound work away from certain CPUs.  Round
> > > robin
> > > these to wq_unbound_cpumask CPUs instead, as perturbation
> > > avoidance
> > > trumps performance.
> > 
> > I don't think doing this by default for everyone is a good idea.  A
> > lot of workqueue usages tend to touch whatever the scheduler was
> > touching after all.  Doing things per-cpu is generally a pretty
> > good
> > thing.
> 
> It doesn't do anything unless the user twiddles the mask to exclude
> certain (think no_hz_full) CPUs, so there are no clueless victims.

(a plus: testers/robots can twiddle mask to help find bugs, _and_
nohz_full people can use it if they so choose)