linux-kernel - Re: workqueue code needing preemption disabled

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Date:	Mon, 18 Mar 2013 15:19:21 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	RT <linux-rt-users@...r.kernel.org>,
	Clark Williams <clark@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: workqueue code needing preemption disabled

On Mon, 2013-03-18 at 12:06 -0700, Tejun Heo wrote:

> Me neither.  Unfortunately, I'm out of ideas at the moment.
> Hmm... last year, there was a similar issue, I think it was in AMD
> cpufreq, which was caused by work function doing
> set_cpus_allowed_ptr(), so the idle worker was on the correct CPU but
> the one issuing local wake up was on the wrong one. 

I should also tell you that -rt is currently based on 3.6.11. Did that
bug get passed on to stable? If not, we could be hitting that same bug
too. Except this is running on Intel. :-/


>  It could be that
> there's another such usage in kernle which doesn't trigger easily w/o
> RT.  As preemption doesn't trigger concurrency management wakeup, as
> long as such user doesn't do something explicitly blocking, upstream
> would be fine as long as it restores affinity before finishing but in
> RT spinlocks become mutexes and can trigger local wakeups, so...

And these wakeups can be triggered by blocking on the gcwq->lock as
well, where it probably happens more often on -rt than mainline.

> 
> Anyways, having a crashdump would go a long way towards identifying
> what's going on.  All we need to know are the work function which was
> being executed, whether the worker was on the right CPU and which
> worker it was trying to wake up.
> 

OK, I'll have my box set up. But I doubt this bug will even trigger
again before I have to return it :-(

-- Steve


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