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Re: [2.6.36-rc3] Workqueues, XFS, dependencies and deadlocks

To: Tejun Heo <tj@xxxxxxxxxx>
Subject: Re: [2.6.36-rc3] Workqueues, XFS, dependencies and deadlocks
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Tue, 7 Sep 2010 23:02:43 +1000
Cc: linux-kernel@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx, linux-fsdevel@xxxxxxxxxxxxxxx
In-reply-to: <4C862F8E.7030507@xxxxxxxxxx>
References: <20100907072954.GM705@dastard> <4C86003B.6090706@xxxxxxxxxx> <20100907100108.GN705@dastard> <4C861582.6080102@xxxxxxxxxx> <4C862F8E.7030507@xxxxxxxxxx>
User-agent: Mutt/1.5.20 (2009-06-14)
On Tue, Sep 07, 2010 at 02:26:54PM +0200, Tejun Heo wrote:
> On 09/07/2010 12:35 PM, Tejun Heo wrote:
> > Can you please help me a bit more?  Are you saying the following?
> > 
> > Work w0 starts execution on wq0.  w0 tries locking but fails.  Does
> > delay(1) and requeues itself on wq0 hoping another work w1 would be
> > queued on wq0 which will release the lock.  The requeueing should make
> > w0 queued and executed after w1, but instead w1 never gets executed
> > while w0 hogs the CPU constantly by re-executing itself.  Also, how
> > does delay(1) help with chewing up CPU?  Are you talking about
> > avoiding constant lock/unlock ops starving other lockers?  In such
> > case, wouldn't cpu_relax() make more sense?
> 
> Ooh, almost forgot.  There was nr_active underflow bug in workqueue
> code which could lead to malfunctioning max_active regulation and
> problems during queue freezing, so you could be hitting that too.  I
> sent out pull request some time ago but hasn't been pulled into
> mainline yet.  Can you please pull from the following branch and add
> WQ_HIGHPRI as discussed before and see whether the problem is still
> reproducible?

I'm currently running with the WQ_HIGHPRI flag. I only change one
thing at a time so I can tell what caused the change in behaviour...

> And if the problem is reproducible, can you please
> trigger sysrq thread dump and attach it?

Well, most of the time the system is 100% unresponsive when the
livelock occurs, so I'll be lucky to get anything at all....

>  git://git.kernel.org/pub/scm/linux/kernel/git/tj/wq.git for-linus

I'll try that next if the probelm still persists.

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx

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