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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: Wed, 8 Sep 2010 18:22:49 +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?

Ok, it looks as if the WQ_HIGHPRI is all that was required to avoid
the log IO completion starvation livelocks. I haven't yet pulled
the tree below, but I've now created about a billion inodes without
seeing any evidence of the livelock occurring.

Hence it looks like I've been seeing two livelocks - one caused by
the VM that Mel's patches fix, and one caused by the workqueue
changeover that is fixed by the WQ_HIGHPRI change.

Thanks for you insights, Tejun - I'll push the workqueue change
through the XFS tree to Linus.

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx

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