linux-kernel - Re: 3.4.4-rt13: btrfs + xfstests 006 = BOOM.. and a bonus rt_mutex deadlock report for absolutely free!

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Message-ID: <20120716163539.GH25961@shiny.int.fusionio.com>
Date:	Mon, 16 Jul 2012 12:35:39 -0400
From:	Chris Mason <chris.mason@...ionio.com>
To:	Mike Galbraith <efault@....de>
CC:	Steven Rostedt <rostedt@...dmis.org>,
	"Chris L. Mason" <clmason@...ionio.com>,
	"linux-rt-users@...r.kernel.org" <linux-rt-users@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: 3.4.4-rt13: btrfs + xfstests 006 = BOOM..  and a bonus rt_mutex
 deadlock report for absolutely free!

On Mon, Jul 16, 2012 at 10:26:08AM -0600, Mike Galbraith wrote:
> On Mon, 2012-07-16 at 12:02 -0400, Steven Rostedt wrote: 
> > On Mon, 2012-07-16 at 04:02 +0200, Mike Galbraith wrote:
> > 
> > > > Great, thanks!  I got stuck in bug land on Friday.  You mentioned
> > > > performance problems earlier on Saturday, did this improve performance?
> > > 
> > > Yeah, the read_trylock() seems to improve throughput.  That's not
> > > heavily tested, but it certainly looks like it does.  No idea why.
> > 
> > Ouch, you just turned the rt_read_lock() into a spin lock. If a higher
> > priority process preempted a lower priority process that holds the same
> > lock, it will deadlock.
> 
> Hm, how, it's doing cpu_chill()?
> 
> > I'm not sure why you would get a performance benefit from this, as the
> > mutex used is an adaptive one (failure to acquire the lock will only
> > sleep if preempted or if the owner is not running).
> 
> I'm not attached to it, can whack it in a heartbeat.. especially so it
> the thing can deadlock.  I've seen enough of those of late.
> 
> > We should look at why this performs better (if it really does).
> 
> Not sure it really does, there's variance, but it looked like it did.
> 

I'd use a benchmark that is more consistent than dbench for this.  I
love dbench for generating load (and the occasional deadlock) but it
tends to steer you in the wrong direction on performance.

-chris

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