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Message-ID: <1342455968.7659.93.camel@marge.simpson.net>
Subject: Re: 3.4.4-rt13: btrfs + xfstests 006 = BOOM..  and a bonus rt_mutex
 deadlock report for absolutely free!
From: Mike Galbraith <efault@gmx.de>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Chris Mason <chris.mason@fusionio.com>,
        "Chris L. Mason" <clmason@fusionio.com>,
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        LKML <linux-kernel@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>
Date: Mon, 16 Jul 2012 18:26:08 +0200
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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.

-Mike