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: <1342500283.7353.46.camel@marge.simpson.net>
Date:	Tue, 17 Jul 2012 06:44:43 +0200
From:	Mike Galbraith <efault@....de>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Chris Mason <chris.mason@...ionio.com>,
	"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 Tue, 2012-07-17 at 00:27 -0400, Steven Rostedt wrote: 
> On Tue, 2012-07-17 at 06:18 +0200, Mike Galbraith wrote:
> >  
> > > There's that too. But the issue I was talking about is with all trylock
> > > loops. As holding an rt-mutex now disables migration, if a high priority
> > > process preempts a task that holds the lock, and then the high prio task
> > > starts spinning waiting for that lock to release, the lower priority
> > > process will never get to run to release it. The cpu_chill() doesn't
> > > help.
> > 
> > Hrm.  I better go make a testcase, this one definitely wants pounding
> > through thick skull.
> 
> Actually, I was mistaken. I forgot that we defined 'cpu_chill()' as
> msleep(1) on RT, which would keep a deadlock from happening.

Whew!  There are no stars and moons on my pointy hat, just plain white
cone, so you had me worried I was missing something critical there.

> It doesn't explain the performance enhancement you get :-/

No, it doesn't.  The only thing I can think of is that while folks are
timed sleeping, they aren't preempting and interleaving IO as much, but
I'm pulling that out of thin air.  Timed sleep should be a lot longer
than regular wakeup, so to my mind, there should be less interleave due
to more thumb twiddling.

-Mike

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