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

[Mike Galbraith <efault@gmx.de>]

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