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

From: Mike Galbraith
Date: Mon Jul 16 2012 - 12:26:17 EST

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.


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