From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mike Galbraith Subject: Re: 3.4.4-rt13: btrfs + xfstests 006 = BOOM.. and a bonus rt_mutex deadlock report for absolutely free! Date: Tue, 17 Jul 2012 06:44:43 +0200 Message-ID: <1342500283.7353.46.camel@marge.simpson.net> References: <1342072060.7338.102.camel@marge.simpson.net> <20120713125043.GH30128@shiny> <1342260883.7368.30.camel@marge.simpson.net> <20120715175612.GF25961@shiny.int.fusionio.com> <1342404140.7659.27.camel@marge.simpson.net> <1342454547.5410.3.camel@gandalf.stny.rr.com> <1342455968.7659.93.camel@marge.simpson.net> <1342456567.7659.97.camel@marge.simpson.net> <1342458195.2783.5.camel@acer.local.home> <1342498722.7353.37.camel@marge.simpson.net> <1342499278.24525.13.camel@gandalf.stny.rr.com> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Cc: Chris Mason , "Chris L. Mason" , "linux-rt-users@vger.kernel.org" , LKML , linux-fsdevel , Thomas Gleixner To: Steven Rostedt Return-path: In-Reply-To: <1342499278.24525.13.camel@gandalf.stny.rr.com> Sender: linux-rt-users-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org 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