Re: RCU related performance regression in 3.3

[Pascal CHAPPERON]

Hello,

I didn't notice any significant slowdown while the system is up and running. A full kernel compilation (make -j 16) takes 14mn with both 3.2.10 and 3.3.0.

Pascal


> Message du 05/04/12 16:01
> De : "Paul E. McKenney" 
> A : "Josh Boyer" 
> Copie à : linux-kernel@xxxxxxxxxxxxxxx, kernel-team@xxxxxxxxxxxxxxxxx, pascal.chapperon@xxxxxxxxxx
> Objet : Re: RCU related performance regression in 3.3
> 
> On Thu, Apr 05, 2012 at 08:37:59AM -0400, Josh Boyer wrote:
> > On Wed, Apr 04, 2012 at 02:36:33PM -0700, Paul E. McKenney wrote:
> > > On Wed, Apr 04, 2012 at 11:27:27AM -0400, Josh Boyer wrote:
> > > > Hi Paul,
> > > > 
> > > > We've had a few reports of some boot slowdowns with the 3.3 rebase we
> > > > did in Fedora 16. One of our users volunteered to bisect a vanilla
> > > > kernel and wound up at "rcu: Permit dyntick-idle with callbacks pending
> > > > (commit 7cb924990)". You can find more details in the bug [1], but it's
> > > > been reported on both physical hardware and in virtual machines.
> > > > 
> > > > Have you seen anything like this in your testing? Given the user used a
> > > > vanilla 3.3 kernel, I'm wondering if there is a targetted fix that might
> > > > be backported. If so, it would be good to get that headed to the 3.3.y
> > > > stable tree.
> > > > 
> > > > josh
> > > > 
> > > > [1] https://bugzilla.redhat.com/show_bug.cgi?id=806548
> > > 
> > > I will look into this. In the meantime, does setting
> > > CONFIG_RCU_FAST_NO_HZ=n get rid of the slowdowns?
> > 
> > Seems so. Pascal tried 3.4-rc1 with and without that set and found that
> > disabling it lead to consistent timings. He put his findings in the
> > bug, and I've also now CC'd him.
> 
> Thank you for the info! Is the performance problem limited to boot time,
> or are there performance problems when the system is up and running?
> 
> Thanx, Paul
> 
>
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