Re: RCU related performance regression in 3.3

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Thu, Apr 05, 2012 at 04:15:33PM +0200, Pascal CHAPPERON wrote:
> 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.

OK, so the natural approach is to disable CONFIG_RCU_FAST_NO_HZ at
boot time.  Unfortunately, you appear to need it to remain disabled
through at least filesystem mounting, which if I understand correctly
happens long after system_state gets set to SYSTEM_RUNNING.

If RCU has some way to find out when init is complete, I can easily
make it so that CONFIG_RCU_FAST_NO_HZ optimizes for speed during boot
and energy efficiency during runtime.

One thing I could easily do would be to provide a sysfs parameter or
some such that allows the boot process to enable energy-efficiency
mode at runtime.  I would much prefer to make this automatic, though.

Other thoughts?

							Thanx, Paul

> Pascal
> 
> 
> > Message du 05/04/12 16:01
> > De : "Paul E. McKenney" 
> > A : "Josh Boyer" 
> > Copie à : linux-kernel@vger.kernel.org, kernel-team@fedoraproject.org, pascal.chapperon@wanadoo.fr
> > 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
> > 
> >
>