Messages in this thread | | | Date | Thu, 5 Apr 2012 07:39:18 -0700 | From | "Paul E. McKenney" <> | Subject | Re: RCU related performance regression in 3.3 |
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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 > > > > >
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