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SubjectRe: RCU related performance regression in 3.3
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.

josh


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