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    SubjectRe: RCU related performance regression in 3.3
    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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