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    SubjectRe: RCU related performance regression in 3.3
    > Message du 05/04/12 16:40
    > De : "Paul E. McKenney"
    > A : "Pascal CHAPPERON"
    > Copie à : "Josh Boyer" , linux-kernel@vger.kernel.org, kernel-team@fedoraproject.org
    > Objet : Re: 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.
    >
    In fact, I need it to remain disable until all the systemd units are completed.
    Some units, such as NetworkManager can take longer time to complete with
    RCU_FAST_NO_HZ enabled.
    And i need it to be disabled at shutdown, as umounting cgroups, sysfs, etc.
    plus old-root mounting can take one plain second for each umounting.

    > 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.
    >
    I said that I didn't noticed significant slowdown during runtime, but my
    laptop usage is basic. Some specific tasks similar to systemd may
    perhaps be impacted by this feature.
    I can test a task/program that could stress RCU_FAST_NO_HZ if any ?

    > 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.
    >
    So the feature is disabled until you trigger a sysfs parameter, and can be
    disabled before shutdown ? It would be fair, at least for hardware like my
    own.

    > Other thoughts?
    >
    Do you think that the culprit is a buggy hardware in my laptop, or the
    number of cpu/threads ?

    > Thanx, Paul
    >

    Resent in plain text because rejected. Sorry, i forgot the rules.

    Pascal
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