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    SubjectRe: [PATCH V3]hrtimer: Fix a performance regression by disable reprogramming in remove_hrtimer
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    On Tue, Jul 30, 2013 at 5:35 PM, Peter Zijlstra <peterz@infradead.org> wrote:
    >
    > On Tue, Jul 30, 2013 at 05:12:49PM +0800, Ethan Zhao wrote:
    > > On Mon, Jul 29, 2013 at 6:18 PM, Thomas Gleixner <tglx@linutronix.de> wrote:
    > > > The test case does not involve anything hrtimer related. Do you have
    > > > CONFIG_SCHED_HRTICK enabled?
    > > >
    > >
    > > Yes. it is default configured in stable release.
    > > CONFIG_SCHED_HRTICK=y
    >
    > Should still be disabled by default even if supported:
    >
    > # grep HRTICK kernel/sched/features.h
    > SCHED_FEAT(HRTICK, false)
    >
    >
    > > > First of all we want to know, which particular hrtimer is causing that
    > > > issue. If it is the hrtick one, then I really have to ask why you want
    > > > to use it at all in such a high performance scenario.
    > > >
    > > > Any advice about the HZ in high performance scenario ? hrtimer tick
    > > Is not fit for high performance ?
    >
    > Hence why its disabled, programming the timer hardware is too expensive.
    > But since you didn't even know that I suspect you aren't in fact using
    > it.
    >
    Got it.
    what tglx and you mean

    SCHED_FEAT(HRTICK, 0)

    then no hrtimer operation in
    void __sched __schedule(void)
    {
    … …
    if (sched_feat(HRTICK))
    hrtick_clear(rq);
    … …

    Yup, So what I am facing is not HRTICK.
    But that doesn't move my eyes away from hrtimer and suspect
    reprogramming delay the scheduling.
    The call stack looks like following :

    cpu_idle()
    {
    … …
    tick_nohz_stop_sched_tick()
    --> hrtimer_start(); --> __hrtimer_start_range_ns() -- > remove_hrtimer()
    -- > raise_softirq_irqoff(TIMER_SOFTIRQ);
    ---->run_timer_softirq() --> tick_sched_timer() -- >
    hrtimer_start_expires
    … …

    … ...
    tick_nohz_restart_sched_tick()
    … ...
    schedule()
    … ...
    }

    So the expensive thing maybe not inside the schedule(), but could
    outside the scheduler(), the more bigger forever loop.

    This is one part of what I am facing.

    Thanks
    Ethan

    >
    > It would be good if you could do what Thomas suggested and look at which
    > timer is actually active during your workload.
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