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    SubjectRe: [patch -mm 09/28] Tick management: spread timer interrupt
    On Sat, 23 Jun 2007 13:32:34 -0000
    Thomas Gleixner <tglx@linutronix.de> wrote:

    > From: john stultz <johnstul@us.ibm.com>
    >
    > After discussing w/ Thomas over IRC, it seems the issue is the sched
    > tick fires on every cpu at the same time, causing extra lock contention.
    >
    > This smaller change, adds an extra offset per cpu so the ticks don't
    > line up. This patch also drops the idle latency from 40us down to under
    > 20us.
    >
    > Signed-off-by: john stultz <johnstul@us.ibm.com>
    > Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
    >
    > ---
    > kernel/time/tick-sched.c | 7 ++++++-
    > 1 file changed, 6 insertions(+), 1 deletion(-)
    >
    > Index: linux-2.6.22-rc4-mm/kernel/time/tick-sched.c
    > ===================================================================
    > --- linux-2.6.22-rc4-mm.orig/kernel/time/tick-sched.c 2007-06-23 14:38:56.000000000 +0200
    > +++ linux-2.6.22-rc4-mm/kernel/time/tick-sched.c 2007-06-23 14:38:58.000000000 +0200
    > @@ -573,6 +573,7 @@ void tick_setup_sched_timer(void)
    > {
    > struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);
    > ktime_t now = ktime_get();
    > + u64 offset;
    >
    > /*
    > * Emulate tick processing via per-CPU hrtimers:
    > @@ -581,8 +582,12 @@ void tick_setup_sched_timer(void)
    > ts->sched_timer.function = tick_sched_timer;
    > ts->sched_timer.cb_mode = HRTIMER_CB_IRQSAFE_NO_SOFTIRQ;
    >
    > - /* Get the next period */
    > + /* Get the next period (per cpu) */
    > ts->sched_timer.expires = tick_init_jiffy_update();
    > + offset = ktime_to_ns(tick_period) >> 1;
    > + do_div(offset, NR_CPUS);

    With CONFIG_NR_CPUS=128 on a 2-way this'll be a pretty lumpy spreading?
    Perhaps it woud be better to use num_possible_cpus() here?


    > + offset *= smp_processor_id();
    > + ts->sched_timer.expires = ktime_add_ns(ts->sched_timer.expires, offset);
    >
    > for (;;) {
    > hrtimer_forward(&ts->sched_timer, now, tick_period);
    >
    > --
    -
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