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SubjectRe: [patch] Remove the per cpu tick skew
On Tue, Jul 27, 2010 at 09:02:10PM -0700, Arjan van de Ven wrote:
> Hi,
>
> the following patch is a win for power management on x86....
> ... but since this touches generic code.. are there any
> other architectures that would be negatively affected by this?
>
>
>
> Subject: [patch] Remove the per cpu tick skew
>
> Historically, Linux has tried to make the regular timer tick on the various
> CPUs not happen at the same time, to avoid contention on xtime_lock.
>
> Nowadays, with the tickless kernel, this contention no longer happens
> since time keeping and updating are done differently. In addition,
> this skew is actually hurting power consumption in a measurable
> way on many-core systems.

Question, how much of a win is it? What does it do that tickless
idle does not, can you explain?

>
> Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
>
> --- linux.trees.git/kernel/time/tick-sched.c~ 2010-07-16 09:40:50.000000000 -0400
> +++ linux.trees.git/kernel/time/tick-sched.c 2010-07-26 11:18:51.138003329 -0400
> @@ -780,7 +780,6 @@
> {
> struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);
> ktime_t now = ktime_get();
> - u64 offset;
>
> /*
> * Emulate tick processing via per-CPU hrtimers:
> @@ -790,10 +789,6 @@
>
> /* Get the next period (per cpu) */
> hrtimer_set_expires(&ts->sched_timer, tick_init_jiffy_update());
> - offset = ktime_to_ns(tick_period) >> 1;
> - do_div(offset, num_possible_cpus());
> - offset *= smp_processor_id();
> - hrtimer_add_expires_ns(&ts->sched_timer, offset);
>
> for (;;) {
> hrtimer_forward(&ts->sched_timer, now, tick_period);
>
> --
> Arjan van de Ven Intel Open Source Technology Centre
> For development, discussion and tips for power savings,
> visit http://www.lesswatts.org
> --
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