Messages in this thread | | | Date | Fri, 30 Jul 2010 17:27:04 +1000 | From | Nick Piggin <> | Subject | Re: [patch] Remove the per cpu tick skew |
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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 > -- > To unsubscribe from this list: send the line "unsubscribe linux-arch" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html
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