Remove the per cpu tick skew
in [Kernel]
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From: john stultz on 28 Jul 2010 16:30 On Tue, Jul 27, 2010 at 9:02 PM, Arjan van de Ven <arjan(a)infradead.org> wrote: > 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? It was added to avoid contention when all the cpus grabbed the xtime_lock (causing latency spikes of timer overhead * number of cpus). We don't grab the xtime lock everywhere anymore, so that shouldn't bite us, but I'm curious if there are not other global locks taken that may see extra contention without this change. > 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. > > Signed-off-by: Arjan van de Ven <arjan(a)linux.intel.com> I'll give it a spin against -rt and see if we show any latency jumps. thanks -john -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: john stultz on 28 Jul 2010 20:00 On Wed, Jul 28, 2010 at 1:26 PM, john stultz <johnstul(a)us.ibm.com> wrote: > On Tue, Jul 27, 2010 at 9:02 PM, Arjan van de Ven <arjan(a)infradead.org> wrote: >> 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. >> > > I'll give it a spin against -rt and see if we show any latency jumps. In my testing on a 8way box, I didn't see any concerning latencies with this patch running cyclictest or a FIFO99 gtod loop. thanks -john -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: Nick Piggin on 30 Jul 2010 03:30 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(a)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(a)vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: Arjan van de Ven on 30 Jul 2010 10:00 On Fri, 30 Jul 2010 17:27:04 +1000 Nick Piggin <npiggin(a)suse.de> wrote: > > 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? tickless idle works great if you're really almost idle if there's "some work but not fully busy" this still matters this is not about 'a few milliwatts', but on a server in our labs (sorry, no hardware details in public) this effect is in the "several dozen Watts" range. -- 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-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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