From: Nick Piggin <npiggin@suse.de>
To: Arjan van de Ven <arjan@infradead.org>
Cc: linux-kernel@vger.kernel.org, tglx@linutronix.de,
linux-arch@vger.kernel.org
Subject: Re: [patch] Remove the per cpu tick skew
Date: Fri, 30 Jul 2010 17:27:04 +1000 [thread overview]
Message-ID: <20100730072704.GA9960@amd> (raw)
In-Reply-To: <20100727210210.58d3118c@infradead.org>
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
next prev parent reply other threads:[~2010-07-30 7:27 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-07-28 4:02 [patch] Remove the per cpu tick skew Arjan van de Ven
2010-07-28 20:26 ` john stultz
2010-07-28 23:50 ` john stultz
2010-07-30 7:27 ` Nick Piggin [this message]
2010-07-30 7:27 ` Nick Piggin
2010-07-30 13:57 ` Arjan van de Ven
2010-07-30 13:57 ` Arjan van de Ven
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