Re: [patch -mm 09/28] Tick management: spread timer interrupt

Andrew Morton
2007-06-26T00:11:55Z

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);
>
> --