2007-08-15 22:48:52

by john stultz

Subject: [PATCH] Use num_possible_cpus() instead of NR_CPUS for timer distribution

Andrew requested this fixup awhile back, and I just now got to it
(apologies for being slow).

To avoid lock contention, we distribute the sched_timer calls across the
cpus so they do not trigger at the same instant. However, I used
NR_CPUS, which can cause needless grouping on small smp systems
depending on your kernel config. This patch converts to using
num_possible_cpus() so we spread it as evenly as possible on every
machine.

Briefly tested w/ NR_CPUS=255 and verified reduced contention.

Signed-off-by: John Stultz <[email protected]>

Index: 2.6-rt/kernel/time/tick-sched.c
===================================================================
--- 2.6-rt.orig/kernel/time/tick-sched.c
+++ 2.6-rt/kernel/time/tick-sched.c
@@ -586,7 +586,7 @@ void tick_setup_sched_timer(void)
/* 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);
+ do_div(offset, num_possible_cpus());
offset *= smp_processor_id();
ts->sched_timer.expires = ktime_add_ns(ts->sched_timer.expires, offset);

2007-08-15 23:04:57

by Thomas Gleixner

Subject: Re: [PATCH] Use num_possible_cpus() instead of NR_CPUS for timer distribution

On Wed, 2007-08-15 at 15:46 -0700, john stultz wrote:
> Andrew requested this fixup awhile back, and I just now got to it
> (apologies for being slow).
>
> To avoid lock contention, we distribute the sched_timer calls across the
> cpus so they do not trigger at the same instant. However, I used
> NR_CPUS, which can cause needless grouping on small smp systems
> depending on your kernel config. This patch converts to using
> num_possible_cpus() so we spread it as evenly as possible on every
> machine.
>
> Briefly tested w/ NR_CPUS=255 and verified reduced contention.
>
> Signed-off-by: John Stultz <[email protected]>

Doh, I missed this as well

Acked-by: Thomas Gleixner <tglx@linutronix,de>

>
> Index: 2.6-rt/kernel/time/tick-sched.c
> ===================================================================
> --- 2.6-rt.orig/kernel/time/tick-sched.c
> +++ 2.6-rt/kernel/time/tick-sched.c
> @@ -586,7 +586,7 @@ void tick_setup_sched_timer(void)
> /* 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);
> + do_div(offset, num_possible_cpus());
> offset *= smp_processor_id();
> ts->sched_timer.expires = ktime_add_ns(ts->sched_timer.expires, offset);