Hi,
On Sun, Jun 10, 2007 at 09:44:05AM -0000, Thomas Gleixner wrote:

quoted text
> From: john stultz

>

> 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.

Hmm, the cpu-specific offset calculation isn't too expensive, hopefully?

(div/mul in patch, maybe this could be done differently)
And is it granted that the do_div() compiles into a nice plain void

on non-SMP? Would be good to verify this.
And calculation order? Do multiply before division to minimize calculation

error?

(for a timer tick it probably doesn't matter, however)

And of course OTOH doing it the other way might lead to overflows...
> This smaller change, adds an extra offset per cpu so the ticks don't

quoted text
> line up. This patch also drops the idle latency from 40us down to under

> 20us.

Very nice, thanks!
> +	/* Get the next period (per cpu)*/

quoted text
>  	ts->sched_timer.expires = tick_init_jiffy_update();

> +	offset = ktime_to_ns(tick_period) >> 1;

> +	do_div(offset, NR_CPUS);

> +	offset *= smp_processor_id();

Andreas Mohr

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