linux-kernel - Re: [patch-mm 07/23] Tick management: spread timer interrupt

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Message-ID: <20070610141617.GA15173@rhlx01.hs-esslingen.de>
Date:	Sun, 10 Jun 2007 16:16:17 +0200
From:	Andreas Mohr <andi@...x01.hs-esslingen.de>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	LKML <linux-kernel@...r.kernel.org>, Andrew Morton <akpm@...l.org>,
	Ingo Molnar <mingo@...e.hu>, Andi Kleen <ak@...e.de>,
	Arjan van de Ven <arjan@...radead.org>,
	Venkatesh Pallipadi <venkatesh.pallipadi@...el.com>,
	Chris Wright <chrisw@...s-sol.org>,
	john stultz <johnstul@...ibm.com>
Subject: Re: [patch-mm 07/23] Tick management: spread timer interrupt

Hi,

On Sun, Jun 10, 2007 at 09:44:05AM -0000, Thomas Gleixner wrote:
> From: john stultz <johnstul@...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.

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
> line up. This patch also drops the idle latency from 40us down to under
> 20us.

Very nice, thanks!

> +	/* 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);
> +	offset *= smp_processor_id();

Andreas Mohr
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