linux-kernel - Re: [RFC PATCH] sched: find the latest idle cpu

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Date:	Wed, 15 Jan 2014 14:45:42 +0800
From:	Alex Shi <alex.shi@...aro.org>
To:	Michael wang <wangyun@...ux.vnet.ibm.com>, mingo@...hat.com,
	peterz@...radead.org, tglx@...utronix.de,
	daniel.lezcano@...aro.org, vincent.guittot@...aro.org,
	morten.rasmussen@....com
CC:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	fengguang.wu@...el.com, linaro-kernel@...ts.linaro.org
Subject: Re: [RFC PATCH] sched: find the latest idle cpu

On 01/15/2014 01:33 PM, Michael wang wrote:
> On 01/15/2014 12:07 PM, Alex Shi wrote:
>> > Currently we just try to find least load cpu. If some cpus idled,
>> > we just pick the first cpu in cpu mask.
>> > 
>> > In fact we can get the interrupted idle cpu or the latest idled cpu,
>> > then we may get the benefit from both latency and power.
>> > The selected cpu maybe not the best, since other cpu may be interrupted
>> > during our selecting. But be captious costs too much.
> So the idea here is we want to choose the latest idle cpu if we have
> multiple idle cpu for choosing, correct?

yes.
> 
> And I guess that was in order to avoid choosing tickless cpu while there
> are un-tickless idle one, is that right?

no, current logical choice least load cpu no matter if it is idle.
> 
> What confused me is, what about those cpu who just going to recover from
> tickless as you mentioned, which means latest idle doesn't mean the best
> choice, or even could be the worst (if just two choice, and the longer
> tickless one is just going to recover while the latest is going to
> tickless).

yes, to save your scenario, we need to know the next timer for idle cpu,
but that is not enough, interrupt is totally unpredictable. So, I'd
rather bear the coarse method now.
> 
> So what about just check 'ts->tick_stopped' and record one ticking idle
> cpu? the cost could be lower than time comparison, we could reduce the
> risk may be...(well, not so risky since the logical only works when
> system is relaxing with several cpu idle)

first, nohz full also stop tick. second, tick_stopped can not reflect
the interrupt. when the idle cpu was interrupted, it's waken, then be a
good candidate for task running.

-- 
Thanks
    Alex
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