linux-kernel - Re: [RFC 2/2] sched/fair: prefer a CPU in the "lowest" idle state

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Date:	Thu, 31 Jan 2013 15:30:02 +0800
From:	Michael Wang <wangyun@...ux.vnet.ibm.com>
To:	Namhyung Kim <namhyung@...nel.org>
CC:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	linux-kernel@...r.kernel.org,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>, tglx@...utronix.de
Subject: Re: [RFC 2/2] sched/fair: prefer a CPU in the "lowest" idle state

On 01/31/2013 02:58 PM, Namhyung Kim wrote:
> On Thu, 31 Jan 2013 14:39:20 +0800, Michael Wang wrote:
>> On 01/31/2013 01:16 PM, Namhyung Kim wrote:
>>> Anyway, I have an idea with this in mind.  It's like adding a new "idle
>>> load" to each idle cpu rather than special casing the idle cpus like
>>> above.  IOW an idle cpu will get very small load weight depends on how
>>> deep it's slept so that it can be compared to other cpus in a same way
>>> but we can find prefered (lowest load) cpu among the idle cpus.
>>>
>>> The simple way I can think of is adding idle_level to a rq load in
>>> weighted_cpuload():
>>>
>>> static unsigned long weighted_cpuload(const int cpu)
>>> {
>>> 	return cpu_rq(cpu)->load.weight + cpuidle_get_state(cpu);
>>> }
>>
>> Hmm... then we don't need changes in find_idlest_cpu(), just compare the
>> load as before, but it works only when the appendix state value is
>> smaller than the lowest load of one task, which is 15 currently, I'm not
>> sure whether we have the promise...
> 
> You said about a nice 19 process, right?  But I found that SCHED_IDLE
> task will have weight of 3. :(
> 
>   #define WEIGHT_IDLEPRIO                3

I missed that policy :)

> 
> 
> But AFAIK the number of states in cpuidle is usually less than 10 so maybe
> we can change the weight then, but there's no promise...

And I just got another case we should take care:

	group 0		cpu 0			cpu 1
			power index 8		power index 8


	group 1		cpu 2			cpu 3
			power index 0		load 15

so load of group 0 is 16 and group 1 is 15, but group 0 is better...

Regards,
Michael Wang

> 
> Thanks,
> Namhyung
> 

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