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

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Message-ID: <510A320D.8010301@linux.vnet.ibm.com>
Date:	Thu, 31 Jan 2013 16:57:49 +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 04:45 PM, Michael Wang wrote:
> On 01/31/2013 04:24 PM, Michael Wang wrote:
>> On 01/31/2013 03:40 PM, Namhyung Kim wrote:
>>> On Thu, 31 Jan 2013 15:30:02 +0800, Michael Wang wrote:
>>>> On 01/31/2013 02:58 PM, Namhyung Kim wrote:
>>>>> 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...
>>>
>>> Maybe it's not.  The cpus in group 0 are in a lower power state so that
>>> there will be a benefit to select cpu 2 from the power' PoV IMHO.  Also
>>> such a low power state has a longer exit latency so that we should
>>> choose cpu2 to get a better performance and it's the basic idea of this
>>> patchset I believe.
>>
>> Well, this case is just to notify that, we may face the comparison
>> between load and index, not between index and index, I just doubt there
>> won't be a rule which could take care both, besides, comparison between
>> load and index is strange...
> 
> Oh, I miss the point that you call it 'idle load', hmm...may be it could
> works, if we could scale the current load number, then we will have more
> 'space' for 'idle load'.

And some tips here:

/*      
 * Increase resolution of nice-level calculations for 64-bit architectures.
 * The extra resolution improves shares distribution and load balancing of
 * low-weight task groups (eg. nice +19 on an autogroup), deeper taskgroup
 * hierarchies, especially on larger systems. This is not a user-visible change
 * and does not change the user-interface for setting shares/weights.
 *      
 * We increase resolution only if we have enough bits to allow this increased
 * resolution (i.e. BITS_PER_LONG > 32). The costs for increasing resolution
 * when BITS_PER_LONG <= 32 are pretty high and the returns do not justify the
 * increased costs.
 */
#if 0 /* BITS_PER_LONG > 32 -- currently broken: it increases power usage under light load  */
# define SCHED_LOAD_RESOLUTION  10
# define scale_load(w)          ((w) << SCHED_LOAD_RESOLUTION)
# define scale_load_down(w)     ((w) >> SCHED_LOAD_RESOLUTION)

It mentioned some regressions, that's the history but
sounds like a lot of testing is needed.

Regards,
Michael Wang

> 
> Regards,
> Michael Wang
> 
>>
>> Regards,
>> Michael Wang
>>
>>>
>>> Thanks,
>>> Namhyung
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>>
> 
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