linux-kernel - Re: [PATCH] sched: Enable arch-specific asym packing option in sched domain

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Date:	Mon, 07 May 2012 10:59:32 +0800
From:	Michael Wang <wangyun@...ux.vnet.ibm.com>
To:	Diwakar Tundlam <dtundlam@...dia.com>
CC:	"'Michael Neuling'" <mikey@...ling.org>,
	"'Peter Zijlstra'" <a.p.zijlstra@...llo.nl>,
	"'Ingo Molnar'" <mingo@...nel.org>,
	"'Andrew Morton'" <akpm@...ux-foundation.org>,
	"'Christoph Lameter'" <cl@...two.org>,
	"'Stephen Rothwell'" <sfr@...b.auug.org.au>,
	"'Benjamin Herrenschmidt'" <benh@...nel.crashing.org>,
	"'David Rientjes'" <rientjes@...gle.com>,
	"'linux-kernel@...r.kernel.org'" <linux-kernel@...r.kernel.org>,
	Peter De Schrijver <pdeschrijver@...dia.com>
Subject: Re: [PATCH] sched: Enable arch-specific asym packing option in sched
 domain

On 05/05/2012 07:25 AM, Diwakar Tundlam wrote:

>>> Arrh, so it's not a performance issue but power savings.. that would this asymmetric packing a lot more appropriate to use.
> 
> Yes, it is for power saving..
> 
> Could you suggest a way t use this flag without affecting Power7, then?
> I was thinking to define a different weak arch_sd_package_asym_packing() in SD_CPU_INIT.
> And the flag is set only for arch/arm/mach-tegra/tegra3.
> 


SD_ASYM_PACKING will help to change the idle processor from higher id to
lower, so if a lower id cpu thread is going to idle, we will move all
the tasks of it's higher id sibling to him.

I don't know how can this way cause power saving? Do you mean on this
tegra3 arch, higher id smt will use more power than lower one?

And could the SD_POWERSAVINGS_BALANCE do some help in your case?

Regards,
Michael Wang

> Thanks,
> --Diwakar.
> 
> -----Original Message-----
> From: Michael Neuling [mailto:mikey@...ling.org] 
> Sent: Friday, May 04, 2012 4:21 PM
> To: Diwakar Tundlam
> Cc: 'Peter Zijlstra'; 'Ingo Molnar'; 'Andrew Morton'; 'Christoph Lameter'; 'Stephen Rothwell'; 'Benjamin Herrenschmidt'; 'David Rientjes'; 'linux-kernel@...r.kernel.org'; Peter De Schrijver
> Subject: Re: [PATCH] sched: Enable arch-specific asym packing option in sched domain
> 
>> Thanks for clarifying the Power7. I see the point - package vs cpu's within a package.
>>
>> Yes, this is for Nvidia's ARM Quad-core Tegra CPU. It is a single 
>> package, organized flatly.  To save power, we want cores to be loaded 
>> up in order from cpu0, 1, etc.  The ASYM_PACKING flag seems to do 
>> exactly what we need if set in the domain flag.
> 
> Arrh, so it's not a performance issue but power savings.. that would this asymmetric packing a lot more appropriate to use.
> 
> Mikey
> 
>>
>> Thanks,
>> --Diwakar.
>>
>> -----Original Message-----
>> From: Peter Zijlstra [mailto:a.p.zijlstra@...llo.nl]
>> Sent: Friday, May 04, 2012 3:30 PM
>> To: Diwakar Tundlam
>> Cc: 'Ingo Molnar'; 'Andrew Morton'; 'Christoph Lameter'; 'Michael 
>> Neuling'; 'Stephen Rothwell'; 'Benjamin Herrenschmidt'; 'David 
>> Rientjes'; 'linux-kernel@...r.kernel.org'; Peter De Schrijver
>> Subject: RE: [PATCH] sched: Enable arch-specific asym packing option 
>> in sched domain
>>
>> On Fri, 2012-05-04 at 15:18 -0700, Diwakar Tundlam wrote:
>>>
>>> arch_sd_sibling_asym_packing() is already present under ifdef CONFIG_SMT.
>>> I didn't touch that. I only added it to SD_CPU_INIT for all cpu's.
>>> I assumed Power7 shouldn't use SD_CPU_INIT.
>>
>> All archs use SD_CPU_INIT, its the default topology level for a package/socket. So now you've made Power7 prefer lower numbered sockets over higher numbered sockets.. not fatal, but not really nice either.
>>
>> [ power7 is the only one that implements arch_sd_sibling_asym_packing 
>> ]
>>
>>> Maybe I should define a separate weak symbol, say arch_sd_bias_to_lower_num_cpu()?
>>> Then Power7 can define arch_sd_sibling_asym_packing() to be '1' and 
>>> it will not break all-cpu init.
>>
>> A slightly saner name would be:
>>
>>   arch_sd_package_asym_packing()
>>
>> FWIW, I suspect you're wanting to use this for some ARM chip (nvidia 
>> doesn't do much else -- aside from this graphics stuff) so that if 
>> there's hardly anything it runs on cpu0. Now, last time I checked, 
>> these ARM things had only 1 package, so I still don't see the point 
>> :-)
>>
>> I suspect you want to modify SD_MC_INIT() with something like:
>>
>>   arch_sd_mc_asym_packing()
>>
>> Or is this the big-little thing and you're representing them as separate packages?
>>
>> See how a little extra information avoids me having to endlessly second guess wtf you're actually wanting to do?
>>
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