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

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Date:	Fri, 4 May 2012 16:18:08 -0700
From:	Diwakar Tundlam <dtundlam@...dia.com>
To:	'Peter Zijlstra' <a.p.zijlstra@...llo.nl>
CC:	'Ingo Molnar' <mingo@...nel.org>,
	'Andrew Morton' <akpm@...ux-foundation.org>,
	'Christoph Lameter' <cl@...two.org>,
	'Michael Neuling' <mikey@...ling.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

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.

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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