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

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Date:	Sat, 05 May 2012 09:41:40 +1000
From:	Michael Neuling <mikey@...ling.org>
To:	Diwakar Tundlam <dtundlam@...dia.com>
cc:	'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

Diwakar Tundlam <dtundlam@...dia.com> 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.

That would work but Peter knows much better than me.  

Mikey

> 
> 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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