linux-kernel - Re: [RFC] sched: CPU topology try

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Date:	Mon, 6 Jan 2014 17:15:30 +0000
From:	Morten Rasmussen <morten.rasmussen@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Dietmar Eggemann <Dietmar.Eggemann@....com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"mingo@...nel.org" <mingo@...nel.org>,
	"pjt@...gle.com" <pjt@...gle.com>,
	"cmetcalf@...era.com" <cmetcalf@...era.com>,
	"tony.luck@...el.com" <tony.luck@...el.com>,
	"alex.shi@...aro.org" <alex.shi@...aro.org>,
	"preeti@...ux.vnet.ibm.com" <preeti@...ux.vnet.ibm.com>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	"rjw@...k.pl" <rjw@...k.pl>,
	"paulmck@...ux.vnet.ibm.com" <paulmck@...ux.vnet.ibm.com>,
	"corbet@....net" <corbet@....net>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"len.brown@...el.com" <len.brown@...el.com>,
	"arjan@...ux.intel.com" <arjan@...ux.intel.com>,
	"amit.kucheria@...aro.org" <amit.kucheria@...aro.org>,
	"james.hogan@...tec.com" <james.hogan@...tec.com>,
	"schwidefsky@...ibm.com" <schwidefsky@...ibm.com>,
	"heiko.carstens@...ibm.com" <heiko.carstens@...ibm.com>
Subject: Re: [RFC] sched: CPU topology try

On Mon, Jan 06, 2014 at 04:28:13PM +0000, Peter Zijlstra wrote:
> On Mon, Dec 23, 2013 at 06:22:17PM +0100, Dietmar Eggemann wrote:
> > I'm not sure if the idea to create a dedicated sched_domain level for every
> > topology flag representing a specific functionality will scale. From the
> > perspective of energy-aware scheduling we need e.g. energy costs (P and C
> > state) which can only be populated towards the scheduler via an additional
> > sub-struct and additional function arch_sd_energy() like depicted in
> > Morten's email:
> > 
> > [2] lkml.org/lkml/2013/11/14/102
> 
> That lkml.org link is actually not working for me (blank page -- maybe
> lkml.org is on the blink again).
> 
> That said, I yet have to sit down and think about the P state stuff, but
> I was thinking we need some rudimentary domain support for that.
> 
> For instance, the big-little thingies seem share their P state per
> cluster, so we need a domain at that level to hang some state off of --
> which we actually have in this case. But we need to ensure we do have
> it -- somehow.

Is there any examples of frequency domains not matching the span of a
sched_domain?

I would have thought that we would have a matching sched_domain to hang
the P and C state information from for most systems. If not, we could
just add it.

I don't think it is safe to assume that big-little always has cluster
P-states. It is implementation dependent. But the most obvious
alternative would be to have per-cpu P-states in which case we would
also have a matching sched_domain.
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