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

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Date:	Wed, 8 Jan 2014 13:45:16 +0000
From:	Morten Rasmussen <morten.rasmussen@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <Dietmar.Eggemann@....com>,
	"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>,
	"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 Wed, Jan 08, 2014 at 01:32:57PM +0000, Peter Zijlstra wrote:
> On Wed, Jan 08, 2014 at 01:27:39PM +0000, Morten Rasmussen wrote:
> > On Wed, Jan 08, 2014 at 12:45:34PM +0000, Peter Zijlstra wrote:
> > > On Wed, Jan 08, 2014 at 12:35:34PM +0000, Morten Rasmussen wrote:
> > > > > Currently we detect overload by sg.nr_running >= sg.capacity, which can
> > > > > be very misleading because while a cpu might have a task running 'now'
> > > > > it might be 99% idle.
> > > > > 
> > > > > At which point I argued we should change the capacity thing anyhow. Ever
> > > > > since the runnable_avg patch set I've been arguing to change that into
> > > > > an actual utilization test.
> > > > > 
> > > > > So I think that if we measure overload by something like >95% utilization
> > > > > on the entire group the load scaling again makes perfect sense.
> > > > 
> > > > I agree that it make more sense to change the overload test to be based
> > > > on some tracked load. How about the non-overloaded case? Load balancing
> > > > would have to be based on unweighted task loads in that case?
> > > 
> > > Yeah, until we're overloaded our goal is to minimize idle time.
> > 
> > I would say, make the most of the available cpu cycles. Minimizing idle
> > time is not always the right thing to do when considering power
> > awareness.
> > 
> > If we know the actual load of the tasks, we may be able to consolidate
> 
> I think we must start to be careful with the word load, I think you
> meant to say utilization.

Indeed, I meant utilization.

> 
> > them on fewer cpus and save power by idling cpus. In that case the idle
> > time (total) is unchanged (unless the P-state is changed). Somewhat
> > similar to the video use-case running on 1, 2, and 4 cpu that I reposted
> > yesterday.
> 
> But fair enough.. Its idle time when you consider CPUs to always run at
> max frequency, but clearly I must stop thinking about CPUs like that :-)

Yes, it opens a whole new world of problems to be solved :-)
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