linux-kernel - Re: [RFC PATCH 02/16] sched: Introduce CONFIG_SCHED

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Message-ID: <20140610144100.GD1581@e103034-lin>
Date:	Tue, 10 Jun 2014 15:41:00 +0100
From:	Morten Rasmussen <morten.rasmussen@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Henrik Austad <henrik@...tad.us>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"mingo@...nel.org" <mingo@...nel.org>,
	"rjw@...ysocki.net" <rjw@...ysocki.net>,
	"vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
	"daniel.lezcano@...aro.org" <daniel.lezcano@...aro.org>,
	"preeti@...ux.vnet.ibm.com" <preeti@...ux.vnet.ibm.com>,
	Dietmar Eggemann <Dietmar.Eggemann@....com>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <Will.Deacon@....com>
Subject: Re: [RFC PATCH 02/16] sched: Introduce CONFIG_SCHED_ENERGY

On Tue, Jun 10, 2014 at 01:24:35PM +0100, Peter Zijlstra wrote:
> On Tue, Jun 10, 2014 at 12:24:03PM +0100, Morten Rasmussen wrote:
> > On Tue, Jun 10, 2014 at 11:23:53AM +0100, Peter Zijlstra wrote:
> > > On Tue, Jun 10, 2014 at 11:06:41AM +0100, Morten Rasmussen wrote:
> > > > How would you like to disable the energy stuff for users for whom
> > > > latency is everything?
> > > > 
> > > > I mean, we are adding some extra load/utilization tracking. While I
> > > > think we should do everything possible to minimize the overhead, I think
> > > > it is unrealistic to assume that it will be zero. Is a some extra 'if
> > > > (energy_enabled)' acceptable?
> > > > 
> > > > I'm open for other suggestions.
> > > 
> > > We have the jump-label stuff to do self modifying code ;-) The only
> > > thing we need to be careful with is data-layout.
> > 
> > Thanks. I can see that it is already used in for various bit in
> > kernel/sched/*. I didn't catch anything in Documentation/static-keys.txt
> > related to data-layout caveats. Is there some other
> > documentation/patches I should read before messing everything up? ;-)
> 
> So the data-layout was mostly referring to things like making sure that
> struct sched_avg doesn't end up straddling a cacheline somewhere by
> accident.
> 
> The most expensive part of the per-task accounting nonsense is the
> amount of memory we need to touch to do so, the actual instructions come
> second, unless of course we go put tons of divisions in there :-)

Make sense.

> BTW, are cachelines 64 bytes for you ARM people too?

Mostly yes, but as with a lot of other things on ARM it is
implementation defined. The cacheline sizes are probeable at runtime,
but for things where we don't know I think 64 bytes is the current
assumption.

Catalin or Will would be able to provide a more detailed answer.
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