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

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Message-ID: <20140610122435.GG6758@twins.programming.kicks-ass.net>
Date:	Tue, 10 Jun 2014 14:24:35 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Morten Rasmussen <morten.rasmussen@....com>
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>
Subject: Re: [RFC PATCH 02/16] sched: Introduce CONFIG_SCHED_ENERGY

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 :-)

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


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