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

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Message-ID: <20140610111732.GA30139@austad.us>
Date:	Tue, 10 Jun 2014 13:17:32 +0200
From:	Henrik Austad <henrik@...tad.us>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Morten Rasmussen <morten.rasmussen@....com>,
	"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 12:23:53PM +0200, 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.

Isn't this asking for trouble?

I do get the point of not introducing more make-ifdeffery, but I'm not
so sure the alternative is much better. Do we really want to spend time
tracing down bugs introduced via a self-modifying process in something
as central as the scheduler?

> So I'm _hoping_ we can do all this without more CONFIG knobs, because
> {PREEMPT*SMP*CGROUP^3*NUMA^2} is already entirely annoying to
> build and run test, not to mention that distro builds will have no other
> option than to enable everything anyhow.

True, but if that is the argument, how is adding this as a dynamic thing
any better, you still end up with a test-matrix of the same size?

Building a kernel isn't _that_ much work and it would make the
test-scripts all the much simpler to maintain if we don't have to rely
on some dynamic tweaking of the core.

Just sayin'

-- 
Henrik Austad
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