Re: [RFC PATCH 02/16] sched: Introduce CONFIG_SCHED_ENERGY

[Peter Zijlstra]

On Tue, Jun 10, 2014 at 01:17:32PM +0200, Henrik Austad wrote:
> 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?

Its already chock full of that stuff ;-)

> > 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?

Test-matrix yes, sadly so and there's nothing we can really do about
that, so that sucks.

But it does reduce the coverage of the tests; everything that is not
uber critical fast path we can do unconditionally. So all the
sched_domain wankery gets tested on every boot / hotplug event, which is
tons better than only when that particular option is build in.

So while the total test matrix does suck rocks, the actual code that
needs testing per option can be siginficantly reduced.

> 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.

its exponential, given that I now already have to build
PREEMPT*SMP*CGROUP^3*NUMA^2 = 2^7 = 128 kernels to cover all options,
adding one more option means I'll have to build another 128 kernels.
Building 128 kernels does take a lot of time, no matter how far you
strip that .config and no matter I can build a kernel in <50 seconds.

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