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

[Morten Rasmussen <morten.rasmussen@arm.com>]

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.