Re: [RFC PATCH 4/6] sched/fair: Introduce an energy estimation helper function

[Juri Lelli <juri.lelli@gmail.com>]

On 21/03/18 13:55, Quentin Perret wrote:
> On Wednesday 21 Mar 2018 at 13:59:25 (+0100), Juri Lelli wrote:
> > On 21/03/18 12:26, Patrick Bellasi wrote:
> > > On 21-Mar 10:04, Juri Lelli wrote:

[...]

> > > > > +	/*
> > > > > +	 * As the goal is to estimate the OPP reached for a specific util
> > > > > +	 * value, mimic the behaviour of schedutil with a 1.25 coefficient
> > > > > +	 */
> > > > > +	util += util >> 2;
> > > > 
> > > > What about other governors (ondemand for example). Is this supposed to
> > > > work only when schedutil is in use (if so we should probably make it
> > > > conditional on that)?
> > > 
> > > Yes, I would say that EAS mostly makes sense when you have a "minimum"
> > > control on OPPs... otherwise all the energy estimations are really
> > > fuzzy.
> > 
> > Make sense to me. Shouldn't we then make all this conditional on using
> > schedutil?
> 
> So, in theory, EAS could make sense even for other governors than
> schedutil. Even with the performance governor it is probably more
> energy efficient (although users using "performance" probably don't care
> about energy, but that's just an example) to place small tasks onto little
> CPUs up to a certain point given by the energy model. The ideal solution
> would be to change the behaviour of find_cap_state() depending on the
> governor being used, but I don't know if this extra complexity is worth
> it really.
> I'm happy to make all this conditional on schedutil as a first step and
> we can see later if that makes sense to extend EAS to other use-cases.

I agree that EAS makes still sense even for !schedutil cases (your
performance example being one of them, powersave maybe another one?).
Making it work with ondemand is tricky, though.

So, not sure what's the best thing to do, but we should be at least aware
of limitations.

> 
> > 
> > > 
> > > > Also, even when schedutil is in use, shouldn't we ask it for a util
> > > > "computation" instead of replicating its _current_ heuristic?
> > > 
> > > Are you proposing to have the 1.25 factor only here and remove it from
> > > schedutil?
> > 
> > I'm only saying that we shouldn't probably have two places where we add
> > this 1.25 factor to utilization before using it, as in the future one of
> > the two might modify that 1.25 to something else and then we'll have
> > problems. So, maybe a common wrapper that adds such factor?
> 
> Ok, I can definitely factorize this code between schedutil and EAS. And
> BTW, would it make sense to make schedutil use "capacity_margin" instead
> of an arbitrary value ? The semantics feels pretty close. Out of curiosity,
> what was the reason to use C=1.25 in the first place ?

I seem to remember it was choosen out of experiments, but I might surely
be wrong and Rafael, Viresh, others will correct me. :)

> 
> > 
> > > 
> > > > I fear  the two might diverge in the future.
> > > 
> > > That could be avoided by factoring out from schedutil the
> > > "compensation" factor into a proper function to be used by all the
> > > interested playes, isn't it?
> > 
> > And I should have read till the end before writing the above paragraph
> > it seems. :)
> > 
> > Thanks,
> > 
> > - Juri
> 
> Thank you very much for the feedback !

No problem. I'm of course very interested in this. Could you please
directly Cc me (juri.lelli@redhat.com) in next versions?

Thanks,

- Juri