Re: [PATCH] sched/fair: Rearrange select_task_rq_fair() to optimize it

[Rohit Jain]

On 04/24/2018 08:47 AM, Joel Fernandes wrote:
> On Tue, Apr 24, 2018 at 8:46 AM, Joel Fernandes <joel.opensrc@xxxxxxxxx> wrote:
> > On Tue, Apr 24, 2018 at 5:35 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > > On Tue, Apr 24, 2018 at 12:19:07PM +0100, Valentin Schneider wrote:
> > > > On 24/04/18 11:43, Peter Zijlstra wrote:
> > > > > On Tue, Apr 24, 2018 at 11:02:26AM +0100, Valentin Schneider wrote:
> > > > > > I'd argue making things easier to read is a non-negligible part as well.
> > > > > Right, so I don't object to either of these (I think); but it would be
> > > > > good to see this in combination with that proposed EAS change.
> > > > True, I would've said the call to find_energy_efficient_cpu() ([1]) could
> > > > simply be added to the if (sd) {} case, but...
> > > I think the proposal was to put it before the for_each_domain() loop
> > > entirely, however...
> > >
> > > > > I think you (valentin) wanted to side-step the entire domain loop in
> > > > > that case or something.
> > > > ...this would change more things. Admittedly I've been sort of out of the loop
> > > > (no pun intended) lately, but this doesn't ring a bell. That might have been
> > > > the other frenchie (Quentin) :)
> > > It does indeed appear I confused the two of you, it was Quentin playing
> > > with that.
> > >
> > > In any case, if there not going to be conflicts here, this all looks
> > > good.
> > Both Viresh's and Valentin's patch looks lovely to me too. I couldn't
> > spot anything wrong with them either. One suggestion I was thinking
> > off is can we add better comments to this code (atleast label fast
> > path vs slow path) ?
> >
> > Also, annotate the conditions for the fast/slow path with
> > likely/unlikely since fast path is the common case? so like:
> >
> > if (unlikely(sd)) {
> >    /* Fast path, common case */
> >    ...
> > } else if (...) {
> >    /* Slow path */
> > }
> Aargh, I messed that up, I meant:
>
> if (unlikely(sd)) {
>     /* Slow path */
>     ...
> } else if (...) {
>     /* Fast path */
> }

Including the "unlikely" suggestion and the original patch, as expected
with a quick hackbench test on a 44 core 2 socket x86 machine causes no
change in performance.

Thanks,
Rohit

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