Re: [RFC PATCH 5/6] sched/fair: Select an energy-efficient CPU on task wake-up

[Quentin Perret <quentin.perret@arm.com>]

On Friday 23 Mar 2018 at 15:47:45 (+0000), Morten Rasmussen wrote:
> On Thu, Mar 22, 2018 at 01:10:22PM -0700, Joel Fernandes wrote:
> > On Wed, Mar 21, 2018 at 8:35 AM, Patrick Bellasi
> > <patrick.bellasi@arm.com> wrote:
> > > [...]
> > >
> > >> @@ -6555,6 +6613,14 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f
> > >>                       break;
> > >>               }
> > >>
> > >> +             /*
> > >> +              * Energy-aware task placement is performed on the highest
> > >> +              * non-overutilized domain spanning over cpu and prev_cpu.
> > >> +              */
> > >> +             if (want_energy && !sd_overutilized(tmp) &&
> > >> +                 cpumask_test_cpu(prev_cpu, sched_domain_span(tmp)))
> > >> +                     energy_sd = tmp;
> > >> +
> > >
> > > Not entirely sure, but I was trying to understand if we can avoid to
> > > modify the definition of want_affine (in the previous chunk) and move
> > > this block before the previous "if (want_affine..." (in mainline but
> > > not in this chunk), which will became an else, e.g.
> > >
> > >         if (want_energy && !sd_overutilized(tmp) &&
> > >                 // ...
> > >         else if (want_energy && !sd_overutilized(tmp) &&
> > >                 // ...
> > >
> > > Isn't that the same?
> > >
> > > Maybe there is a code path I'm missing... but otherwise it seems a
> > > more self contained modification of select_task_rq_fair...
> > 
> > Just replying to this here Patrick instead of the other thread.
> > 
> > I think this is the right place for the block from Quentin quoted
> > above because we want to search for the highest domain that is
> > !overutilized and look among those for the candidates. So from that
> > perspective, we can't move the block to the beginning and it seems to
> > be in the right place. My main concern on the other thread was
> > different, I was talking about the cases where sd_flag & tmp->flags
> > don't match. In that case, sd = NULL would trump EAS and I was
> > wondering if that's the right thing to do...
> 
> You mean if SD_BALANCE_WAKE isn't set on sched_domains?
> 
> The current code seems to rely on that flag to be set to work correctly.
> Otherwise, the loop might bail out on !want_affine and we end up doing
> the find_energy_efficient_cpu() on the lowest level sched_domain even if
> there is higher level one which isn't over-utilized.
> 
> However, SD_BALANCE_WAKE should be set if SD_ASYM_CPUCAPACITY is set so
> sd == NULL shouldn't be possible? This only holds as long as we only
> want EAS for asymmetric systems.

That's correct, we are under the assumption that the SD_ASYM_CPUCAPACITY
flag is set somewhere in the hierarchy here. If a sched domain has this
flag set, SD_BALANCE_WAKE is propagated to all lower sched domains
(see sd_init() in kernel/sched/topology.c) so we should be fine.