On Thu, Jan 20, 2022 at 10:15:07PM +0530, Chitti Babu Theegala wrote:
>
>
> On 1/13/2022 10:05 PM, Vincent Donnefort wrote:
> > On Wed, Jan 12, 2022 at 08:09:02PM +0530, Chitti Babu Theegala wrote:
> > > Newly forked threads don't have any useful utilization data yet and
> > > it's not possible to forecast their impact on energy consumption.
> > > update_pick_idlest These forkees (though very small, most times) end up waking big
> > > cores from deep sleep for that very small durations.
> > >
> > > Bias all forkees to small cores to prevent waking big cores from deep
> > > sleep to save power.
> >
> > This bias might be interesting for some workloads, but what about the
> > others? (see find_energy_efficient_cpu() comment, which discusses forkees).
> >
>
> Yes, I agree with the find_energy_efficient_cpu() comment that we don't have
> any useful utilization data yet and hence not possible to forecast. However,
> I don't see any point in penalizing the power by waking up bigger cores
> which are in deep sleep state for very small workloads.
>
> This patch helps lighter workloads during idle conditions w.r.t power POV.
> For active (interactive or heavier) workloads, on most big.Little systems'
> these foreground tasks get pulled into gold affined cpu-sets where this
> patch would not play any spoilsport. Even for systems with such cpu-sets not
> defined, heavy workloads might need just another 1 or 2 scheduling windows
> for ramping to better freq or core.
Scheduling windows? I suppose you do not refer to PELT here, so I'm not sure
this argument applies here.
Beside, CFS always bias toward performance (except feec(), which does it in a
lesser extent).
>
> > >
> > > Signed-off-by: Chitti Babu Theegala <quic_ctheegal@xxxxxxxxxxx>
> > > ---
> > > kernel/sched/fair.c | 16 +++++++++++-----
> > > 1 file changed, 11 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > index 6e476f6..d407bbc 100644
> > > --- a/kernel/sched/fair.c
> > > +++ b/kernel/sched/fair.c
> > > @@ -5976,7 +5976,7 @@ static int wake_affine(struct sched_domain *sd, struct task_struct *p,
> > > }
> > > static struct sched_group *
> > > -find_idlest_group(struct sched_domain *sd, struct task_struct *p, int this_cpu);
> > > +find_idlest_group(struct sched_domain *sd, struct task_struct *p, int this_cpu, int sd_flag);
> > > /*
> > > * find_idlest_group_cpu - find the idlest CPU among the CPUs in the group.
> > > @@ -6063,7 +6063,7 @@ static inline int find_idlest_cpu(struct sched_domain *sd, struct task_struct *p
> > > continue;
> > > }
> > > - group = find_idlest_group(sd, p, cpu);
> > > + group = find_idlest_group(sd, p, cpu, sd_flag);
> > > if (!group) {
> > > sd = sd->child;
> > > continue;
> > > @@ -8997,7 +8997,8 @@ static inline void update_sg_wakeup_stats(struct sched_domain *sd,
> > > static bool update_pick_idlest(struct sched_group *idlest,
> > > struct sg_lb_stats *idlest_sgs,
> > > struct sched_group *group,
> > > - struct sg_lb_stats *sgs)
> > > + struct sg_lb_stats *sgs,
> > > + int sd_flag)
> > > {
> > > if (sgs->group_type < idlest_sgs->group_type)
> > > return true;
> > > @@ -9034,6 +9035,11 @@ static bool update_pick_idlest(struct sched_group *idlest,
> > > if (idlest_sgs->idle_cpus > sgs->idle_cpus)
> > > return false;
> > > + /* Select smaller cpu group for newly woken up forkees */
> > > + if ((sd_flag & SD_BALANCE_FORK) && (idlest_sgs->idle_cpus &&
> > > + !capacity_greater(idlest->sgc->max_capacity, group->sgc->max_capacity)))
> > > + return false;
> > > +
> >
> > Energy biased placement should probably be applied only when EAS is enabled.
> >
> > It's especially true here, if all CPUs have the same capacity, capacity_greater
> > would be always false. So unless I missed something, we wouldn't let the group_util
> > evaluation happen, would we?
>
> True. I am uploading new version patch with a EAS enablement check in place.
>
> >
> > [...]
