linux-kernel - Re: [PATCH] sched/fair: Prefer small idle cores for forkees

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Date:   Fri, 21 Jan 2022 10:17:20 +0000
From:   Vincent Donnefort <vincent.donnefort@....com>
To:     Chitti Babu Theegala <quic_ctheegal@...cinc.com>
Cc:     mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, joel@...lfernandes.org,
        linux-arm-msm@...r.kernel.org, quic_lingutla@...cinc.com,
        linux-kernel@...r.kernel.org, quic_rjendra@...cinc.com
Subject: Re: [PATCH] sched/fair: Prefer small idle cores for forkees

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@...cinc.com>
> > > ---
> > >   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.
> 
> > 
> > [...]