Re: [RFC patch v3 04/20] sched: Avoid calculating the cpumask if the system is overloaded

[Tim Chen]

On Fri, 2025-07-04 at 01:09 +0530, Shrikanth Hegde wrote:
> 
> On 6/18/25 23:57, Tim Chen wrote:
> > From: K Prateek Nayak <kprateek.nayak@xxxxxxx>
> > 
> > If the SIS_UTIL cuts off idle cpu search, result of the cpumask_and() is
> > of no use. Since select_idle_cpu() can now be called twice per wake up
> > in the select_idle_sibling() due to cache aware wake up, this overhead
> > can be visible in benchmarks like hackbench.
> > 
> > To save some additional cycles, especially in cases where we target
> > the LLC frequently and the search bails out because the LLC is busy,
> > only calculate the cpumask if the system is not overloaded.
> > 
> 
> This patch could be independent and should help in general.
> But changelog needs to be updated.
> 
> 

Yes, that makes sense.

Tim

> > Signed-off-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
> > ---
> >   kernel/sched/fair.c | 4 ++--
> >   1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 567ad2a0cfa2..6a2678f9d44a 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -7918,8 +7918,6 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
> >   	int i, cpu, idle_cpu = -1, nr = INT_MAX;
> >   	struct sched_domain_shared *sd_share;
> >   
> > -	cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
> > -
> >   	if (sched_feat(SIS_UTIL)) {
> >   		sd_share = rcu_dereference(per_cpu(sd_llc_shared, target));
> >   		if (sd_share) {
> > @@ -7931,6 +7929,8 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
> >   		}
> >   	}
> >   
> > +	cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
> > +
> >   	if (static_branch_unlikely(&sched_cluster_active)) {
> >   		struct sched_group *sg = sd->groups;
> >   
>