Re: [PATCH v2 09/23] sched/cache: Count tasks prefering destination LLC in a sched group

[Chen, Yu C]

On 12/10/2025 9:52 PM, Peter Zijlstra wrote:
> On Wed, Dec 03, 2025 at 03:07:28PM -0800, Tim Chen wrote:
> > During LLC load balancing, tabulate the number of tasks on each runqueue
> > that prefer the LLC contains the env->dst_cpu in a sched group.
> >
> > For example, consider a system with 4 LLC sched groups (LLC0 to LLC3)
> > balancing towards LLC3. LLC0 has 3 tasks preferring LLC3, LLC1 has
> > 2, and LLC2 has 1. LLC0, having the most tasks preferring LLC3, is
> > selected as the busiest source to pick tasks from.
> >
> > Within a source LLC, the total number of tasks preferring a destination
> > LLC is computed by summing counts across all CPUs in that LLC. For
> > instance, if LLC0 has CPU0 with 2 tasks and CPU1 with 1 task preferring
> > LLC3, the total for LLC0 is 3.
> >
> > These statistics allow the load balancer to choose tasks from source
> > sched groups that best match their preferred LLCs.
> >
> > Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> > ---
> >
> > Notes:
> >      v1->v2:
> >          Convert nr_pref_llc array in sg_lb_stats to a single
> >          variable as only the dst LLC stat is needed.
> >          (K Prateek Nayak)
> >
> >   kernel/sched/fair.c | 12 ++++++++++++
> >   1 file changed, 12 insertions(+)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index b0e87616e377..4d7803f69a74 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -10445,6 +10445,9 @@ struct sg_lb_stats {
> >   	unsigned int nr_numa_running;
> >   	unsigned int nr_preferred_running;
> >   #endif
> > +#ifdef CONFIG_SCHED_CACHE
> > +	unsigned int nr_pref_llc;
> > +#endif
>
> At this point I have to note that rq->nr_pref_llc seems like a horrible
> misnomer, for it being an array, and not an actual number like the
> naming suggests.

In the v2 it seems that rq->nr_pref_llc is not an array anymore, it 
indicates
the number of tasks that want to be migrated to the env->dst_cpu 
(dst_llc), because
these tasks' preferred LLC are env->dst_cpu(dst_llc). Maybe renaming it to
rq->nr_pref_dst_llc?

thanks,
Chenyu