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

[Peter Zijlstra]

On Wed, Dec 10, 2025 at 11:05:33PM +0900, Chen, Yu C wrote:
> 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

>From two patches ago:

+       unsigned int            *nr_pref_llc;

Its a pointer of some sort.


> 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?

Like I said in:

  https://lkml.kernel.org/r/20251210125114.GS3707891@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

it might make sense to put it in struct sched_domain instead of struct
rq, since then you can allocate and swap it right along with the rest of
the domain tree.