[Patch v4 10/22] sched/cache: Count tasks prefering destination LLC in a sched group

Tim Chen
2026-04-01T21:46:58Z

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.

Co-developed-by: Chen Yu <yu.c.chen@intel.com>
Signed-off-by: Chen Yu <yu.c.chen@intel.com>
Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
---

Notes:
v3->v4:
Use rcu_dereference_all() for protection(Peter Zijlstra)
Move the declaration of dst_llc variable under sched_cache_enabled(),
and remove the corresponding ifdeffery(Peter Zijlstra)

kernel/sched/fair.c | 18 ++++++++++++++++++
1 file changed, 18 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index e6474e61f4aa..62a628a1594c 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -10681,6 +10681,9 @@ struct sg_lb_stats {
unsigned int nr_numa_running;
unsigned int nr_preferred_running;
#endif
+#ifdef CONFIG_SCHED_CACHE
+ unsigned int nr_pref_dst_llc;
+#endif
};

/*
@@ -11170,6 +11173,21 @@ static inline void update_sg_lb_stats(struct lb_env *env,
if (cpu_overutilized(i))
*sg_overutilized = 1;

+#ifdef CONFIG_SCHED_CACHE
+ if (sched_cache_enabled()) {
+ struct sched_domain *sd_tmp;
+ int dst_llc;
+
+ dst_llc = llc_id(env->dst_cpu);
+ if (llc_id(i) != dst_llc) {
+ sd_tmp = rcu_dereference_all(rq->sd);
+ if (valid_llc_buf(sd_tmp, dst_llc))
+ sgs->nr_pref_dst_llc +=
+ sd_tmp->llc_counts[dst_llc];
+ }
+ }
+#endif
+
/*
* No need to call idle_cpu() if nr_running is not 0
*/
--
2.32.0