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

[Tim Chen]

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@xxxxxxxxx>
Signed-off-by: Chen Yu <yu.c.chen@xxxxxxxxx>
Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
---

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