[RFC PATCH v4 21/28] sched: Introduce a static key to enable cache aware only for multi LLCs

[Chen Yu]

If there are more than one LLCs in the node, the
cache aware scheduling is enabled. Otherwise, the
cache aware scheduling is disabled.

The definition of multiple LLCs in a node is that
every node in the system should have more than one
LLC. For example, if node0, node1, and node2 each
have 4 LLCs, while node3 has 1 LLC (possibly due
to CPU hotplug), cache-aware scheduling should be
disabled.

Suggested-by: Libo Chen <libo.chen@xxxxxxxxxx>
Co-developed-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
Signed-off-by: Chen Yu <yu.c.chen@xxxxxxxxx>
---
 kernel/sched/fair.c     | 31 ++++++++++++++++++++++---------
 kernel/sched/sched.h    |  1 +
 kernel/sched/topology.c | 22 ++++++++++++++++++++--
 3 files changed, 43 insertions(+), 11 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 6e61f9e1f628..194ec594561b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1175,6 +1175,8 @@ static s64 update_curr_se(struct rq *rq, struct sched_entity *curr)
 #define EPOCH_PERIOD	(HZ/100)	/* 10 ms */
 #define EPOCH_OLD	5		/* 50 ms */
 
+DEFINE_STATIC_KEY_FALSE(sched_cache_present);
+
 static int llc_id(int cpu)
 {
 	if (cpu < 0)
@@ -1318,7 +1320,8 @@ void account_mm_sched(struct rq *rq, struct task_struct *p, s64 delta_exec)
 	unsigned long epoch;
 	int mm_sched_llc = -1;
 
-	if (!sched_feat(SCHED_CACHE))
+	if (!sched_feat(SCHED_CACHE) ||
+	    !static_branch_likely(&sched_cache_present))
 		return;
 
 	if (p->sched_class != &fair_sched_class)
@@ -1366,7 +1369,8 @@ static void task_tick_cache(struct rq *rq, struct task_struct *p)
 	struct callback_head *work = &p->cache_work;
 	struct mm_struct *mm = p->mm;
 
-	if (!sched_feat(SCHED_CACHE))
+	if (!sched_feat(SCHED_CACHE) ||
+	    !static_branch_likely(&sched_cache_present))
 		return;
 
 	if (!mm || !mm->pcpu_sched)
@@ -9063,7 +9067,8 @@ static int select_cache_cpu(struct task_struct *p, int prev_cpu)
 	struct mm_struct *mm = p->mm;
 	int cpu;
 
-	if (!sched_feat(SCHED_CACHE) || !sched_feat(SCHED_CACHE_WAKE))
+	if (!sched_feat(SCHED_CACHE) || !sched_feat(SCHED_CACHE_WAKE) ||
+	    !static_branch_likely(&sched_cache_present))
 		return prev_cpu;
 
 	if (!mm || p->nr_cpus_allowed == 1)
@@ -10024,6 +10029,7 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
 
 #ifdef CONFIG_SCHED_CACHE
 	if (sched_feat(SCHED_CACHE) && sched_feat(SCHED_CACHE_LB) &&
+	   static_branch_likely(&sched_cache_present) &&
 	    get_migrate_hint(env->src_cpu, env->dst_cpu, p) == mig_forbid)
 		return 0;
 #endif
@@ -10109,7 +10115,8 @@ static struct list_head
 	LIST_HEAD(no_pref_llc);
 	LIST_HEAD(pref_other_llc);
 
-	if (!sched_feat(SCHED_CACHE) || !sched_feat(SCHED_CACHE_LB))
+	if (!sched_feat(SCHED_CACHE) || !sched_feat(SCHED_CACHE_LB) ||
+	    !static_branch_likely(&sched_cache_present))
 		return tasks;
 
 	if (cpus_share_cache(env->dst_cpu, env->src_cpu))
@@ -10295,6 +10302,7 @@ static int detach_tasks(struct lb_env *env)
 		 * they are tasks that prefer the current LLC.
 		 */
 		if (sched_feat(SCHED_CACHE) && sched_feat(SCHED_CACHE_LB) &&
+		    static_branch_likely(&sched_cache_present) &&
 		    p->preferred_llc != -1 &&
 		    llc_id(env->src_cpu) == p->preferred_llc)
 			break;
@@ -10952,7 +10960,8 @@ static inline bool llc_balance(struct lb_env *env, struct sg_lb_stats *sgs,
 	struct sched_domain *child = env->sd->child;
 	int llc;
 
-	if (!sched_feat(SCHED_CACHE) || !sched_feat(SCHED_CACHE_LB))
+	if (!sched_feat(SCHED_CACHE) || !sched_feat(SCHED_CACHE_LB) ||
+	    !static_branch_likely(&sched_cache_present))
 		return false;
 
 	if (env->sd->flags & SD_SHARE_LLC)
@@ -11064,7 +11073,8 @@ static void update_sg_if_llc(struct lb_env *env, struct sg_lb_stats *sgs,
 	struct sched_domain_shared *sd_share;
 
 	if (!sched_feat(SCHED_CACHE) || env->idle == CPU_NEWLY_IDLE ||
-	    !sched_feat(SCHED_CACHE_LB))
+	    !sched_feat(SCHED_CACHE_LB) ||
+	    !static_branch_likely(&sched_cache_present))
 		return;
 
 	/* only care the sched domain that spans 1 LLC */
@@ -11126,7 +11136,8 @@ static inline void update_sg_lb_stats(struct lb_env *env,
 			*sg_overutilized = 1;
 
 #ifdef CONFIG_SCHED_CACHE
-		if (sched_feat(SCHED_CACHE) && sched_feat(SCHED_CACHE_LB)) {
+		if (sched_feat(SCHED_CACHE) && sched_feat(SCHED_CACHE_LB) &&
+		    static_branch_likely(&sched_cache_present)) {
 			int j;
 
 			for (j = 0; j < max_llcs; ++j)
@@ -12412,7 +12423,8 @@ imbalanced_active_balance(struct lb_env *env)
 static inline bool
 break_llc_locality(struct lb_env *env)
 {
-	if (!sched_feat(SCHED_CACHE) || !sched_feat(SCHED_CACHE_LB))
+	if (!sched_feat(SCHED_CACHE) || !sched_feat(SCHED_CACHE_LB) ||
+	    !static_branch_likely(&sched_cache_present))
 		return 0;
 
 	if (cpus_share_cache(env->src_cpu, env->dst_cpu))
@@ -12914,7 +12926,8 @@ static int active_load_balance_cpu_stop(void *data)
 #ifdef CONFIG_SCHED_CACHE
 		int llc = llc_idx(target_cpu);
 
-		if (!sched_feat(SCHED_CACHE) || !sched_feat(SCHED_CACHE_LB))
+		if (!sched_feat(SCHED_CACHE) || !sched_feat(SCHED_CACHE_LB) ||
+		    !static_branch_likely(&sched_cache_present))
 			goto out_unlock;
 
 		if (llc < 0)
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 4464b92767ad..3e60618a88e9 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2857,6 +2857,7 @@ extern unsigned int sysctl_numa_balancing_hot_threshold;
 #ifdef CONFIG_SCHED_CACHE
 extern unsigned int sysctl_llc_aggr_cap;
 extern unsigned int sysctl_llc_aggr_imb;
+extern struct static_key_false sched_cache_present;
 #endif
 
 #ifdef CONFIG_SCHED_HRTICK
diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 91a2b7f65fee..8483c02b4d28 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -2476,6 +2476,8 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
 	int i, ret = -ENOMEM;
 	bool has_asym = false;
 	bool has_cluster = false;
+	bool llc_has_parent_sd = false;
+	unsigned int multi_llcs_node = 1;
 
 #ifdef CONFIG_SCHED_CACHE
 	if (max_llcs < 0) {
@@ -2545,6 +2547,8 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
 				struct sched_domain __rcu *top_p;
 				unsigned int nr_llcs;
 
+				if (!llc_has_parent_sd)
+					llc_has_parent_sd = true;
 				/*
 				 * For a single LLC per node, allow an
 				 * imbalance up to 12.5% of the node. This is
@@ -2566,10 +2570,19 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
 				 * between LLCs and memory channels.
 				 */
 				nr_llcs = sd->span_weight / child->span_weight;
-				if (nr_llcs == 1)
+				/*
+				 * iff all nodes have multiple LLCs, the
+				 * multi_llcs_node will be set to 1. If
+				 * there is at least 1 node having 1 single
+				 * LLC, the multi_llcs_node remains 0.
+				 */
+				if (nr_llcs == 1) {
 					imb = sd->span_weight >> 3;
-				else
+					multi_llcs_node = 0;
+				} else {
 					imb = nr_llcs;
+					multi_llcs_node &= 1;
+				}
 				imb = max(1U, imb);
 				sd->imb_numa_nr = imb;
 
@@ -2617,6 +2630,11 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
 	if (has_cluster)
 		static_branch_inc_cpuslocked(&sched_cluster_active);
 
+#ifdef CONFIG_SCHED_CACHE
+	if (llc_has_parent_sd && multi_llcs_node && !sched_asym_cpucap_active())
+		static_branch_inc_cpuslocked(&sched_cache_present);
+#endif
+
 	if (rq && sched_debug_verbose)
 		pr_info("root domain span: %*pbl\n", cpumask_pr_args(cpu_map));
 
-- 
2.25.1