[PATCH v3 13/21] sched/cache: Handle moving single tasks to/from their preferred LLC

[Tim Chen]

In the generic load balance(non-cache-aware-load-balance),
if the busiest runqueue has only one task, active balancing may be
invoked to move it. However, this migration might break LLC locality.

Before migration, check whether the task is running on its preferred
LLC: Do not move a lone task to another LLC if it would move the task
away from its preferred LLC or cause excessive imbalance between LLCs.

On the other hand, if the migration type is migrate_llc_task, it means
that there are tasks on the env->src_cpu that want to be migrated to
their preferred LLC,  launch the active load balance anyway.

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:
    v2->v3:
    Remove redundant rcu read lock in break_llc_locality().

 kernel/sched/fair.c | 54 ++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 53 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 1697791ef11c..03959a701514 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9999,12 +9999,60 @@ static __maybe_unused enum llc_mig can_migrate_llc_task(int src_cpu, int dst_cpu
 			       task_util(p), to_pref);
 }
 
+/*
+ * Check if active load balance breaks LLC locality in
+ * terms of cache aware load balance.
+ */
+static inline bool
+alb_break_llc(struct lb_env *env)
+{
+	if (!sched_cache_enabled())
+		return false;
+
+	if (cpus_share_cache(env->src_cpu, env->dst_cpu))
+		return false;
+	/*
+	 * All tasks prefer to stay on their current CPU.
+	 * Do not pull a task from its preferred CPU if:
+	 * 1. It is the only task running there; OR
+	 * 2. Migrating it away from its preferred LLC would violate
+	 *    the cache-aware scheduling policy.
+	 */
+	if (env->src_rq->nr_pref_llc_running &&
+	    env->src_rq->nr_pref_llc_running == env->src_rq->cfs.h_nr_runnable) {
+		unsigned long util = 0;
+		struct task_struct *cur;
+
+		if (env->src_rq->nr_running <= 1)
+			return true;
+
+		/*
+		 * Reach here in load balance with
+		 * rcu_read_lock() protected.
+		 */
+		cur = rcu_dereference(env->src_rq->curr);
+		if (cur)
+			util = task_util(cur);
+
+		if (can_migrate_llc(env->src_cpu, env->dst_cpu,
+				    util, false) == mig_forbid)
+			return true;
+	}
+
+	return false;
+}
 #else
 static inline bool get_llc_stats(int cpu, unsigned long *util,
 				 unsigned long *cap)
 {
 	return false;
 }
+
+static inline bool
+alb_break_llc(struct lb_env *env)
+{
+	return false;
+}
 #endif
 /*
  * can_migrate_task - may task p from runqueue rq be migrated to this_cpu?
@@ -12421,6 +12469,9 @@ static int need_active_balance(struct lb_env *env)
 {
 	struct sched_domain *sd = env->sd;
 
+	if (alb_break_llc(env))
+		return 0;
+
 	if (asym_active_balance(env))
 		return 1;
 
@@ -12440,7 +12491,8 @@ static int need_active_balance(struct lb_env *env)
 			return 1;
 	}
 
-	if (env->migration_type == migrate_misfit)
+	if (env->migration_type == migrate_misfit ||
+	    env->migration_type == migrate_llc_task)
 		return 1;
 
 	return 0;
-- 
2.32.0