[PATCH 13/13] sched/numa: Stop an exhastive search if a reasonable swap candidate or idle CPU is found

[Mel Gorman]

When domains are imbalanced or overloaded a search of all CPUs on the
target domain is searched and compared with task_numa_compare. In some
circumstances, a candidate is found that is an obvious win.

o A task can move to an idle CPU and an idle CPU is found
o A swap candidate is found that would move to its preferred domain

This patch terminates the search when either condition is met.

Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
---
 kernel/sched/fair.c | 31 +++++++++++++++++++++++++++----
 1 file changed, 27 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 935baf529f10..a1d5760f7985 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1707,7 +1707,7 @@ static bool load_too_imbalanced(long src_load, long dst_load,
  * into account that it might be best if task running on the dst_cpu should
  * be exchanged with the source task
  */
-static void task_numa_compare(struct task_numa_env *env,
+static bool task_numa_compare(struct task_numa_env *env,
 			      long taskimp, long groupimp, bool maymove)
 {
 	struct numa_group *cur_ng, *p_ng = deref_curr_numa_group(env->p);
@@ -1718,9 +1718,10 @@ static void task_numa_compare(struct task_numa_env *env,
 	int dist = env->dist;
 	long moveimp = imp;
 	long load;
+	bool stopsearch = false;
 
 	if (READ_ONCE(dst_rq->numa_migrate_on))
-		return;
+		return false;
 
 	rcu_read_lock();
 	cur = rcu_dereference(dst_rq->curr);
@@ -1731,8 +1732,10 @@ static void task_numa_compare(struct task_numa_env *env,
 	 * Because we have preemption enabled we can get migrated around and
 	 * end try selecting ourselves (current == env->p) as a swap candidate.
 	 */
-	if (cur == env->p)
+	if (cur == env->p) {
+		stopsearch = true;
 		goto unlock;
+	}
 
 	if (!cur) {
 		if (maymove && moveimp >= env->best_imp)
@@ -1860,8 +1863,27 @@ static void task_numa_compare(struct task_numa_env *env,
 	}
 
 	task_numa_assign(env, cur, imp);
+
+	/*
+	 * If a move to idle is allowed because there is capacity or load
+	 * balance improves then stop the search. While a better swap
+	 * candidate may exist, a search is not free.
+	 */
+	if (maymove && !cur && env->best_cpu >= 0 && idle_cpu(env->best_cpu))
+		stopsearch = true;
+
+	/*
+	 * If a swap candidate must be identified and the current best task
+	 * moves its preferred node then stop the search.
+	 */
+	if (!maymove && env->best_task &&
+	    env->best_task->numa_preferred_nid == env->src_nid) {
+		stopsearch = true;
+	}
 unlock:
 	rcu_read_unlock();
+
+	return stopsearch;
 }
 
 static void task_numa_find_cpu(struct task_numa_env *env,
@@ -1911,7 +1933,8 @@ static void task_numa_find_cpu(struct task_numa_env *env,
 			continue;
 
 		env->dst_cpu = cpu;
-		task_numa_compare(env, taskimp, groupimp, maymove);
+		if (task_numa_compare(env, taskimp, groupimp, maymove))
+			break;
 	}
 }
 
-- 
2.16.4