[PATCH 12/13] sched/numa: Bias swapping tasks based on their preferred node

[Mel Gorman <mgorman@techsingularity.net>]

When swapping tasks for NUMA balancing, it is preferred that tasks move
to or remain on their preferred node. When considering an imbalance,
encourage tasks to move to their preferred node and discourage tasks from
moving away from their preferred node.

Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
---
 kernel/sched/fair.c | 43 +++++++++++++++++++++++++++++++++++++------
 1 file changed, 37 insertions(+), 6 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index ee14cd9815ec..63f22b1a5f0b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1741,18 +1741,27 @@ static void task_numa_compare(struct task_numa_env *env,
 			goto unlock;
 	}
 
+	/* Skip this swap candidate if cannot move to the source cpu. */
+	if (!cpumask_test_cpu(env->src_cpu, cur->cpus_ptr))
+		goto unlock;
+
+	/*
+	 * Skip this swap candidate if it is not moving to its preferred
+	 * node and the best task is.
+	 */
+	if (env->best_task &&
+	    env->best_task->numa_preferred_nid == env->src_nid &&
+	    cur->numa_preferred_nid != env->src_nid) {
+		goto unlock;
+	}
+
 	/*
 	 * "imp" is the fault differential for the source task between the
 	 * source and destination node. Calculate the total differential for
 	 * the source task and potential destination task. The more negative
 	 * the value is, the more remote accesses that would be expected to
 	 * be incurred if the tasks were swapped.
-	 */
-	/* Skip this swap candidate if cannot move to the source cpu */
-	if (!cpumask_test_cpu(env->src_cpu, cur->cpus_ptr))
-		goto unlock;
-
-	/*
+	 *
 	 * If dst and source tasks are in the same NUMA group, or not
 	 * in any group then look only at task weights.
 	 */
@@ -1779,12 +1788,34 @@ static void task_numa_compare(struct task_numa_env *env,
 			       task_weight(cur, env->dst_nid, dist);
 	}
 
+	/* Discourage picking a task already on its preferred node */
+	if (cur->numa_preferred_nid == env->dst_nid)
+		imp -= imp / 16;
+
+	/*
+	 * Encourage picking a task that moves to its preferred node.
+	 * This potentially makes imp larger than it's maximum of
+	 * 1998 (see SMALLIMP and task_weight for why) but in this
+	 * case, it does not matter.
+	 */
+	if (cur->numa_preferred_nid == env->src_nid)
+		imp += imp / 8;
+
 	if (maymove && moveimp > imp && moveimp > env->best_imp) {
 		imp = moveimp;
 		cur = NULL;
 		goto assign;
 	}
 
+	/*
+	 * Prefer swapping with a task moving to its preferred node over a
+	 * task that is not.
+	 */
+	if (env->best_task && cur->numa_preferred_nid == env->src_nid &&
+	    env->best_task->numa_preferred_nid != env->src_nid) {
+		goto assign;
+	}
+
 	/*
 	 * If the NUMA importance is less than SMALLIMP,
 	 * task migration might only result in ping pong
-- 
2.16.4