sched/numa: Favor placing a task on the preferred node

A tasks preferred node is selected based on the number of faults
recorded for a node but the actual task_numa_migate() conducts a global
search regardless of the preferred nid. This patch checks if the
preferred nid has capacity and if so, searches for a CPU within that
node. This avoids a global search when the preferred node is not
overloaded.

Signed-off-by: Mel Gorman <mgorman@suse.de>
Reviewed-by: Rik van Riel <riel@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1381141781-10992-41-git-send-email-mgorman@suse.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 35 insertions, 19 deletions

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 1422765d4b86..09aac90df89e 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1052,6 +1052,20 @@ unlock:
         rcu_read_unlock();
 }
 
+static void task_numa_find_cpu(struct task_numa_env *env, long imp)
+{
+        int cpu;
+
+        for_each_cpu(cpu, cpumask_of_node(env->dst_nid)) {
+                /* Skip this CPU if the source task cannot migrate */
+                if (!cpumask_test_cpu(cpu, tsk_cpus_allowed(env->p)))
+                        continue;
+
+                env->dst_cpu = cpu;
+                task_numa_compare(env, imp);
+        }
+}
+
 static int task_numa_migrate(struct task_struct *p)
 {
         struct task_numa_env env = {
@@ -1068,7 +1082,8 @@ static int task_numa_migrate(struct task_struct *p)
         };
         struct sched_domain *sd;
         unsigned long faults;
-        int nid, cpu, ret;
+        int nid, ret;
+        long imp;
 
         /*
          * Pick the lowest SD_NUMA domain, as that would have the smallest
@@ -1085,28 +1100,29 @@ static int task_numa_migrate(struct task_struct *p)
 
         faults = task_faults(p, env.src_nid);
         update_numa_stats(&env.src_stats, env.src_nid);
+        env.dst_nid = p->numa_preferred_nid;
+        imp = task_faults(env.p, env.dst_nid) - faults;
+        update_numa_stats(&env.dst_stats, env.dst_nid);
 
-        /* Find an alternative node with relatively better statistics */
-        for_each_online_node(nid) {
-                long imp;
-
-                if (nid == env.src_nid)
-                        continue;
-
-                /* Only consider nodes that recorded more faults */
-                imp = task_faults(p, nid) - faults;
-                if (imp < 0)
-                        continue;
+        /*
+         * If the preferred nid has capacity then use it. Otherwise find an
+         * alternative node with relatively better statistics.
+         */
+        if (env.dst_stats.has_capacity) {
+                task_numa_find_cpu(&env, imp);
+        } else {
+                for_each_online_node(nid) {
+                        if (nid == env.src_nid || nid == p->numa_preferred_nid)
+                                continue;
 
-                env.dst_nid = nid;
-                update_numa_stats(&env.dst_stats, env.dst_nid);
-                for_each_cpu(cpu, cpumask_of_node(nid)) {
-                        /* Skip this CPU if the source task cannot migrate */
-                        if (!cpumask_test_cpu(cpu, tsk_cpus_allowed(p)))
+                        /* Only consider nodes that recorded more faults */
+                        imp = task_faults(env.p, nid) - faults;
+                        if (imp < 0)
                                 continue;
 
-                        env.dst_cpu = cpu;
-                        task_numa_compare(&env, imp);
+                        env.dst_nid = nid;
+                        update_numa_stats(&env.dst_stats, env.dst_nid);
+                        task_numa_find_cpu(&env, imp);
                 }
         }