sched/numa: Select a preferred node with the most numa hinting faults

This patch selects a preferred node for a task to run on based on the
NUMA hinting faults. This information is later used to migrate tasks
towards the node during balancing.

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/1381141781-10992-21-git-send-email-mgorman@suse.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>

include/linux/sched.h

@@ -1344,6 +1344,7 @@ struct task_struct {
 	struct callback_head numa_work;
 
 	unsigned long *numa_faults;
+	int numa_preferred_nid;
 #endif /* CONFIG_NUMA_BALANCING */
 
 	struct rcu_head rcu;

kernel/sched/core.c

@@ -1633,6 +1633,7 @@ static void __sched_fork(struct task_struct *p)
 	p->numa_scan_seq = p->mm ? p->mm->numa_scan_seq : 0;
 	p->numa_migrate_seq = p->mm ? p->mm->numa_scan_seq - 1 : 0;
 	p->numa_scan_period = sysctl_numa_balancing_scan_delay;
+	p->numa_preferred_nid = -1;
 	p->numa_work.next = &p->numa_work;
 	p->numa_faults = NULL;
 #endif /* CONFIG_NUMA_BALANCING */

kernel/sched/fair.c

@@ -879,7 +879,8 @@ static unsigned int task_scan_max(struct task_struct *p)
 
 static void task_numa_placement(struct task_struct *p)
 {
-	int seq;
+	int seq, nid, max_nid = -1;
+	unsigned long max_faults = 0;
 
 	if (!p->mm)	/* for example, ksmd faulting in a user's mm */
 		return;
@@ -889,7 +890,19 @@ static void task_numa_placement(struct task_struct *p)
 	p->numa_scan_seq = seq;
 	p->numa_scan_period_max = task_scan_max(p);
 
-	/* FIXME: Scheduling placement policy hints go here */
+	/* Find the node with the highest number of faults */
+	for_each_online_node(nid) {
+		unsigned long faults = p->numa_faults[nid];
+		p->numa_faults[nid] >>= 1;
+		if (faults > max_faults) {
+			max_faults = faults;
+			max_nid = nid;
+		}
+	}
+
+	/* Update the tasks preferred node if necessary */
+	if (max_faults && max_nid != p->numa_preferred_nid)
+		p->numa_preferred_nid = max_nid;
 }
 
 /*