sched/numa: Prevent parallel updates to group stats during placement

Having multiple tasks in a group go through task_numa_placement
simultaneously can lead to a task picking a wrong node to run on, because
the group stats may be in the middle of an update. This patch avoids
parallel updates by holding the numa_group lock during placement
decisions.

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/1381141781-10992-52-git-send-email-mgorman@suse.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 23 insertions, 12 deletions

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index c4df2de6ca4a..147349987bfe 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1233,6 +1233,7 @@ static void task_numa_placement(struct task_struct *p)
 {
         int seq, nid, max_nid = -1, max_group_nid = -1;
         unsigned long max_faults = 0, max_group_faults = 0;
+        spinlock_t *group_lock = NULL;
 
         seq = ACCESS_ONCE(p->mm->numa_scan_seq);
         if (p->numa_scan_seq == seq)
@@ -1241,6 +1242,12 @@ static void task_numa_placement(struct task_struct *p)
         p->numa_migrate_seq++;
         p->numa_scan_period_max = task_scan_max(p);
 
+        /* If the task is part of a group prevent parallel updates to group stats */
+        if (p->numa_group) {
+                group_lock = &p->numa_group->lock;
+                spin_lock(group_lock);
+        }
+
         /* Find the node with the highest number of faults */
         for_each_online_node(nid) {
                 unsigned long faults = 0, group_faults = 0;
@@ -1279,20 +1286,24 @@ static void task_numa_placement(struct task_struct *p)
                 }
         }
 
-        /*
-         * If the preferred task and group nids are different,
-         * iterate over the nodes again to find the best place.
-         */
-        if (p->numa_group && max_nid != max_group_nid) {
-                unsigned long weight, max_weight = 0;
-
-                for_each_online_node(nid) {
-                        weight = task_weight(p, nid) + group_weight(p, nid);
-                        if (weight > max_weight) {
-                                max_weight = weight;
-                                max_nid = nid;
+        if (p->numa_group) {
+                /*
+                 * If the preferred task and group nids are different,
+                 * iterate over the nodes again to find the best place.
+                 */
+                if (max_nid != max_group_nid) {
+                        unsigned long weight, max_weight = 0;
+
+                        for_each_online_node(nid) {
+                                weight = task_weight(p, nid) + group_weight(p, nid);
+                                if (weight > max_weight) {
+                                        max_weight = weight;
+                                        max_nid = nid;
+                                }
                         }
                 }
+
+                spin_unlock(group_lock);
         }
 
         /* Preferred node as the node with the most faults */