[PATCH] sched,numa: document and fix numa_preferred_nid setting

From: Rik van Riel
Date: Tue Jun 16 2015 - 15:55:03 EST

There are two places where the numa balancing code sets a task's

The primary location is task_numa_placement(), where the kernel
examines the NUMA fault statistics to determine the location where
most of the memory that the task (or numa_group) accesses is.

The second location is only used for large workloads, where a
numa_group has enough tasks that the tasks are spread out over
several NUMA nodes, and multiple nodes are in the numa group's
active_nodes mask.

In order to allow those workloads to settle down, we pretend
that any node inside the numa_group's active_nodes mask is the
task's new preferred node. This dissuades task_numa_fault()
from continuously retrying to migrate the task to the group's
preferred node, and allows a multi-node workload to settle down.
This in turn improves locality of private faults inside a numa

Reported-by: Shrikar Dronamraju <srikar@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Rik van Riel <riel@xxxxxxxxxx>
kernel/sched/fair.c | 23 +++++++++++++++--------
1 file changed, 15 insertions(+), 8 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index c2980e8733bc..54bb57f09e75 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1485,7 +1485,12 @@ static int task_numa_migrate(struct task_struct *p)
groupweight = group_weight(p, env.src_nid, dist);

- /* Only consider nodes where both task and groups benefit */
+ /*
+ * Only consider nodes where placement is better for
+ * either the group (help large workloads converge),
+ * or the task (placement of tasks within a numa group,
+ * and single threaded processes).
+ */
taskimp = task_weight(p, nid, dist) - taskweight;
groupimp = group_weight(p, nid, dist) - groupweight;
if (taskimp < 0 && groupimp < 0)
@@ -1499,12 +1504,14 @@ static int task_numa_migrate(struct task_struct *p)

- * If the task is part of a workload that spans multiple NUMA nodes,
- * and is migrating into one of the workload's active nodes, remember
- * this node as the task's preferred numa node, so the workload can
- * settle down.
- * A task that migrated to a second choice node will be better off
- * trying for a better one later. Do not set the preferred node here.
+ * The primary place for setting a task's numa_preferred_nid is in
+ * task_numa_placement(). If a task is moved to a sub-optimal node,
+ * leave numa_preferred_nid alone, so task_numa_fault() will retry
+ * migrating the task to where it really belongs.
+ * The exception is a task that belongs to a large numa_group, which
+ * spans multiple NUMA nodes. If that task migrates into one of the
+ * workload's active nodes, remember that node as the task's
+ * numa_preferred_nid, so the workload can settle down.
if (p->numa_group) {
if (env.best_cpu == -1)
@@ -1513,7 +1520,7 @@ static int task_numa_migrate(struct task_struct *p)
nid = env.dst_nid;

if (node_isset(nid, p->numa_group->active_nodes))
- sched_setnuma(p, env.dst_nid);
+ sched_setnuma(p, nid);

/* No better CPU than the current one was found. */

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