[PATCH 0/6] sched,numa: weigh nearby nodes for task placement on complex NUMA topologies (v2)

[PATCH 0/6] sched,numa: weigh nearby nodes for task placement on complex NUMA topologies (v2)

[riel]

This patch set integrates two algorithms I have previously tested,
one for glueless mesh NUMA topologies, where NUMA nodes communicate
with far-away nodes through intermediary nodes, and backplane
topologies, where communication with far-away NUMA nodes happens
through backplane controllers (which cannot run tasks).

Due to the inavailability of 8 node systems, and the fact that I
am flying out to Linuxcon Europe / Plumbers / KVM Forum on Friday,
I have not tested these patches yet. However, with a conference (and
many familiar faces) coming up, it seemed like a good idea to get
the code out there, anyway.

Vinod tested the v1 series + patch 6 on a backplane topology
system, and I changed v2 on a glueless mesh topology system.
The patches appear to behave.

Placement of tasks on smaller, directly connected, NUMA systems
should not be affected at all by this patch serie


v2: remove the hops table, and use numa_distance instead, tested
    on a system with glueless mesh topology, not yet tested on
    a backplane style system (but the algorithm should be unchanged)

[PATCH 1/6] sched,numa: export info needed for NUMA balancing on complex topologies

[riel]

From: Rik van Riel <riel@redhat.com>

Export some information that is necessary to do placement of
tasks on systems with multi-level NUMA topologies.

Signed-off-by: Rik van Riel <riel@redhat.com>
---
 kernel/sched/core.c  | 4 +++-
 kernel/sched/sched.h | 2 ++
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 5a4ad05..ed427f9 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6076,6 +6076,7 @@ static void claim_allocations(int cpu, struct sched_domain *sd)
 #ifdef CONFIG_NUMA
 static int sched_domains_numa_levels;
 static int *sched_domains_numa_distance;
+int sched_max_numa_distance;
 static struct cpumask ***sched_domains_numa_masks;
 static int sched_domains_curr_level;
 #endif
@@ -6247,7 +6248,7 @@ static void sched_numa_warn(const char *str)
 	printk(KERN_WARNING "\n");
 }
 
-static bool find_numa_distance(int distance)
+bool find_numa_distance(int distance)
 {
 	int i;
 
@@ -6394,6 +6395,7 @@ static void sched_init_numa(void)
 	sched_domain_topology = tl;
 
 	sched_domains_numa_levels = level;
+	sched_max_numa_distance = sched_domains_numa_distance[level - 1];
 }
 
 static void sched_domains_numa_masks_set(int cpu)
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 6130251..515402e 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -680,6 +680,8 @@ static inline u64 rq_clock_task(struct rq *rq)
 
 #ifdef CONFIG_NUMA_BALANCING
 extern void sched_setnuma(struct task_struct *p, int node);
+extern int sched_max_numa_distance;
+extern bool find_numa_distance(int distance);
 extern int migrate_task_to(struct task_struct *p, int cpu);
 extern int migrate_swap(struct task_struct *, struct task_struct *);
 #endif /* CONFIG_NUMA_BALANCING */
-- 
1.9.3

[PATCH 2/6] sched,numa: classify the NUMA topology of a system

[riel]

From: Rik van Riel <riel@redhat.com>

Smaller NUMA systems tend to have all NUMA nodes directly connected
to each other. This includes the degenerate case of a system with just
one node, ie. a non-NUMA system.

Larger systems can have two kinds of NUMA topology, which affects how
tasks and memory should be placed on the system.

On glueless mesh systems, nodes that are not directly connected to
each other will bounce traffic through intermediary nodes. Task groups
can be run closer to each other by moving tasks from a node to an
intermediary node between it and the task's preferred node.

On NUMA systems with backplane controllers, the intermediary hops
are incapable of running programs. This creates "islands" of nodes
that are at an equal distance to anywhere else in the system.

Each kind of topology requires a slightly different placement
algorithm; this patch provides the mechanism to detect the kind
of NUMA topology of a system.

Signed-off-by: Rik van Riel <riel@redhat.com>
Tested-by: Chegu Vinod <chegu_vinod@hp.com>
---
 include/linux/topology.h |  7 +++++++
 kernel/sched/core.c      | 53 ++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 60 insertions(+)

diff --git a/include/linux/topology.h b/include/linux/topology.h
index dda6ee5..40d6cea 100644
--- a/include/linux/topology.h
+++ b/include/linux/topology.h
@@ -48,6 +48,13 @@
 
 int arch_update_cpu_topology(void);
 
+enum numa_topology_type {
+	NUMA_DIRECT,
+	NUMA_GLUELESS_MESH,
+	NUMA_BACKPLANE,
+};
+extern enum numa_topology_type sched_numa_topology_type;
+
 /* Conform to ACPI 2.0 SLIT distance definitions */
 #define LOCAL_DISTANCE		10
 #define REMOTE_DISTANCE		20
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index ed427f9..19e6c1b 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6075,6 +6075,7 @@ static void claim_allocations(int cpu, struct sched_domain *sd)
 
 #ifdef CONFIG_NUMA
 static int sched_domains_numa_levels;
+enum numa_topology_type sched_numa_topology_type;
 static int *sched_domains_numa_distance;
 int sched_max_numa_distance;
 static struct cpumask ***sched_domains_numa_masks;
@@ -6263,6 +6264,56 @@ bool find_numa_distance(int distance)
 	return false;
 }
 
+/*
+ * A system can have three types of NUMA topology:
+ * NUMA_DIRECT: all nodes are directly connected, or not a NUMA system
+ * NUMA_GLUELESS_MESH: some nodes reachable through intermediary nodes
+ * NUMA_BACKPLANE: nodes can reach other nodes through a backplane
+ *
+ * The difference between a glueless mesh topology and a backplane
+ * topology lies in whether communication between not directly
+ * connected nodes goes through intermediary nodes (where programs
+ * could run), or through backplane controllers. This affects
+ * placement of programs.
+ *
+ * The type of topology can be discerned with the following tests:
+ * - If the maximum distance between any nodes is 1 hop, the system
+ *   is directly connected.
+ * - If for two nodes A and B, located N > 1 hops away from each other,
+ *   there is an intermediary node C, which is < N hops away from both
+ *   nodes A and B, the system is a glueless mesh.
+ */
+static void init_numa_topology_type(void)
+{
+	int a, b, c, n;
+
+	n = sched_max_numa_distance;
+
+	if (n <= 1)
+		sched_numa_topology_type = NUMA_DIRECT;
+
+	for_each_online_node(a) {
+		for_each_online_node(b) {
+			/* Find two nodes furthest removed from each other. */
+			if (node_distance(a, b) < n)
+				continue;
+
+			/* Is there an intermediary node between a and b? */
+			for_each_online_node(c) {
+				if (node_distance(a, c) < n &&
+				    node_distance(b, c) < n) {
+					sched_numa_topology_type =
+							NUMA_GLUELESS_MESH;
+					return;
+				}
+			}
+
+			sched_numa_topology_type = NUMA_BACKPLANE;
+			return;
+		}
+	}
+}
+
 static void sched_init_numa(void)
 {
 	int next_distance, curr_distance = node_distance(0, 0);
@@ -6396,6 +6447,8 @@ static void sched_init_numa(void)
 
 	sched_domains_numa_levels = level;
 	sched_max_numa_distance = sched_domains_numa_distance[level - 1];
+
+	init_numa_topology_type();
 }
 
 static void sched_domains_numa_masks_set(int cpu)
-- 
1.9.3

[PATCH 3/6] sched,numa: preparations for complex topology placement

[riel]

From: Rik van Riel <riel@redhat.com>

Preparatory patch for adding NUMA placement on systems with
complex NUMA topology. Also fix a potential divide by zero
in group_weight()

Signed-off-by: Rik van Riel <riel@redhat.com>
Tested-by: Chegu Vinod <chegu_vinod@hp.com>
---
 kernel/sched/fair.c | 57 ++++++++++++++++++++++++++++++++++-------------------
 1 file changed, 37 insertions(+), 20 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 6d44052..1c540f5 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -930,9 +930,10 @@ static inline unsigned long group_faults_cpu(struct numa_group *group, int nid)
  * larger multiplier, in order to group tasks together that are almost
  * evenly spread out between numa nodes.
  */
-static inline unsigned long task_weight(struct task_struct *p, int nid)
+static inline unsigned long task_weight(struct task_struct *p, int nid,
+					int dist)
 {
-	unsigned long total_faults;
+	unsigned long faults, total_faults;
 
 	if (!p->numa_faults_memory)
 		return 0;
@@ -942,15 +943,25 @@ static inline unsigned long task_weight(struct task_struct *p, int nid)
 	if (!total_faults)
 		return 0;
 
-	return 1000 * task_faults(p, nid) / total_faults;
+	faults = task_faults(p, nid);
+	return 1000 * faults / total_faults;
 }
 
-static inline unsigned long group_weight(struct task_struct *p, int nid)
+static inline unsigned long group_weight(struct task_struct *p, int nid,
+					 int dist)
 {
-	if (!p->numa_group || !p->numa_group->total_faults)
+	unsigned long faults, total_faults;
+
+	if (!p->numa_group)
+		return 0;
+
+	total_faults = p->numa_group->total_faults;
+
+	if (!total_faults)
 		return 0;
 
-	return 1000 * group_faults(p, nid) / p->numa_group->total_faults;
+	faults = group_faults(p, nid);
+	return 1000 * faults / total_faults;
 }
 
 bool should_numa_migrate_memory(struct task_struct *p, struct page * page,
@@ -1083,6 +1094,7 @@ struct task_numa_env {
 	struct numa_stats src_stats, dst_stats;
 
 	int imbalance_pct;
+	int dist;
 
 	struct task_struct *best_task;
 	long best_imp;
@@ -1162,6 +1174,7 @@ static void task_numa_compare(struct task_numa_env *env,
 	long load;
 	long imp = env->p->numa_group ? groupimp : taskimp;
 	long moveimp = imp;
+	int dist = env->dist;
 
 	rcu_read_lock();
 	cur = ACCESS_ONCE(dst_rq->curr);
@@ -1185,8 +1198,8 @@ static void task_numa_compare(struct task_numa_env *env,
 		 * in any group then look only at task weights.
 		 */
 		if (cur->numa_group == env->p->numa_group) {
-			imp = taskimp + task_weight(cur, env->src_nid) -
-			      task_weight(cur, env->dst_nid);
+			imp = taskimp + task_weight(cur, env->src_nid, dist) -
+			      task_weight(cur, env->dst_nid, dist);
 			/*
 			 * Add some hysteresis to prevent swapping the
 			 * tasks within a group over tiny differences.
@@ -1200,11 +1213,11 @@ static void task_numa_compare(struct task_numa_env *env,
 			 * instead.
 			 */
 			if (cur->numa_group)
-				imp += group_weight(cur, env->src_nid) -
-				       group_weight(cur, env->dst_nid);
+				imp += group_weight(cur, env->src_nid, dist) -
+				       group_weight(cur, env->dst_nid, dist);
 			else
-				imp += task_weight(cur, env->src_nid) -
-				       task_weight(cur, env->dst_nid);
+				imp += task_weight(cur, env->src_nid, dist) -
+				       task_weight(cur, env->dst_nid, dist);
 		}
 	}
 
@@ -1303,7 +1316,7 @@ static int task_numa_migrate(struct task_struct *p)
 	};
 	struct sched_domain *sd;
 	unsigned long taskweight, groupweight;
-	int nid, ret;
+	int nid, ret, dist;
 	long taskimp, groupimp;
 
 	/*
@@ -1331,12 +1344,13 @@ static int task_numa_migrate(struct task_struct *p)
 		return -EINVAL;
 	}
 
-	taskweight = task_weight(p, env.src_nid);
-	groupweight = group_weight(p, env.src_nid);
-	update_numa_stats(&env.src_stats, env.src_nid);
 	env.dst_nid = p->numa_preferred_nid;
-	taskimp = task_weight(p, env.dst_nid) - taskweight;
-	groupimp = group_weight(p, env.dst_nid) - groupweight;
+	dist = env.dist = node_distance(env.src_nid, env.dst_nid);
+	taskweight = task_weight(p, env.src_nid, dist);
+	groupweight = group_weight(p, env.src_nid, dist);
+	update_numa_stats(&env.src_stats, env.src_nid);
+	taskimp = task_weight(p, env.dst_nid, dist) - taskweight;
+	groupimp = group_weight(p, env.dst_nid, dist) - groupweight;
 	update_numa_stats(&env.dst_stats, env.dst_nid);
 
 	/* Try to find a spot on the preferred nid. */
@@ -1348,12 +1362,15 @@ static int task_numa_migrate(struct task_struct *p)
 			if (nid == env.src_nid || nid == p->numa_preferred_nid)
 				continue;
 
+			dist = node_distance(env.src_nid, env.dst_nid);
+
 			/* Only consider nodes where both task and groups benefit */
-			taskimp = task_weight(p, nid) - taskweight;
-			groupimp = group_weight(p, nid) - groupweight;
+			taskimp = task_weight(p, nid, dist) - taskweight;
+			groupimp = group_weight(p, nid, dist) - groupweight;
 			if (taskimp < 0 && groupimp < 0)
 				continue;
 
+			env.dist = dist;
 			env.dst_nid = nid;
 			update_numa_stats(&env.dst_stats, env.dst_nid);
 			task_numa_find_cpu(&env, taskimp, groupimp);
-- 
1.9.3

[PATCH 4/6] sched,numa: calculate node scores in complex NUMA topologies

[riel]

From: Rik van Riel <riel@redhat.com>

In order to do task placement on systems with complex NUMA topologies,
it is necessary to count the faults on nodes nearby the node that is
being examined for a potential move.

In case of a system with a backplane interconnect, we are dealing with
groups of NUMA nodes; each of the nodes within a group is the same number
of hops away from nodes in other groups in the system. Optimal placement
on this topology is achieved by counting all nearby nodes equally. When
comparing nodes A and B at distance N, nearby nodes are those at distances
smaller than N from nodes A or B.

Placement strategy on a system with a glueless mesh NUMA topology needs
to be different, because there are no natural groups of nodes determined
by the hardware. Instead, when dealing with two nodes A and B at distance
N, N >= 2, there will be intermediate nodes at distance < N from both nodes
A and B. Good placement can be achieved by right shifting the faults on
nearby nodes by the number of hops from the node being scored. In this
context, a nearby node is any node less than the maximum distance in the
system away from the node. Those nodes are skipped for efficiency reasons,
there is no real policy reason to do so.

Placement policy on directly connected NUMA systems is not affected.

Signed-off-by: Rik van Riel <riel@redhat.com>
Tested-by: Chegu Vinod <chegu_vinod@hp.com>
---
 kernel/sched/fair.c | 74 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 74 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 1c540f5..6df460f 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -924,6 +924,71 @@ static inline unsigned long group_faults_cpu(struct numa_group *group, int nid)
 		group->faults_cpu[task_faults_idx(nid, 1)];
 }
 
+/* Handle placement on systems where not all nodes are directly connected. */
+static unsigned long score_nearby_nodes(struct task_struct *p, int nid,
+					int maxdist, bool task)
+{
+	unsigned long score = 0;
+	int node;
+
+	/*
+	 * All nodes are directly connected, and the same distance
+	 * from each other. No need for fancy placement algorithms.
+	 */
+	if (sched_numa_topology_type == NUMA_DIRECT)
+		return 0;
+
+	/*
+	 * This code is called for each node, introducing N^2 complexity,
+	 * which should be ok given the number of nodes rarely exceeds 8.
+	 */
+	for_each_online_node(node) {
+		unsigned long faults;
+		int dist = node_distance(nid, node);
+
+		/*
+		 * The furthest away nodes in the system are not interesting
+		 * for placement; nid was already counted.
+		 */
+		if (dist == sched_max_numa_distance || node == nid)
+			continue;
+
+		/*
+		 * On systems with a backplane NUMA topology, compare groups
+		 * of nodes, and move tasks towards the group with the most
+		 * memory accesses. When comparing two nodes at distance
+		 * "hoplimit", only nodes closer by than "hoplimit" are part
+		 * of each group. Skip other nodes.
+		 */
+		if (sched_numa_topology_type == NUMA_BACKPLANE &&
+					dist > maxdist)
+			continue;
+
+		/* Add up the faults from nearby nodes. */
+		if (task)
+			faults = task_faults(p, node);
+		else
+			faults = group_faults(p, node);
+
+		/*
+		 * On systems with a glueless mesh NUMA topology, there are
+		 * no fixed "groups of nodes". Instead, nodes that are not
+		 * directly connected bounce traffic through intermediate
+		 * nodes; a numa_group can occupy any set of nodes.
+		 * The further away a node is, the less the faults count.
+		 * This seems to result in good task placement.
+		 */
+		if (sched_numa_topology_type == NUMA_GLUELESS_MESH) {
+			faults *= (sched_max_numa_distance - dist);
+			faults /= (sched_max_numa_distance - LOCAL_DISTANCE);
+		}
+
+		score += faults;
+	}
+
+	return score;
+}
+
 /*
  * These return the fraction of accesses done by a particular task, or
  * task group, on a particular numa node.  The group weight is given a
@@ -944,6 +1009,8 @@ static inline unsigned long task_weight(struct task_struct *p, int nid,
 		return 0;
 
 	faults = task_faults(p, nid);
+	faults += score_nearby_nodes(p, nid, dist, true);
+
 	return 1000 * faults / total_faults;
 }
 
@@ -961,6 +1028,8 @@ static inline unsigned long group_weight(struct task_struct *p, int nid,
 		return 0;
 
 	faults = group_faults(p, nid);
+	faults += score_nearby_nodes(p, nid, dist, false);
+
 	return 1000 * faults / total_faults;
 }
 
@@ -1363,6 +1432,11 @@ static int task_numa_migrate(struct task_struct *p)
 				continue;
 
 			dist = node_distance(env.src_nid, env.dst_nid);
+			if (sched_numa_topology_type == NUMA_BACKPLANE &&
+						dist != env.dist) {
+				taskweight = task_weight(p, env.src_nid, dist);
+				groupweight = group_weight(p, env.src_nid, dist);
+			}
 
 			/* Only consider nodes where both task and groups benefit */
 			taskimp = task_weight(p, nid, dist) - taskweight;
-- 
1.9.3

[PATCH 5/6] sched,numa: find the preferred nid with complex NUMA topology

[riel]

From: Rik van Riel <riel@redhat.com>

On systems with complex NUMA topologies, the node scoring is adjusted
to allow workloads to converge on nodes that are near each other.

The way a task group's preferred nid is determined needs to be adjusted,
in order for the preferred_nid to be consistent with group_weight scoring.
This ensures that we actually try to converge workloads on adjacent nodes.

Signed-off-by: Rik van Riel <riel@redhat.com>
Tested-by: Chegu Vinod <chegu_vinod@hp.com>
---
 kernel/sched/fair.c | 88 ++++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 87 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 6df460f..b973d77 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1648,6 +1648,92 @@ static u64 numa_get_avg_runtime(struct task_struct *p, u64 *period)
 	return delta;
 }
 
+/*
+ * Determine the preferred nid for a task in a numa_group. This needs to
+ * be done in a way that produces consistent results with group_weight,
+ * otherwise workloads might not converge.
+ */ 
+static int preferred_group_nid(struct task_struct *p, int nid)
+{
+	nodemask_t nodes;
+	int dist;
+
+	/* Direct connections between all NUMA nodes. */
+	if (sched_numa_topology_type == NUMA_DIRECT)
+		return nid;
+
+	/*
+	 * On a system with glueless mesh NUMA topology, group_weight
+	 * scores nodes according to the number of NUMA hinting faults on
+	 * both the node itself, and on nearby nodes.
+	 */
+	if (sched_numa_topology_type == NUMA_GLUELESS_MESH) {
+		unsigned long score, max_score = 0;
+		int node, max_node = nid;
+
+		dist = sched_max_numa_distance;
+
+		for_each_online_node(node) {
+			score = group_weight(p, node, dist);
+			if (score > max_score) {
+				max_score = score;
+				max_node = node;
+			}
+		}
+		return max_node;
+	}
+
+	/*
+	 * Finding the preferred nid in a system with NUMA backplane
+	 * interconnect topology is more involved. The goal is to locate
+	 * tasks from numa_groups near each other in the system, and
+	 * untangle workloads from different sides of the system. This requires
+	 * searching down the hierarchy of node groups, recursively searching
+	 * inside the highest scoring group of nodes. The nodemask tricks
+	 * keep the complexity of the search down.
+	 */
+	nodes = node_online_map;
+	for (dist = sched_max_numa_distance; dist > LOCAL_DISTANCE; dist--) {
+		unsigned long max_faults = 0;
+		nodemask_t max_group;
+		int a, b;
+
+		/* Are there nodes at this distance from each other? */
+		if (!find_numa_distance(dist))
+			continue;
+
+		for_each_node_mask(a, nodes) {
+			unsigned long faults = 0;
+			nodemask_t this_group;
+			nodes_clear(this_group);
+
+			/* Sum group's NUMA faults; includes a==b case. */
+			for_each_node_mask(b, nodes) {
+				if (node_distance(a, b) < dist) {
+					faults += group_faults(p, b);
+					node_set(b, this_group);
+					node_clear(b, nodes);
+				}
+			}
+
+			/* Remember the top group. */
+			if (faults > max_faults) {
+				max_faults = faults;
+				max_group = this_group;
+				/*
+				 * subtle: at the smallest distance there is
+				 * just one node left in each "group", the
+				 * winner is the preferred nid.
+				 */
+				nid = a;
+			}
+		}
+		/* Next round, evaluate the nodes within max_group. */
+		nodes = max_group;
+	}
+	return nid;
+}
+
 static void task_numa_placement(struct task_struct *p)
 {
 	int seq, nid, max_nid = -1, max_group_nid = -1;
@@ -1730,7 +1816,7 @@ static void task_numa_placement(struct task_struct *p)
 	if (p->numa_group) {
 		update_numa_active_node_mask(p->numa_group);
 		spin_unlock_irq(group_lock);
-		max_nid = max_group_nid;
+		max_nid = preferred_group_nid(p, max_group_nid);
 	}
 
 	if (max_faults) {
-- 
1.9.3

[PATCH 6/6] sched,numa: check all nodes when placing a pseudo-interleaved group

[riel]

From: Rik van Riel <riel@redhat.com>

In pseudo-interleaved numa_groups, all tasks try to relocate to
the group's preferred_nid.  When a group is spread across multiple
NUMA nodes, this can lead to tasks swapping their location with
other tasks inside the same group, instead of having the group
converge on a few nodes.

When placing a task in a pseudo-interleaved numa_group, it pays
to examine all nodes, to see if it isn't better to move to eg.
the #2 node, so the group locality can be improved.

Signed-off-by: Rik van Riel <riel@redhat.com>
Tested-by: Chegu Vinod <chegu_vinod@hp.com>
---
 kernel/sched/fair.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index b973d77..00c1137 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1425,8 +1425,15 @@ static int task_numa_migrate(struct task_struct *p)
 	/* Try to find a spot on the preferred nid. */
 	task_numa_find_cpu(&env, taskimp, groupimp);
 
-	/* No space available on the preferred nid. Look elsewhere. */
-	if (env.best_cpu == -1) {
+	/*
+	 * Look at other nodes in these cases:
+	 * - there is no space available on the preferred_nid
+	 * - the task is part of a numa_group that is interleaved across
+	 *   multiple NUMA nodes; in order to better consolidate the group,
+	 *   we need to check other locations.
+	 */
+	if (env.best_cpu == -1 || (p->numa_group &&
+			nodes_weight(p->numa_group->active_nodes) > 1)) {
 		for_each_online_node(nid) {
 			if (nid == env.src_nid || nid == p->numa_preferred_nid)
 				continue;
-- 
1.9.3

[tip:sched/core] sched/numa: Export info needed for NUMA balancing on complex topologies

[tip-bot for Rik van Riel]

Commit-ID:  9942f79baaaf111d63ebf0862a819278d84fccc4
Gitweb:     http://git.kernel.org/tip/9942f79baaaf111d63ebf0862a819278d84fccc4
Author:     Rik van Riel <riel@redhat.com>
AuthorDate: Fri, 17 Oct 2014 03:29:49 -0400
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Tue, 28 Oct 2014 10:47:47 +0100

sched/numa: Export info needed for NUMA balancing on complex topologies

Export some information that is necessary to do placement of
tasks on systems with multi-level NUMA topologies.

Signed-off-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: mgorman@suse.de
Cc: chegu_vinod@hp.com
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/1413530994-9732-2-git-send-email-riel@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 kernel/sched/core.c  | 4 +++-
 kernel/sched/sched.h | 5 +++++
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 240157c..4007595 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6129,6 +6129,7 @@ static void claim_allocations(int cpu, struct sched_domain *sd)
 #ifdef CONFIG_NUMA
 static int sched_domains_numa_levels;
 static int *sched_domains_numa_distance;
+int sched_max_numa_distance;
 static struct cpumask ***sched_domains_numa_masks;
 static int sched_domains_curr_level;
 #endif
@@ -6300,7 +6301,7 @@ static void sched_numa_warn(const char *str)
 	printk(KERN_WARNING "\n");
 }
 
-static bool find_numa_distance(int distance)
+bool find_numa_distance(int distance)
 {
 	int i;
 
@@ -6447,6 +6448,7 @@ static void sched_init_numa(void)
 	sched_domain_topology = tl;
 
 	sched_domains_numa_levels = level;
+	sched_max_numa_distance = sched_domains_numa_distance[level - 1];
 }
 
 static void sched_domains_numa_masks_set(int cpu)
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 24156c84..443d6e1 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -678,6 +678,11 @@ static inline u64 rq_clock_task(struct rq *rq)
 	return rq->clock_task;
 }
 
+#ifdef CONFIG_NUMA
+extern int sched_max_numa_distance;
+extern bool find_numa_distance(int distance);
+#endif
+
 #ifdef CONFIG_NUMA_BALANCING
 extern void sched_setnuma(struct task_struct *p, int node);
 extern int migrate_task_to(struct task_struct *p, int cpu);

[tip:sched/core] sched/numa: Classify the NUMA topology of a system

[tip-bot for Rik van Riel]

Commit-ID:  e3fe70b1f72e3f83a00d9c332ec09ab347a981e2
Gitweb:     http://git.kernel.org/tip/e3fe70b1f72e3f83a00d9c332ec09ab347a981e2
Author:     Rik van Riel <riel@redhat.com>
AuthorDate: Fri, 17 Oct 2014 03:29:50 -0400
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Tue, 28 Oct 2014 10:47:48 +0100

sched/numa: Classify the NUMA topology of a system

Smaller NUMA systems tend to have all NUMA nodes directly connected
to each other. This includes the degenerate case of a system with just
one node, ie. a non-NUMA system.

Larger systems can have two kinds of NUMA topology, which affects how
tasks and memory should be placed on the system.

On glueless mesh systems, nodes that are not directly connected to
each other will bounce traffic through intermediary nodes. Task groups
can be run closer to each other by moving tasks from a node to an
intermediary node between it and the task's preferred node.

On NUMA systems with backplane controllers, the intermediary hops
are incapable of running programs. This creates "islands" of nodes
that are at an equal distance to anywhere else in the system.

Each kind of topology requires a slightly different placement
algorithm; this patch provides the mechanism to detect the kind
of NUMA topology of a system.

Signed-off-by: Rik van Riel <riel@redhat.com>
Tested-by: Chegu Vinod <chegu_vinod@hp.com>
[ Changed to use kernel/sched/sched.h ]
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: mgorman@suse.de
Cc: chegu_vinod@hp.com
Link: http://lkml.kernel.org/r/1413530994-9732-3-git-send-email-riel@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 kernel/sched/core.c  | 53 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 kernel/sched/sched.h |  6 ++++++
 2 files changed, 59 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 4007595..cde8481 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6128,6 +6128,7 @@ static void claim_allocations(int cpu, struct sched_domain *sd)
 
 #ifdef CONFIG_NUMA
 static int sched_domains_numa_levels;
+enum numa_topology_type sched_numa_topology_type;
 static int *sched_domains_numa_distance;
 int sched_max_numa_distance;
 static struct cpumask ***sched_domains_numa_masks;
@@ -6316,6 +6317,56 @@ bool find_numa_distance(int distance)
 	return false;
 }
 
+/*
+ * A system can have three types of NUMA topology:
+ * NUMA_DIRECT: all nodes are directly connected, or not a NUMA system
+ * NUMA_GLUELESS_MESH: some nodes reachable through intermediary nodes
+ * NUMA_BACKPLANE: nodes can reach other nodes through a backplane
+ *
+ * The difference between a glueless mesh topology and a backplane
+ * topology lies in whether communication between not directly
+ * connected nodes goes through intermediary nodes (where programs
+ * could run), or through backplane controllers. This affects
+ * placement of programs.
+ *
+ * The type of topology can be discerned with the following tests:
+ * - If the maximum distance between any nodes is 1 hop, the system
+ *   is directly connected.
+ * - If for two nodes A and B, located N > 1 hops away from each other,
+ *   there is an intermediary node C, which is < N hops away from both
+ *   nodes A and B, the system is a glueless mesh.
+ */
+static void init_numa_topology_type(void)
+{
+	int a, b, c, n;
+
+	n = sched_max_numa_distance;
+
+	if (n <= 1)
+		sched_numa_topology_type = NUMA_DIRECT;
+
+	for_each_online_node(a) {
+		for_each_online_node(b) {
+			/* Find two nodes furthest removed from each other. */
+			if (node_distance(a, b) < n)
+				continue;
+
+			/* Is there an intermediary node between a and b? */
+			for_each_online_node(c) {
+				if (node_distance(a, c) < n &&
+				    node_distance(b, c) < n) {
+					sched_numa_topology_type =
+							NUMA_GLUELESS_MESH;
+					return;
+				}
+			}
+
+			sched_numa_topology_type = NUMA_BACKPLANE;
+			return;
+		}
+	}
+}
+
 static void sched_init_numa(void)
 {
 	int next_distance, curr_distance = node_distance(0, 0);
@@ -6449,6 +6500,8 @@ static void sched_init_numa(void)
 
 	sched_domains_numa_levels = level;
 	sched_max_numa_distance = sched_domains_numa_distance[level - 1];
+
+	init_numa_topology_type();
 }
 
 static void sched_domains_numa_masks_set(int cpu)
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 443d6e1..57aacea 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -679,6 +679,12 @@ static inline u64 rq_clock_task(struct rq *rq)
 }
 
 #ifdef CONFIG_NUMA
+enum numa_topology_type {
+	NUMA_DIRECT,
+	NUMA_GLUELESS_MESH,
+	NUMA_BACKPLANE,
+};
+extern enum numa_topology_type sched_numa_topology_type;
 extern int sched_max_numa_distance;
 extern bool find_numa_distance(int distance);
 #endif

[tip:sched/core] sched/numa: Prepare for complex topology placement

[tip-bot for Rik van Riel]

Commit-ID:  7bd953206b0b5e0a3aded871982367410b42e1b1
Gitweb:     http://git.kernel.org/tip/7bd953206b0b5e0a3aded871982367410b42e1b1
Author:     Rik van Riel <riel@redhat.com>
AuthorDate: Fri, 17 Oct 2014 03:29:51 -0400
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Tue, 28 Oct 2014 10:47:49 +0100

sched/numa: Prepare for complex topology placement

Preparatory patch for adding NUMA placement on systems with
complex NUMA topology. Also fix a potential divide by zero
in group_weight()

Signed-off-by: Rik van Riel <riel@redhat.com>
Tested-by: Chegu Vinod <chegu_vinod@hp.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: mgorman@suse.de
Cc: chegu_vinod@hp.com
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/1413530994-9732-4-git-send-email-riel@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 kernel/sched/fair.c | 57 ++++++++++++++++++++++++++++++++++-------------------
 1 file changed, 37 insertions(+), 20 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 34baa60..0af3bed 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -931,9 +931,10 @@ static inline unsigned long group_faults_cpu(struct numa_group *group, int nid)
  * larger multiplier, in order to group tasks together that are almost
  * evenly spread out between numa nodes.
  */
-static inline unsigned long task_weight(struct task_struct *p, int nid)
+static inline unsigned long task_weight(struct task_struct *p, int nid,
+					int dist)
 {
-	unsigned long total_faults;
+	unsigned long faults, total_faults;
 
 	if (!p->numa_faults_memory)
 		return 0;
@@ -943,15 +944,25 @@ static inline unsigned long task_weight(struct task_struct *p, int nid)
 	if (!total_faults)
 		return 0;
 
-	return 1000 * task_faults(p, nid) / total_faults;
+	faults = task_faults(p, nid);
+	return 1000 * faults / total_faults;
 }
 
-static inline unsigned long group_weight(struct task_struct *p, int nid)
+static inline unsigned long group_weight(struct task_struct *p, int nid,
+					 int dist)
 {
-	if (!p->numa_group || !p->numa_group->total_faults)
+	unsigned long faults, total_faults;
+
+	if (!p->numa_group)
+		return 0;
+
+	total_faults = p->numa_group->total_faults;
+
+	if (!total_faults)
 		return 0;
 
-	return 1000 * group_faults(p, nid) / p->numa_group->total_faults;
+	faults = group_faults(p, nid);
+	return 1000 * faults / total_faults;
 }
 
 bool should_numa_migrate_memory(struct task_struct *p, struct page * page,
@@ -1084,6 +1095,7 @@ struct task_numa_env {
 	struct numa_stats src_stats, dst_stats;
 
 	int imbalance_pct;
+	int dist;
 
 	struct task_struct *best_task;
 	long best_imp;
@@ -1163,6 +1175,7 @@ static void task_numa_compare(struct task_numa_env *env,
 	long load;
 	long imp = env->p->numa_group ? groupimp : taskimp;
 	long moveimp = imp;
+	int dist = env->dist;
 
 	rcu_read_lock();
 
@@ -1196,8 +1209,8 @@ static void task_numa_compare(struct task_numa_env *env,
 		 * in any group then look only at task weights.
 		 */
 		if (cur->numa_group == env->p->numa_group) {
-			imp = taskimp + task_weight(cur, env->src_nid) -
-			      task_weight(cur, env->dst_nid);
+			imp = taskimp + task_weight(cur, env->src_nid, dist) -
+			      task_weight(cur, env->dst_nid, dist);
 			/*
 			 * Add some hysteresis to prevent swapping the
 			 * tasks within a group over tiny differences.
@@ -1211,11 +1224,11 @@ static void task_numa_compare(struct task_numa_env *env,
 			 * instead.
 			 */
 			if (cur->numa_group)
-				imp += group_weight(cur, env->src_nid) -
-				       group_weight(cur, env->dst_nid);
+				imp += group_weight(cur, env->src_nid, dist) -
+				       group_weight(cur, env->dst_nid, dist);
 			else
-				imp += task_weight(cur, env->src_nid) -
-				       task_weight(cur, env->dst_nid);
+				imp += task_weight(cur, env->src_nid, dist) -
+				       task_weight(cur, env->dst_nid, dist);
 		}
 	}
 
@@ -1314,7 +1327,7 @@ static int task_numa_migrate(struct task_struct *p)
 	};
 	struct sched_domain *sd;
 	unsigned long taskweight, groupweight;
-	int nid, ret;
+	int nid, ret, dist;
 	long taskimp, groupimp;
 
 	/*
@@ -1342,12 +1355,13 @@ static int task_numa_migrate(struct task_struct *p)
 		return -EINVAL;
 	}
 
-	taskweight = task_weight(p, env.src_nid);
-	groupweight = group_weight(p, env.src_nid);
-	update_numa_stats(&env.src_stats, env.src_nid);
 	env.dst_nid = p->numa_preferred_nid;
-	taskimp = task_weight(p, env.dst_nid) - taskweight;
-	groupimp = group_weight(p, env.dst_nid) - groupweight;
+	dist = env.dist = node_distance(env.src_nid, env.dst_nid);
+	taskweight = task_weight(p, env.src_nid, dist);
+	groupweight = group_weight(p, env.src_nid, dist);
+	update_numa_stats(&env.src_stats, env.src_nid);
+	taskimp = task_weight(p, env.dst_nid, dist) - taskweight;
+	groupimp = group_weight(p, env.dst_nid, dist) - groupweight;
 	update_numa_stats(&env.dst_stats, env.dst_nid);
 
 	/* Try to find a spot on the preferred nid. */
@@ -1359,12 +1373,15 @@ static int task_numa_migrate(struct task_struct *p)
 			if (nid == env.src_nid || nid == p->numa_preferred_nid)
 				continue;
 
+			dist = node_distance(env.src_nid, env.dst_nid);
+
 			/* Only consider nodes where both task and groups benefit */
-			taskimp = task_weight(p, nid) - taskweight;
-			groupimp = group_weight(p, nid) - groupweight;
+			taskimp = task_weight(p, nid, dist) - taskweight;
+			groupimp = group_weight(p, nid, dist) - groupweight;
 			if (taskimp < 0 && groupimp < 0)
 				continue;
 
+			env.dist = dist;
 			env.dst_nid = nid;
 			update_numa_stats(&env.dst_stats, env.dst_nid);
 			task_numa_find_cpu(&env, taskimp, groupimp);

[tip:sched/core] sched/numa: Calculate node scores in complex NUMA topologies

[tip-bot for Rik van Riel]

Commit-ID:  6c6b1193e71fed1a58dc3fab9d967d245177f87b
Gitweb:     http://git.kernel.org/tip/6c6b1193e71fed1a58dc3fab9d967d245177f87b
Author:     Rik van Riel <riel@redhat.com>
AuthorDate: Fri, 17 Oct 2014 03:29:52 -0400
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Tue, 28 Oct 2014 10:47:50 +0100

sched/numa: Calculate node scores in complex NUMA topologies

In order to do task placement on systems with complex NUMA topologies,
it is necessary to count the faults on nodes nearby the node that is
being examined for a potential move.

In case of a system with a backplane interconnect, we are dealing with
groups of NUMA nodes; each of the nodes within a group is the same number
of hops away from nodes in other groups in the system. Optimal placement
on this topology is achieved by counting all nearby nodes equally. When
comparing nodes A and B at distance N, nearby nodes are those at distances
smaller than N from nodes A or B.

Placement strategy on a system with a glueless mesh NUMA topology needs
to be different, because there are no natural groups of nodes determined
by the hardware. Instead, when dealing with two nodes A and B at distance
N, N >= 2, there will be intermediate nodes at distance < N from both nodes
A and B. Good placement can be achieved by right shifting the faults on
nearby nodes by the number of hops from the node being scored. In this
context, a nearby node is any node less than the maximum distance in the
system away from the node. Those nodes are skipped for efficiency reasons,
there is no real policy reason to do so.

Placement policy on directly connected NUMA systems is not affected.

Signed-off-by: Rik van Riel <riel@redhat.com>
Tested-by: Chegu Vinod <chegu_vinod@hp.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: mgorman@suse.de
Cc: chegu_vinod@hp.com
Link: http://lkml.kernel.org/r/1413530994-9732-5-git-send-email-riel@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 kernel/sched/fair.c | 74 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 74 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 0af3bed..7e5712a 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -925,6 +925,71 @@ static inline unsigned long group_faults_cpu(struct numa_group *group, int nid)
 		group->faults_cpu[task_faults_idx(nid, 1)];
 }
 
+/* Handle placement on systems where not all nodes are directly connected. */
+static unsigned long score_nearby_nodes(struct task_struct *p, int nid,
+					int maxdist, bool task)
+{
+	unsigned long score = 0;
+	int node;
+
+	/*
+	 * All nodes are directly connected, and the same distance
+	 * from each other. No need for fancy placement algorithms.
+	 */
+	if (sched_numa_topology_type == NUMA_DIRECT)
+		return 0;
+
+	/*
+	 * This code is called for each node, introducing N^2 complexity,
+	 * which should be ok given the number of nodes rarely exceeds 8.
+	 */
+	for_each_online_node(node) {
+		unsigned long faults;
+		int dist = node_distance(nid, node);
+
+		/*
+		 * The furthest away nodes in the system are not interesting
+		 * for placement; nid was already counted.
+		 */
+		if (dist == sched_max_numa_distance || node == nid)
+			continue;
+
+		/*
+		 * On systems with a backplane NUMA topology, compare groups
+		 * of nodes, and move tasks towards the group with the most
+		 * memory accesses. When comparing two nodes at distance
+		 * "hoplimit", only nodes closer by than "hoplimit" are part
+		 * of each group. Skip other nodes.
+		 */
+		if (sched_numa_topology_type == NUMA_BACKPLANE &&
+					dist > maxdist)
+			continue;
+
+		/* Add up the faults from nearby nodes. */
+		if (task)
+			faults = task_faults(p, node);
+		else
+			faults = group_faults(p, node);
+
+		/*
+		 * On systems with a glueless mesh NUMA topology, there are
+		 * no fixed "groups of nodes". Instead, nodes that are not
+		 * directly connected bounce traffic through intermediate
+		 * nodes; a numa_group can occupy any set of nodes.
+		 * The further away a node is, the less the faults count.
+		 * This seems to result in good task placement.
+		 */
+		if (sched_numa_topology_type == NUMA_GLUELESS_MESH) {
+			faults *= (sched_max_numa_distance - dist);
+			faults /= (sched_max_numa_distance - LOCAL_DISTANCE);
+		}
+
+		score += faults;
+	}
+
+	return score;
+}
+
 /*
  * These return the fraction of accesses done by a particular task, or
  * task group, on a particular numa node.  The group weight is given a
@@ -945,6 +1010,8 @@ static inline unsigned long task_weight(struct task_struct *p, int nid,
 		return 0;
 
 	faults = task_faults(p, nid);
+	faults += score_nearby_nodes(p, nid, dist, true);
+
 	return 1000 * faults / total_faults;
 }
 
@@ -962,6 +1029,8 @@ static inline unsigned long group_weight(struct task_struct *p, int nid,
 		return 0;
 
 	faults = group_faults(p, nid);
+	faults += score_nearby_nodes(p, nid, dist, false);
+
 	return 1000 * faults / total_faults;
 }
 
@@ -1374,6 +1443,11 @@ static int task_numa_migrate(struct task_struct *p)
 				continue;
 
 			dist = node_distance(env.src_nid, env.dst_nid);
+			if (sched_numa_topology_type == NUMA_BACKPLANE &&
+						dist != env.dist) {
+				taskweight = task_weight(p, env.src_nid, dist);
+				groupweight = group_weight(p, env.src_nid, dist);
+			}
 
 			/* Only consider nodes where both task and groups benefit */
 			taskimp = task_weight(p, nid, dist) - taskweight;

[tip:sched/core] sched/numa: Find the preferred nid with complex NUMA topology

[tip-bot for Rik van Riel]

Commit-ID:  54009416ac3b5f219c0df68559ce534287ae97b1
Gitweb:     http://git.kernel.org/tip/54009416ac3b5f219c0df68559ce534287ae97b1
Author:     Rik van Riel <riel@redhat.com>
AuthorDate: Fri, 17 Oct 2014 03:29:53 -0400
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Tue, 28 Oct 2014 10:47:51 +0100

sched/numa: Find the preferred nid with complex NUMA topology

On systems with complex NUMA topologies, the node scoring is adjusted
to allow workloads to converge on nodes that are near each other.

The way a task group's preferred nid is determined needs to be adjusted,
in order for the preferred_nid to be consistent with group_weight scoring.
This ensures that we actually try to converge workloads on adjacent nodes.

Signed-off-by: Rik van Riel <riel@redhat.com>
Tested-by: Chegu Vinod <chegu_vinod@hp.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: mgorman@suse.de
Cc: chegu_vinod@hp.com
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/1413530994-9732-6-git-send-email-riel@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 kernel/sched/fair.c | 88 ++++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 87 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 7e5712a..7760c2a 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1659,6 +1659,92 @@ static u64 numa_get_avg_runtime(struct task_struct *p, u64 *period)
 	return delta;
 }
 
+/*
+ * Determine the preferred nid for a task in a numa_group. This needs to
+ * be done in a way that produces consistent results with group_weight,
+ * otherwise workloads might not converge.
+ */
+static int preferred_group_nid(struct task_struct *p, int nid)
+{
+	nodemask_t nodes;
+	int dist;
+
+	/* Direct connections between all NUMA nodes. */
+	if (sched_numa_topology_type == NUMA_DIRECT)
+		return nid;
+
+	/*
+	 * On a system with glueless mesh NUMA topology, group_weight
+	 * scores nodes according to the number of NUMA hinting faults on
+	 * both the node itself, and on nearby nodes.
+	 */
+	if (sched_numa_topology_type == NUMA_GLUELESS_MESH) {
+		unsigned long score, max_score = 0;
+		int node, max_node = nid;
+
+		dist = sched_max_numa_distance;
+
+		for_each_online_node(node) {
+			score = group_weight(p, node, dist);
+			if (score > max_score) {
+				max_score = score;
+				max_node = node;
+			}
+		}
+		return max_node;
+	}
+
+	/*
+	 * Finding the preferred nid in a system with NUMA backplane
+	 * interconnect topology is more involved. The goal is to locate
+	 * tasks from numa_groups near each other in the system, and
+	 * untangle workloads from different sides of the system. This requires
+	 * searching down the hierarchy of node groups, recursively searching
+	 * inside the highest scoring group of nodes. The nodemask tricks
+	 * keep the complexity of the search down.
+	 */
+	nodes = node_online_map;
+	for (dist = sched_max_numa_distance; dist > LOCAL_DISTANCE; dist--) {
+		unsigned long max_faults = 0;
+		nodemask_t max_group;
+		int a, b;
+
+		/* Are there nodes at this distance from each other? */
+		if (!find_numa_distance(dist))
+			continue;
+
+		for_each_node_mask(a, nodes) {
+			unsigned long faults = 0;
+			nodemask_t this_group;
+			nodes_clear(this_group);
+
+			/* Sum group's NUMA faults; includes a==b case. */
+			for_each_node_mask(b, nodes) {
+				if (node_distance(a, b) < dist) {
+					faults += group_faults(p, b);
+					node_set(b, this_group);
+					node_clear(b, nodes);
+				}
+			}
+
+			/* Remember the top group. */
+			if (faults > max_faults) {
+				max_faults = faults;
+				max_group = this_group;
+				/*
+				 * subtle: at the smallest distance there is
+				 * just one node left in each "group", the
+				 * winner is the preferred nid.
+				 */
+				nid = a;
+			}
+		}
+		/* Next round, evaluate the nodes within max_group. */
+		nodes = max_group;
+	}
+	return nid;
+}
+
 static void task_numa_placement(struct task_struct *p)
 {
 	int seq, nid, max_nid = -1, max_group_nid = -1;
@@ -1741,7 +1827,7 @@ static void task_numa_placement(struct task_struct *p)
 	if (p->numa_group) {
 		update_numa_active_node_mask(p->numa_group);
 		spin_unlock_irq(group_lock);
-		max_nid = max_group_nid;
+		max_nid = preferred_group_nid(p, max_group_nid);
 	}
 
 	if (max_faults) {