[PATCH 06/15] sched: Reschedule task on preferred NUMA node once selected

[Mel Gorman]

A preferred node is selected based on the node the most NUMA hinting
faults was incurred on. There is no guarantee that the task is running
on that node at the time so this patch rescheules the task to run on
the most idle CPU of the selected node when selected. This avoids
waiting for the balancer to make a decision.

Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
---
 kernel/sched/core.c  | 17 ++++++++++++++++
 kernel/sched/fair.c  | 55 +++++++++++++++++++++++++++++++++++++++++++++++++++-
 kernel/sched/sched.h |  1 +
 3 files changed, 72 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 5e02507..e4c1832 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -992,6 +992,23 @@ struct migration_arg {
 
 static int migration_cpu_stop(void *data);
 
+#ifdef CONFIG_NUMA_BALANCING
+/* Migrate current task p to target_cpu */
+int migrate_task_to(struct task_struct *p, int target_cpu)
+{
+	struct migration_arg arg = { p, target_cpu };
+	int curr_cpu = task_cpu(p);
+
+	if (curr_cpu == target_cpu)
+		return 0;
+
+	if (!cpumask_test_cpu(target_cpu, tsk_cpus_allowed(p)))
+		return -EINVAL;
+
+	return stop_one_cpu(curr_cpu, migration_cpu_stop, &arg);
+}
+#endif
+
 /*
  * wait_task_inactive - wait for a thread to unschedule.
  *
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 5055bf9..5a01dcb 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -800,6 +800,40 @@ unsigned int sysctl_numa_balancing_scan_delay = 1000;
  */
 unsigned int sysctl_numa_balancing_settle_count __read_mostly = 3;
 
+static unsigned long weighted_cpuload(const int cpu);
+
+
+static int
+find_idlest_cpu_node(int this_cpu, int nid)
+{
+	unsigned long load, min_load = ULONG_MAX;
+	int i, idlest_cpu = this_cpu;
+
+	BUG_ON(cpu_to_node(this_cpu) == nid);
+
+	rcu_read_lock();
+	for_each_cpu(i, cpumask_of_node(nid)) {
+		load = weighted_cpuload(i);
+
+		if (load < min_load) {
+			/*
+			 * Kernel threads can be preempted. For others, do
+			 * not preempt if running on their preferred node
+			 * or pinned.
+			 */
+			struct task_struct *p = cpu_rq(i)->curr;
+			if ((p->flags & PF_KTHREAD) ||
+			    (p->numa_preferred_nid != nid && p->nr_cpus_allowed > 1)) {
+				min_load = load;
+				idlest_cpu = i;
+			}
+		}
+	}
+	rcu_read_unlock();
+
+	return idlest_cpu;
+}
+
 static void task_numa_placement(struct task_struct *p)
 {
 	int seq, nid, max_nid = 0;
@@ -829,10 +863,29 @@ static void task_numa_placement(struct task_struct *p)
 		}
 	}
 
-	/* Update the tasks preferred node if necessary */
+	/*
+	 * Record the preferred node as the node with the most faults,
+	 * requeue the task to be running on the idlest CPU on the
+	 * preferred node and reset the scanning rate to recheck
+	 * the working set placement.
+	 */
 	if (max_faults && max_nid != p->numa_preferred_nid) {
+		int preferred_cpu;
+
+		/*
+		 * If the task is not on the preferred node then find the most
+		 * idle CPU to migrate to.
+		 */
+		preferred_cpu = task_cpu(p);
+		if (cpu_to_node(preferred_cpu) != max_nid) {
+			preferred_cpu = find_idlest_cpu_node(preferred_cpu,
+							     max_nid);
+		}
+
+		/* Update the preferred nid and migrate task if possible */
 		p->numa_preferred_nid = max_nid;
 		p->numa_migrate_seq = 0;
+		migrate_task_to(p, preferred_cpu);
 	}
 }
 
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index c5f773d..795346d 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -504,6 +504,7 @@ DECLARE_PER_CPU(struct rq, runqueues);
 #define raw_rq()		(&__raw_get_cpu_var(runqueues))
 
 #ifdef CONFIG_NUMA_BALANCING
+extern int migrate_task_to(struct task_struct *p, int cpu);
 static inline void task_numa_free(struct task_struct *p)
 {
 	kfree(p->numa_faults);
-- 
1.8.1.4

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