[PATCH] sched,numa: ensure task_numa_migrate checks the preferred node

[Rik van Riel <riel@redhat.com>]

The first thing task_numa_migrate does is check to see if there is
CPU capacity available on the preferred node, in order to move the
task there.

However, if the preferred node is all busy, we would skip considering
that node for tasks swaps in the subsequent loop. This prevents NUMA
convergence of tasks on busy systems.

However, swapping locations with a task on our preferred nid, when
the preferred nid is busy, is perfectly fine.

The fix is to also look for a CPU on our preferred nid when it is
totally busy.

This changes "perf bench numa mem -p 4 -t 20 -m -0 -P 1000" from
not converging in 15 minutes on my 4 node system, to converging in
10-20 seconds.

Signed-off-by: Rik van Riel <riel@redhat.com>
Cc: stable@vger.kernel.org
---
This is a safe, simple patch to fix NUMA convergence, which is why
it should go to -stable, IMHO.

 kernel/sched/fair.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index e5884d8..824e241 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1425,9 +1425,8 @@ static int task_numa_migrate(struct task_struct *p)
 	groupimp = group_weight(p, env.dst_nid, false) - groupweight;
 	update_numa_stats(&env.dst_stats, env.dst_nid);
 
-	/* If the preferred nid has capacity, try to use it. */
-	if (env.dst_stats.has_capacity)
-		task_numa_find_cpu(&env, taskimp, groupimp);
+	/* 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) {