[PATCH] numa,sched: only consider less busy nodes as numa balancing destination

[Rik van Riel]

On Wed, 06 May 2015 13:35:30 +0300
Artem Bityutskiy <dedekind1@xxxxxxxxx> wrote:

> we observe a tremendous regression between kernel version 3.16 and 3.17
> (and up), and I've bisected it to this commit:
> 
> a43455a sched/numa: Ensure task_numa_migrate() checks the preferred node

Artem, Jirka, does this patch fix (or at least improve) the issues you
have been seeing?  Does it introduce any new regressions?

Peter, Mel, I think it may be time to stop waiting for the impedance
mismatch between the load balancer and NUMA balancing to be resolved,
and try to just avoid the issue in the NUMA balancing code...

----8<----

Subject: numa,sched: only consider less busy nodes as numa balancing destination

Changeset a43455a1 ("sched/numa: Ensure task_numa_migrate() checks the
preferred node") fixes an issue where workloads would never converge
on a fully loaded (or overloaded) system.

However, it introduces a regression on less than fully loaded systems,
where workloads converge on a few NUMA nodes, instead of properly staying
spread out across the whole system. This leads to a reduction in available
memory bandwidth, and usable CPU cache, with predictable performance problems.

The root cause appears to be an interaction between the load balancer and
NUMA balancing, where the short term load represented by the load balancer
differs from the long term load the NUMA balancing code would like to base
its decisions on.

Simply reverting a43455a1 would re-introduce the non-convergence of
workloads on fully loaded systems, so that is not a good option. As
an aside, the check done before a43455a1 only applied to a task's
preferred node, not to other candidate nodes in the system, so the
converge-on-too-few-nodes problem still happens, just to a lesser
degree.

Instead, try to compensate for the impedance mismatch between the
load balancer and NUMA balancing by only ever considering a lesser
loaded node as a destination for NUMA balancing, regardless of
whether the task is trying to move to the preferred node, or to another
node.

Signed-off-by: Rik van Riel <riel@xxxxxxxxxx>
Reported-by: Artem Bityutski <dedekind1@xxxxxxxxx>
Reported-by: Jirka Hladky <jhladky@xxxxxxxxxx>
---
 kernel/sched/fair.c | 30 ++++++++++++++++++++++++++++--
 1 file changed, 28 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index ffeaa4105e48..480e6a35ab35 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1409,6 +1409,30 @@ static void task_numa_find_cpu(struct task_numa_env *env,
 	}
 }
 
+/* Only move tasks to a NUMA node less busy than the current node. */
+static bool numa_has_capacity(struct task_numa_env *env)
+{
+	struct numa_stats *src = &env->src_stats;
+	struct numa_stats *dst = &env->dst_stats;
+
+	if (src->has_free_capacity && !dst->has_free_capacity)
+		return false;
+
+	/*
+	 * Only consider a task move if the source has a higher destination
+	 * than the destination, corrected for CPU capacity on each node.
+	 *
+	 *      src->load                dst->load
+	 * --------------------- vs ---------------------
+	 * src->compute_capacity    dst->compute_capacity
+	 */
+	if (src->load * dst->compute_capacity >
+	    dst->load * src->compute_capacity)
+		return true;
+
+	return false;
+}
+
 static int task_numa_migrate(struct task_struct *p)
 {
 	struct task_numa_env env = {
@@ -1463,7 +1487,8 @@ static int task_numa_migrate(struct task_struct *p)
 	update_numa_stats(&env.dst_stats, env.dst_nid);
 
 	/* Try to find a spot on the preferred nid. */
-	task_numa_find_cpu(&env, taskimp, groupimp);
+	if (numa_has_capacity(&env))
+		task_numa_find_cpu(&env, taskimp, groupimp);
 
 	/*
 	 * Look at other nodes in these cases:
@@ -1494,7 +1519,8 @@ static int task_numa_migrate(struct task_struct *p)
 			env.dist = dist;
 			env.dst_nid = nid;
 			update_numa_stats(&env.dst_stats, env.dst_nid);
-			task_numa_find_cpu(&env, taskimp, groupimp);
+			if (numa_has_capacity(&env))
+				task_numa_find_cpu(&env, taskimp, groupimp);
 		}
 	}
 
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