[PATCH] sched/topology: Warn when NUMA diameter > 2

* [PATCH] sched/topology: Warn when NUMA diameter > 2
@ 2020-11-10 18:43 Valentin Schneider
  2020-11-11  8:43 ` Mel Gorman
  0 siblings, 1 reply; 3+ messages in thread
From: Valentin Schneider @ 2020-11-10 18:43 UTC (permalink / raw)
  To: linux-kernel
  Cc: Peter Zijlstra, Ingo Molnar, Vincent Guittot, Morten Rasmussen,
	Dietmar Eggemann, Mel Gorman, Rik van Riel, Barry Song

NUMA topologies where the shortest path between some two nodes requires
three or more hops (i.e. diameter > 2) end up being misrepresented in the
scheduler topology structures.

This is currently detected when booting a kernel with CONFIG_SCHED_DEBUG=y
+ sched_debug on the cmdline, although this will only yield a warning about
sched_group spans not matching sched_domain spans:

  ERROR: groups don't span domain->span

Add an explicit warning for that case, triggered regardless of
CONFIG_SCHED_DEBUG, and decorate it with an appropriate comment.

The topology described in the comment can be booted up on QEMU by appending
the following to your usual QEMU incantation:

    -smp cores=4 \
    -numa node,cpus=0,nodeid=0 -numa node,cpus=1,nodeid=1, \
    -numa node,cpus=2,nodeid=2, -numa node,cpus=3,nodeid=3, \
    -numa dist,src=0,dst=1,val=20, -numa dist,src=0,dst=2,val=30, \
    -numa dist,src=0,dst=3,val=40, -numa dist,src=1,dst=2,val=20, \
    -numa dist,src=1,dst=3,val=30, -numa dist,src=2,dst=3,val=20

A somewhat more realistic topology (6-node mesh) with the same affliction
can be conjured with:

    -smp cores=6 \
    -numa node,cpus=0,nodeid=0 -numa node,cpus=1,nodeid=1, \
    -numa node,cpus=2,nodeid=2, -numa node,cpus=3,nodeid=3, \
    -numa node,cpus=4,nodeid=4, -numa node,cpus=5,nodeid=5, \
    -numa dist,src=0,dst=1,val=20, -numa dist,src=0,dst=2,val=30, \
    -numa dist,src=0,dst=3,val=40, -numa dist,src=0,dst=4,val=30, \
    -numa dist,src=0,dst=5,val=20, \
    -numa dist,src=1,dst=2,val=20, -numa dist,src=1,dst=3,val=30, \
    -numa dist,src=1,dst=4,val=20, -numa dist,src=1,dst=5,val=30, \
    -numa dist,src=2,dst=3,val=20, -numa dist,src=2,dst=4,val=30, \
    -numa dist,src=2,dst=5,val=40, \
    -numa dist,src=3,dst=4,val=20, -numa dist,src=3,dst=5,val=30, \
    -numa dist,src=4,dst=5,val=20

Link: https://lore.kernel.org/lkml/jhjtux5edo2.mognet@arm.com
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
---
 kernel/sched/topology.c | 33 +++++++++++++++++++++++++++++++++
 1 file changed, 33 insertions(+)

diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 90f3e5558fa2..b296c1c6b961 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -675,6 +675,7 @@ cpu_attach_domain(struct sched_domain *sd, struct root_domain *rd, int cpu)
 {
 	struct rq *rq = cpu_rq(cpu);
 	struct sched_domain *tmp;
+	int numa_distance = 0;
 
 	/* Remove the sched domains which do not contribute to scheduling. */
 	for (tmp = sd; tmp; ) {
@@ -706,6 +707,38 @@ cpu_attach_domain(struct sched_domain *sd, struct root_domain *rd, int cpu)
 			sd->child = NULL;
 	}
 
+	for (tmp = sd; tmp; tmp = tmp->parent)
+		numa_distance += !!(tmp->flags & SD_NUMA);
+
+	/*
+	 * FIXME: Diameter >=3 is misrepresented.
+	 *
+	 * Smallest diameter=3 topology is:
+	 *
+	 *   node   0   1   2   3
+	 *     0:  10  20  30  40
+	 *     1:  20  10  20  30
+	 *     2:  30  20  10  20
+	 *     3:  40  30  20  10
+	 *
+	 *   0 --- 1 --- 2 --- 3
+	 *
+	 * NUMA-3	0-3		N/A		N/A		0-3
+	 *  groups:	{0-2},{1-3}					{1-3},{0-2}
+	 *
+	 * NUMA-2	0-2		0-3		0-3		1-3
+	 *  groups:	{0-1},{1-3}	{0-2},{2-3}	{1-3},{0-1}	{2-3},{0-2}
+	 *
+	 * NUMA-1	0-1		0-2		1-3		2-3
+	 *  groups:	{0},{1}		{1},{2},{0}	{2},{3},{1}	{3},{2}
+	 *
+	 * NUMA-0	0		1		2		3
+	 *
+	 * The NUMA-2 groups for nodes 0 and 3 are obviously buggered, as the
+	 * group span isn't a subset of the domain span.
+	 */
+	WARN_ONCE(numa_distance > 2, "Shortest NUMA path spans too many nodes\n");
+
 	sched_domain_debug(sd, cpu);
 
 	rq_attach_root(rq, rd);
-- 
2.27.0


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