Commit b5b21734 authored Nov 10, 2020 by Valentin Schneider Committed by Peter Zijlstra Nov 19, 2020

sched/topology: Warn when NUMA diameter > 2



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

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Mel Gorman <mgorman@techsingularity.net>
Link: https://lore.kernel.org/lkml/jhjtux5edo2.mognet@arm.com

parent 406100f3

kernel/sched/topology.c

+33 −0

Original line number	Diff line number	Diff line
		@@ -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);