On Tue, Jul 23, 2019 at 12:42:48PM +0100, Mel Gorman wrote:
> On Tue, Jul 23, 2019 at 11:48:30AM +0100, Matt Fleming wrote:
> > SD_BALANCE_{FORK,EXEC} and SD_WAKE_AFFINE are stripped in sd_init()
> > for any sched domains with a NUMA distance greater than 2 hops
> > (RECLAIM_DISTANCE). The idea being that it's expensive to balance
> > across domains that far apart.
> >
> > However, as is rather unfortunately explained in
> >
> > commit 32e45ff43eaf ("mm: increase RECLAIM_DISTANCE to 30")
> >
> > the value for RECLAIM_DISTANCE is based on node distance tables from
> > 2011-era hardware.
> >
> > Current AMD EPYC machines have the following NUMA node distances:
> >
> > node distances:
> > node 0 1 2 3 4 5 6 7
> > 0: 10 16 16 16 32 32 32 32
> > 1: 16 10 16 16 32 32 32 32
> > 2: 16 16 10 16 32 32 32 32
> > 3: 16 16 16 10 32 32 32 32
> > 4: 32 32 32 32 10 16 16 16
> > 5: 32 32 32 32 16 10 16 16
> > 6: 32 32 32 32 16 16 10 16
> > 7: 32 32 32 32 16 16 16 10
> >
> > where 2 hops is 32.
> >
> > The result is that the scheduler fails to load balance properly across
> > NUMA nodes on different sockets -- 2 hops apart.
> >
> > For example, pinning 16 busy threads to NUMA nodes 0 (CPUs 0-7) and 4
> > (CPUs 32-39) like so,
> >
> > $ numactl -C 0-7,32-39 ./spinner 16
> >
> > causes all threads to fork and remain on node 0 until the active
> > balancer kicks in after a few seconds and forcibly moves some threads
> > to node 4.
> >
> > Override node_reclaim_distance for AMD Zen.
> >
> > Signed-off-by: Matt Fleming <[email protected]>
> > Cc: "Suthikulpanit, Suravee" <[email protected]>
> > Cc: Mel Gorman <[email protected]>
> > Cc: "Lendacky, Thomas" <[email protected]>
> > Cc: Borislav Petkov <[email protected]>
>
> Acked-by: Mel Gorman <[email protected]>
>
> The only caveat I can think of is that a future generation of Zen might
> take a different magic number than 32 as their remote distance. If or
> when this happens, it'll need additional smarts but lacking a crystal
> ball, we can cross that bridge when we come to it.
I just suggested to Matt on IRC we could do something along these lines,
but we can do that later.
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -1611,6 +1611,12 @@ void sched_init_numa(void)
}
/*
+ * Set the reclaim distance at 2 hops instead of at a fixed distance
value.
+ */
+ if (level >= 2)
+ node_reclaim_distance = sched_domains_numa_distance[2];
+
+ /*
* 'level' contains the number of unique distances
*
* The sched_domains_numa_distance[] array includes the actual distance
