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.
--
Mel Gorman
SUSE Labs
