linux-kernel - Re: [PATCH v4 2/2] sched/topology: Improve load balancing on AMD EPYC

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Date:   Mon, 7 Oct 2019 08:28:16 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Matt Fleming <matt@...eblueprint.co.uk>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, Tony Luck <tony.luck@...el.com>,
        Rik van Riel <riel@...riel.com>,
        Suravee.Suthikulpanit@....com, Borislav Petkov <bp@...en8.de>,
        Thomas.Lendacky@....com, Mel Gorman <mgorman@...hsingularity.net>
Subject: Re: [PATCH v4 2/2] sched/topology: Improve load balancing on AMD EPYC

Hi,

On Thu, Aug 08, 2019 at 08:53:01PM +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 <matt@...eblueprint.co.uk>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Acked-by: Mel Gorman <mgorman@...hsingularity.net>
> Cc: Suravee.Suthikulpanit@....com
> Cc: Borislav Petkov <bp@...en8.de>
> Cc: Thomas.Lendacky@....com

This patch causes build errors on systems where NUMA does not depend on SMP,
for example MIPS and PPC. For example, building mips:ip27_defconfig with SMP
disabled results in

mips-linux-ld: mm/page_alloc.o: in function `get_page_from_freelist':
page_alloc.c:(.text+0x5018): undefined reference to `node_reclaim_distance'
mips-linux-ld: page_alloc.c:(.text+0x5020): undefined reference to `node_reclaim_distance'
mips-linux-ld: page_alloc.c:(.text+0x5028): undefined reference to `node_reclaim_distance'
mips-linux-ld: page_alloc.c:(.text+0x5040): undefined reference to `node_reclaim_distance'
Makefile:1074: recipe for target 'vmlinux' failed
make: *** [vmlinux] Error 1

I have seen a similar problem with one of my PPC test builds.

powerpc64-linux-ld: mm/page_alloc.o:(.toc+0x18): undefined reference to `node_reclaim_distance'

Guenter