Re: [PATCH v3] sched/topology: Improve load balancing on AMD EPYC

["Suthikulpanit, Suravee" <Suravee.Suthikulpanit@amd.com>]

Matt,

On 7/23/2019 5:48 AM, 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.

I am testing this patch on the Linux-5.2, and I actually do not
notice difference pre vs post patch.

Besides the case above, I have also run an experiment with
a different number of threads across two sockets:

(Note: I only focus on thread0 of each core.)

sXnY = Socket X Node Y

     * s0n0 + s0n1 + s1n0 + s1n1
     numactl -C 0-15,32-47 ./spinner 32

     * s0n2 + s0n3 + s1n2 + s1n3
     numactl -C 16-31,48-63 ./spinner 32

     * s0 + s1
     numactl -C 0-63 ./spinner 64

My observations are:

     * I still notice improper load-balance on one of the task initially
       for a few seconds before they are load-balanced correctly.

     * It is taking longer to load balance w/ more number of tasks.

I wonder if you have tried with a different kernel base?

Regards,
Suravee