Re: [PATCH v4] sched/fair: Use sched_domain_span() for topology_span_sane()

[Leon Romanovsky <leon@kernel.org>]

On Wed, Jul 09, 2025 at 04:19:17PM +0000, K Prateek Nayak wrote:
> Leon noted a topology_span_sane() warning in their guest deployment
> starting from v6.16-rc1 [1]. Debug that followed pointed to the
> tl->mask() for the NODE domain being incorrectly resolved to that of the
> highest NUMA domain.
> 
> tl->mask() for NODE is set to the sd_numa_mask() which depends on the
> global "sched_domains_curr_level" hack. "sched_domains_curr_level" is
> set to the "tl->numa_level" during tl traversal in build_sched_domains()
> calling sd_init() but was not reset before topology_span_sane().
> 
> Since "tl->numa_level" still reflected the old value from
> build_sched_domains(), topology_span_sane() for the NODE domain trips
> when the span of the last NUMA domain overlaps.
> 
> Instead of replicating the "sched_domains_curr_level" hack, Valentin
> suggested using the spans from the sched_domain objects constructed
> during build_sched_domains() which can also catch overlaps when the
> domain spans are fixed up by build_sched_domain().
> 
> Since build_sched_domain() is skipped when tl->mask() of a child domain
> already covers the entire cpumap, skip the domains that have an empty
> span.
> 
> The original warning was reproducible on the following NUMA topology
> reported by Leon:
> 
>     $ sudo numactl -H
>     available: 5 nodes (0-4)
>     node 0 cpus: 0 1
>     node 0 size: 2927 MB
>     node 0 free: 1603 MB
>     node 1 cpus: 2 3
>     node 1 size: 3023 MB
>     node 1 free: 3008 MB
>     node 2 cpus: 4 5
>     node 2 size: 3023 MB
>     node 2 free: 3007 MB
>     node 3 cpus: 6 7
>     node 3 size: 3023 MB
>     node 3 free: 3002 MB
>     node 4 cpus: 8 9
>     node 4 size: 3022 MB
>     node 4 free: 2718 MB
>     node distances:
>     node   0   1   2   3   4
>       0:  10  39  38  37  36
>       1:  39  10  38  37  36
>       2:  38  38  10  37  36
>       3:  37  37  37  10  36
>       4:  36  36  36  36  10
> 
> The above topology can be mimicked using the following QEMU cmd that was
> used to reproduce the warning and test the fix:
> 
>      sudo qemu-system-x86_64 -enable-kvm -cpu host \
>      -m 20G -smp cpus=10,sockets=10 -machine q35 \
>      -object memory-backend-ram,size=4G,id=m0 \
>      -object memory-backend-ram,size=4G,id=m1 \
>      -object memory-backend-ram,size=4G,id=m2 \
>      -object memory-backend-ram,size=4G,id=m3 \
>      -object memory-backend-ram,size=4G,id=m4 \
>      -numa node,cpus=0-1,memdev=m0,nodeid=0 \
>      -numa node,cpus=2-3,memdev=m1,nodeid=1 \
>      -numa node,cpus=4-5,memdev=m2,nodeid=2 \
>      -numa node,cpus=6-7,memdev=m3,nodeid=3 \
>      -numa node,cpus=8-9,memdev=m4,nodeid=4 \
>      -numa dist,src=0,dst=1,val=39 \
>      -numa dist,src=0,dst=2,val=38 \
>      -numa dist,src=0,dst=3,val=37 \
>      -numa dist,src=0,dst=4,val=36 \
>      -numa dist,src=1,dst=0,val=39 \
>      -numa dist,src=1,dst=2,val=38 \
>      -numa dist,src=1,dst=3,val=37 \
>      -numa dist,src=1,dst=4,val=36 \
>      -numa dist,src=2,dst=0,val=38 \
>      -numa dist,src=2,dst=1,val=38 \
>      -numa dist,src=2,dst=3,val=37 \
>      -numa dist,src=2,dst=4,val=36 \
>      -numa dist,src=3,dst=0,val=37 \
>      -numa dist,src=3,dst=1,val=37 \
>      -numa dist,src=3,dst=2,val=37 \
>      -numa dist,src=3,dst=4,val=36 \
>      -numa dist,src=4,dst=0,val=36 \
>      -numa dist,src=4,dst=1,val=36 \
>      -numa dist,src=4,dst=2,val=36 \
>      -numa dist,src=4,dst=3,val=36 \
>      ...
> 
> Suggested-by: Valentin Schneider <vschneid@redhat.com>
> Reported-by: Leon Romanovsky <leon@kernel.org>
> Closes: https://lore.kernel.org/lkml/20250610110701.GA256154@unreal/ [1]
> Fixes: ccf74128d66c ("sched/topology: Assert non-NUMA topology masks don't (partially) overlap") # ce29a7da84cd, f55dac1dafb3
> Reviewed-by: Steve Wahl <steve.wahl@hpe.com>
> Tested-by: Valentin Schneider <vschneid@redhat.com>
> Reviewed-by: Valentin Schneider <vschneid@redhat.com>
> Signed-off-by: K Prateek Nayak <kprateek.nayak@amd.com>
> ---
> Changes are based on tip:sched/urgent at commit fc975cfb3639
> ("sched/deadline: Fix dl_server runtime calculation formula")

Was this patch picked?

Thanks,
Tested-by: Leon Romanovsky <leon@kernel.org>