Re: [tip: sched/core] sched/topology: Compute sd_weight considering cpuset partitions

[K Prateek Nayak]

Hello Nathan,

Thank you for the report.

On 3/21/2026 5:28 AM, Nathan Chancellor wrote:
>   $ cat kernel/configs/schedstats.config
>   CONFIG_SCHEDSTATS=y

Is the "schedstats.config" available somewhere? I tried these
steps on my end but couldn't reproduce the crash with my config.

Also, are you saying it is necessary to enable CONFIG_SCHEDSTATS
to observe the crash?

> 
>   $ make -skj"$(nproc)" ARCH=arm CROSS_COMPILE=arm-linux-gnueabi- mrproper defconfig schedstats.config zImage
> 
>   $ curl -LSs https://github.com/ClangBuiltLinux/boot-utils/releases/download/20241120-044434/arm-rootfs.cpio.zst | zstd -d >rootfs.cpio
> 
>   $ qemu-system-arm \
>       -display none \
>       -nodefaults \
>       -no-reboot \
>       -machine virt \
>       -append 'console=ttyAMA0 earlycon' \
>       -kernel arch/arm/boot/zImage \
>       -initrd rootfs.cpio \
>       -m 1G \
>       -serial mon:stdio
>   [    0.000000] Booting Linux on physical CPU 0x0
>   [    0.000000] Linux version 7.0.0-rc4-00017-g8e8e23dea43e (nathan@framework-amd-ryzen-maxplus-395) (arm-linux-gnueabi-gcc (GCC) 15.2.0, GNU ld (GNU Binutils) 2.45) #1 SMP Fri Mar 20 16:12:05 MST 2026
>   ...
>   [    0.031929] 8<--- cut here ---
>   [    0.031999] Unable to handle kernel NULL pointer dereference at virtual address 00000000 when write
>   [    0.032172] [00000000] *pgd=00000000
>   [    0.032459] Internal error: Oops: 805 [#1] SMP ARM
>   [    0.032902] Modules linked in:
>   [    0.033466] CPU: 0 UID: 0 PID: 1 Comm: swapper/0 Not tainted 7.0.0-rc4-00017-g8e8e23dea43e #1 VOLUNTARY
>   [    0.033658] Hardware name: Generic DT based system
>   [    0.033770] PC is at build_sched_domains+0x7d0/0x1628

For me, this points to:

  $ scripts/faddr2line vmlinux build_sched_domains+0x7d0/0x1628
  build_sched_domains+0x7d0/0x1628:
  find_next_bit_wrap at include/linux/find.h:455
  (inlined by) build_sched_groups at kernel/sched/topology.c:1255
  (inlined by) build_sched_domains at kernel/sched/topology.c:2603

which is the:

  span = sched_domain_span(sd);

  for_each_cpu_wrap(i, span, cpu) /* Here */ {
    ...
  }

in build_sched_groups() so we are likely going off the allocated
cpumask size but before that, we do this in the caller:

  sd->span_weight = cpumask_weight(sched_domain_span(sd));

which should have crashed too if we had a NULL pointer in the
cpumask range. So I'm at a loss. Maybe the pc points to a
different location in your build?

-- 
Thanks and Regards,
Prateek