Re: [BUG] sched/cache: "Make LLC id continuous" causes NULL cpumask dereference in build_sched_domains on POWER9

[Ritesh Harjani (IBM) <ritesh.list@gmail.com>]

Venkat Rao Bagalkote <venkat88@linux.ibm.com> writes:

> Greetings!!!
>
> I am seeing an early boot kernel panic due to NULL pointer dereference 
> on a POWER9 (pSeries) system when testing linux-next (next-20260522).
>
>
> [    0.039029] NIP [c000000000e58504] _find_first_bit+0x44/0x130
> [    0.039043] LR [c000000000e58500] _find_first_bit+0x40/0x130
> [    0.039054] Call Trace:
> [    0.039060] [c0000000090e7b80] [c00000000416af20] 
> schedutil_gov+0x0/0xa0 (unreliable)
> [    0.039076] [c0000000090e7bc0] [c00000000038b3b8] 
> build_sched_domains+0xad8/0xe50
> [    0.039089] [c0000000090e7ce0] [c000000003045d78] 
> sched_init_smp+0xa8/0x164
> [    0.039102] [c0000000090e7d30] [c00000000300f374] 
> kernel_init_freeable+0x250/0x370
> [    0.039117] [c0000000090e7de0] [c000000000011f90] kernel_init+0x34/0x1e4
> [    0.039129] [c0000000090e7e50] [c00000000000debc] 
> ret_from_kernel_user_thread+0x14/0x1c
> [    0.039142] ---- interrupt: 0 at 0x0
> [    0.039150] Code: 41820090 7c0802a6 393cffff fbe10038 7c7f1b78 
> fba10028 fbc10030 3bc00000 793dd7e2 f8010050 4bae6e9d 60000000 
> <e93f0000> 2c290000 408200bc 283c0040
> [    0.039196] ---[ end trace 0000000000000000 ]---
>
>

Well, I am hitting this on 7.1.0-rc5-next-20260526-00010-gbfac43765a97
with Qemu Pseries TCG power10/11.

[    0.342868][    T1] smp: Bringing up secondary CPUs ...
[    0.342868][    T1] smp: Bringing up secondary CPUs ...
[    0.525419][    T1] smp: Brought up 1 node, 4 CPUs
[    0.525419][    T1] smp: Brought up 1 node, 4 CPUs
[    0.527992][    T1] numa: Node 0 CPUs: 0-3
[    0.527992][    T1] numa: Node 0 CPUs: 0-3
[    0.552787][    T1] BUG: Kernel NULL pointer dereference on read at 0x00000000
[    0.552787][    T1] BUG: Kernel NULL pointer dereference on read at 0x00000000
[    0.557446][    T1] Faulting instruction address: 0xc000000000fe3f1c
[    0.557446][    T1] Faulting instruction address: 0xc000000000fe3f1c
cpu 0x0: Vector: 300 (Data Access) at [c000000006607800]
    pc: c000000000fe3f1c: _find_first_bit+0xc/0xc0
    lr: c00000000027b7d8: build_sched_domains+0xbb4/0x1938
    sp: c000000006607ac0
   msr: 8000000002009033
   dar: 0
 dsisr: 80000
  current = 0xc000000006f9fb00
  paca    = 0xc000000005670000   irqmask: 0x03   irq_happened: 0x09
    pid   = 1, comm = swapper/0
Linux version 7.1.0-rc5-next-20260526-00010-gbfac43765a97-dirty (powerpc64le-linux-gnu-gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40) #19 SMP PREEMPT Thu May 28 12:29:51 IST 2026
enter ? for help
[link register   ] c00000000027b7d8 build_sched_domains+0xbb4/0x1938
[c000000006607ac0] c00000000027b0a8 build_sched_domains+0x484/0x1938 (unreliable)
[c000000006607c20] c000000004053180 sched_init_domains+0x114/0x1cc
[c000000006607c70] c0000000040515e0 sched_init_smp+0x5c/0x17c
[c000000006607cc0] c000000004012888 kernel_init_freeable+0x258/0x790
[c000000006607dc0] c000000000011f3c kernel_init+0x34/0x268
[c000000006607e30] c00000000000debc ret_from_kernel_user_thread+0x14/0x1c
---- Exception: 0  at 0000000000000000



-ritesh