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>]

Shrikanth Hegde <sshegde@linux.ibm.com> writes:

> On 5/25/26 7:37 PM, Venkat Rao Bagalkote wrote:
>> 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).
>> 
>
> Hi Venkat, Ritesh,
> Could you please try the below diff and see if it helps.
> This helps to fix boot problem for SPLPAR for me.
>
> Hi Chenyu,
> Let me know if I have to send the patch. Or
> if you want to add more comments or change it feel free to pick it up and send it.
> Either way is fine. Let me know.
>
> Hi Prateek, Srikar,
> I hope the below diff makes sense. Please check.
>
> nit: llc_mask is still under CONFIG_SCHED_MC, for ppc it is set to true
> always for SMP systems, and for others it is LLC domain. So not a concern i guess.
> ---
>
>  From 10e9413cef063446d67dc02c2b44e1ea582e5d53 Mon Sep 17 00:00:00 2001
> From: Shrikanth Hegde <sshegde@linux.ibm.com>
> Date: Thu, 28 May 2026 06:16:44 -0400
> Subject: [PATCH] topology: Provide arch_llc_mask for cache aware scheduling
>
> Venkat Reported a boot kernel panic next-20260522. Git bisect pointed to
> b5ea300a17e3 ("sched/cache: Make LLC id continuous")
>
> Stacktrace points to llc_mask being null.
>
> NIP [c000000000e58504] _find_first_bit+0x44/0x130
> LR [c000000000e58500] _find_first_bit+0x40/0x130
> Call Trace:
> build_sched_domains+0xad8/0xe50
> sched_init_smp+0xa8/0x164
> kernel_init_freeable+0x250/0x370
> ret_from_kernel_user_thread+0x14/0x1c
>
> On powerpc, cpu_coregroup_mask is available only when the underlying
> hardware support coregroup. In shared LPAR, QEMU guest or power9 etc
> coregroup isn;t supported. In such cases llc_mask was being referrenced
> when it was null leading to panic.
>
> on powerpc, LLC is at SMT core level. So assumption that coregroup(MC)
> domain point to LLC is wrong. Provide a way for archs to say where its
> LLC is if it not at MC domain.
>
> Fixes: b5ea300a17e3 ("sched/cache: Make LLC id continuous")
> Reported-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
> Closes: https://lore.kernel.org/all/51154de7-3700-4cb4-82f2-1b3a8fa427f7@linux.ibm.com/
> Suggested-by: Chen, Yu C <yu.c.chen@intel.com>
> Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
> ---
>   arch/powerpc/include/asm/topology.h |  3 +++
>   arch/powerpc/kernel/smp.c           | 10 ++++++++++
>   kernel/sched/topology.c             |  9 +++++++++
>   3 files changed, 22 insertions(+)
>

I had to apply this change manually - as it wasn't cleanly applicable
for me on linux-next. But since the changes were straight forward so it
wasn't a problem.

With this change I don't see the panic anymore, which I was observing
earlier.

Thanks for the quick fix. Feel free to add:
Tested-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>