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

["Chen, Yu C" <yu.c.chen@intel.com>]

Hi Shrikanth,

On 5/28/2026 7:27 PM, Shrikanth Hegde wrote:
> 
> 
>  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>

It looks good to me. Let's see if this resolves the issue for Venkat
and Ritesh. Feel free to send the formal version :)

Reviewed-by: Chen Yu <yu.c.chen@intel.com>


thanks,
Chenyu