Re: [PATCH] sched/topology: Provide arch_llc_mask for cache aware scheduling

[Peter Zijlstra <peterz@infradead.org>]

On Fri, May 29, 2026 at 01:27:12PM +0530, Shrikanth Hegde wrote:
> 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 referenced
> 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. 
> 
> Based on tip/master at 5c89783224e9 ("Merge branch into tip/master: 'x86/tdx'")
> Cc: stable@vger.kernel.org

This seems unwarranted, the patch breaking stuff is in tip:sched/core.

> 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/
> Reviewed-by: Chen Yu <yu.c.chen@intel.com>
> Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com> 
> Tested-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
> Co-developed-by: Chen, Yu C <yu.c.chen@intel.com>
> Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>

Thanks all! I'll stick this in tip:sched/core to go along with the
patches that broke stuff.