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

[Shrikanth Hegde <sshegde@linux.ibm.com>]

Hello. Sorry for too many mails.

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).
> 
> 

Based on srikar's suggestion to keep the below,
#define arch_llc_mask(cpu)     cpu_l2_cache_mask(cpu)

which makes it pretty much what chenyu had here
https://lore.kernel.org/all/8d14c844-b4a8-4af6-acab-2cfdd42225be@intel.com/

I added the changelog and comments. removed the changes in !CONFIG_MC case since powerpc
defines it always. I have changed the chenyu tag to Co-developed-by: instead.

I have carried the tested-by and reviewed-by tags since patch is
still more or less the same.

This is based on tip/master at
5c89783224e9  Merge branch into tip/master: 'x86/tdx'
I am planning to send it based on tip tree.
Let me know if has to be against a any different tree.

Please let me know if there are any concerns.
verified below too fixes the panic seen in shared LPAR.

============================================

From: Shrikanth Hegde <sshegde@linux.ibm.com>
Date: Thu, 28 May 2026 23:23:43 -0400
Subject: [PATCH] sched/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 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.

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: 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>
---
  arch/powerpc/include/asm/topology.h |  6 ++++++
  kernel/sched/topology.c             | 13 +++++++++++--
  2 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/include/asm/topology.h b/arch/powerpc/include/asm/topology.h
index 66ed5fe1b718..e3de0f3d8b86 100644
--- a/arch/powerpc/include/asm/topology.h
+++ b/arch/powerpc/include/asm/topology.h
@@ -135,6 +135,12 @@ struct cpumask *cpu_coregroup_mask(int cpu);
  const struct cpumask *cpu_die_mask(int cpu);
  int cpu_die_id(int cpu);
  
+/* Points to where the LLC is. On power9 this will point at CACHE
+ * domain, On others it will point to SMT domain. In all cases
+ * cpu_l2_cache_mask points to where LLC is.
+ */
+#define arch_llc_mask(cpu)     cpu_l2_cache_mask(cpu)
+
  #ifdef CONFIG_PPC64
  #include <asm/smp.h>
  
diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index df2ceb54c970..622e2e01974c 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -2063,12 +2063,21 @@ const struct cpumask *tl_mc_mask(struct sched_domain_topology_level *tl, int cpu
  	return cpu_coregroup_mask(cpu);
  }
  
-#define llc_mask(cpu) cpu_coregroup_mask(cpu)
+/*
+ * Majority of architectures have LLC at MC domain level with exception
+ * such as powerpc. Provide a way for arch to specify where its LLC is
+ * if it falls in exception category
+ */
+# ifndef arch_llc_mask
+#define arch_llc_mask(cpu) cpu_coregroup_mask(cpu)
+# endif
  
  #else
-#define llc_mask(cpu) cpumask_of(cpu)
+#define arch_llc_mask(cpu) cpumask_of(cpu)
  #endif
  
+#define llc_mask(cpu) arch_llc_mask(cpu)
+
  const struct cpumask *tl_pkg_mask(struct sched_domain_topology_level *tl, int cpu)
  {
  	return cpu_node_mask(cpu);
-- 
2.47.3