On 29/05/26 9:28 am, Shrikanth Hegde wrote:
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/[email protected]/
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 <[email protected]>
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 <[email protected]>
Closes:
https://lore.kernel.org/all/[email protected]/
Reviewed-by: Chen Yu <[email protected]>
Tested-by: Ritesh Harjani (IBM) <[email protected]>
Co-developed-by: Chen, Yu C <[email protected]>
Signed-off-by: Shrikanth Hegde <[email protected]>
---
Tested this fix, by applying on top of commit
b5ea300a17e37eada7a98561fbd34a3054578713 and on P9, its booting fine.
Please add the below tag.
Tested-by: Venkat Rao Bagalkote <[email protected]>
Regards,
Venkat.
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);
