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3qf/jeAzAtJiwlXv0ug2u+Ly7UGE/1OoYKjgAS6zwTtFtn2JzESIU8Ly9qE/uHe12S5dLU3mdvUmXY7tTsjxlA== X-MS-Exchange-Transport-CrossTenantHeadersStamped: MW6PR11MB8340 X-OriginatorOrg: intel.com Hi Venkat, On 5/25/2026 10:07 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). > Thanks for the test. > > Traces: > > [    0.038567] Big cores detected but using small core scheduling > [    0.038796] BUG: Kernel NULL pointer dereference at 0x00000000 > [    0.038804] Faulting instruction address: 0xc000000000e58504 > [    0.038812] Oops: Kernel access of bad area, sig: 11 [#1] > [    0.038819] LE PAGE_SIZE=64K MMU=Hash  SMP NR_CPUS=8192 NUMA pSeries > [    0.038830] Modules linked in: > [    0.038840] CPU: 0 UID: 0 PID: 1 Comm: swapper/0 Not tainted 7.0.0- > rc6+ #14 PREEMPTLAZY > [    0.038851] Hardware name: IBM,8375-42A POWER9 (architected) 0x4e0202 > 0xf000005 of:IBM,FW950.80 (VL950_131) hv:phyp pSeries > [    0.039029] NIP [c000000000e58504] _find_first_bit+0x44/0x130 > [    0.039043] LR [c000000000e58500] _find_first_bit+0x40/0x130 > [    0.039076] [c0000000090e7bc0] [c00000000038b3b8] > build_sched_domains+0xad8/0xe50 It seems that cpumask_first(llc_mask(i)) is accessing NULL cpu_coregroup_mask(): has_coregroup_support() is false, thus cpu_coregroup_map is never allocated in smp_prepare_cpus(). This machine is a "shared system" VM. We should probably let the LLC id generation fall back to using L2 id if cpu_coregroup_mask is unavailable (which restores the behavior before this patch). I'm wondering if the following change would help(need IBM friends' help on this): diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c index 3467f86fd78f..cf6c2e4190ab 100644 --- a/arch/powerpc/kernel/smp.c +++ b/arch/powerpc/kernel/smp.c @@ -1042,11 +1042,6 @@ static const struct cpumask *tl_smallcore_smt_mask(struct sched_domain_topology_ } #endif -struct cpumask *cpu_coregroup_mask(int cpu) -{ - return per_cpu(cpu_coregroup_map, cpu); -} - static bool has_coregroup_support(void) { /* Coregroup identification not available on shared systems */ @@ -1056,6 +1051,14 @@ static bool has_coregroup_support(void) return coregroup_enabled; } +struct cpumask *cpu_coregroup_mask(int cpu) +{ + if (!has_coregroup_support()) + return cpu_l2_cache_mask(cpu); + + return per_cpu(cpu_coregroup_map, cpu); +} + static int __init init_big_cores(void) { int cpu;