Shrikanth Hegde <[email protected]> writes: > 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). >> > > Hi Venkat, Ritesh, > Could you please try the below diff and see if it helps. > This helps to fix boot problem for SPLPAR for me. > > Hi Chenyu, > Let me know if I have to send the patch. Or > if you want to add more comments or change it feel free to pick it up and > send it. > Either way is fine. Let me know. > > Hi Prateek, Srikar, > I hope the below diff makes sense. Please check. > > nit: llc_mask is still under CONFIG_SCHED_MC, for ppc it is set to true > always for SMP systems, and for others it is LLC domain. So not a concern i > guess. > --- > > From 10e9413cef063446d67dc02c2b44e1ea582e5d53 Mon Sep 17 00:00:00 2001 > From: Shrikanth Hegde <[email protected]> > 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 <[email protected]> > Closes: > https://lore.kernel.org/all/[email protected]/ > Suggested-by: Chen, Yu C <[email protected]> > Signed-off-by: Shrikanth Hegde <[email protected]> > --- > arch/powerpc/include/asm/topology.h | 3 +++ > arch/powerpc/kernel/smp.c | 10 ++++++++++ > kernel/sched/topology.c | 9 +++++++++ > 3 files changed, 22 insertions(+) >
I had to apply this change manually - as it wasn't cleanly applicable for me on linux-next. But since the changes were straight forward so it wasn't a problem. With this change I don't see the panic anymore, which I was observing earlier. Thanks for the quick fix. Feel free to add: Tested-by: Ritesh Harjani (IBM) <[email protected]>
