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by DM4PR11MB6020.namprd11.prod.outlook.com ([fe80::3058:1480:e4ac:5765%4]) with mapi id 15.21.0071.011; Thu, 28 May 2026 13:21:17 +0000 Message-ID: <8935ba3c-4ced-43ce-a5b7-c5df91b466d3@intel.com> Date: Thu, 28 May 2026 21:21:08 +0800 User-Agent: Mozilla Thunderbird Subject: Re: [BUG] sched/cache: "Make LLC id continuous" causes NULL cpumask dereference in build_sched_domains on POWER9 To: Shrikanth Hegde CC: Madhavan Srinivasan , "Christophe Leroy (CS GROUP)" , LKML , linuxppc-dev , , "Srikar Dronamraju" , Peter Zijlstra , "K Prateek Nayak" , Venkat Rao Bagalkote , Ritesh Harjani References: <51154de7-3700-4cb4-82f2-1b3a8fa427f7@linux.ibm.com> <74a61427-b5d8-41ad-bc5b-508ee246d510@linux.ibm.com> Content-Language: en-US From: "Chen, Yu C" In-Reply-To: <74a61427-b5d8-41ad-bc5b-508ee246d510@linux.ibm.com> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: KL1PR0401CA0032.apcprd04.prod.outlook.com (2603:1096:820:e::19) To 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X-OriginatorOrg: intel.com Hi Shrikanth, On 5/28/2026 7:27 PM, Shrikanth Hegde wrote: > > > From 10e9413cef063446d67dc02c2b44e1ea582e5d53 Mon Sep 17 00:00:00 2001 > From: Shrikanth Hegde > 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 > Closes: https://lore.kernel.org/ > all/51154de7-3700-4cb4-82f2-1b3a8fa427f7@linux.ibm.com/ > Suggested-by: Chen, Yu C > Signed-off-by: Shrikanth Hegde It looks good to me. Let's see if this resolves the issue for Venkat and Ritesh. Feel free to send the formal version :) Reviewed-by: Chen Yu thanks, Chenyu