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Since v1: Edited the comment according to Srivatsa's suggestion. During the testing, we encounter below WARN followed by Oops: WARNING: at kernel/sched/core.c:6218 ... NIP [c000000000101660] .build_sched_domains+0x11d0/0x1200 LR [c000000000101358] .build_sched_domains+0xec8/0x1200 PACATMSCRATCH [800000000000f032] Call Trace: [c00000001b103850] [c000000000101358] .build_sched_domains+0xec8/0x1200 [c00000001b1039a0] [c00000000010aad4] .partition_sched_domains+0x484/0x510 [c00000001b103aa0] [c00000000016d0a8] .rebuild_sched_domains+0x68/0xa0 [c00000001b103b30] [c00000000005cbf0] .topology_work_fn+0x10/0x30 ... Oops: Kernel access of bad area, sig: 11 [#1] ... NIP [c00000000045c000] .__bitmap_weight+0x60/0xf0 LR [c00000000010132c] .build_sched_domains+0xe9c/0x1200 PACATMSCRATCH [8000000000029032] Call Trace: [c00000001b1037a0] [c000000000288ff4] .kmem_cache_alloc_node_trace+0x184/0x3a0 [c00000001b103850] [c00000000010132c] .build_sched_domains+0xe9c/0x1200 [c00000001b1039a0] [c00000000010aad4] .partition_sched_domains+0x484/0x510 [c00000001b103aa0] [c00000000016d0a8] .rebuild_sched_domains+0x68/0xa0 [c00000001b103b30] [c00000000005cbf0] .topology_work_fn+0x10/0x30 ... This was caused by that 'sd->groups == NULL' after building groups, which was caused by the empty 'sd->span'. The cpu's domain contained nothing because the cpu was assigned to a wrong node, due to the following unfortunate sequence of events: 1. The hypervisor sent a topology update to the guest OS, to notify changes to the cpu-node mapping. However, the update was actually redundant - i.e., the "new" mapping was exactly the same as the old one. 2. Due to this, the 'updated_cpus' mask turned out to be empty after exiting the 'for-loop' in arch_update_cpu_topology(). 3. So we ended up calling stop-machine() with an empty cpumask list, which made stop-machine internally elect cpumask_first(cpu_online_mask), i.e., CPU0 as the cpu to run the payload (the update_cpu_topology() function). 4. This causes update_cpu_topology() to be run by CPU0. And since 'updates' is kzalloc()'ed inside arch_update_cpu_topology(), update_cpu_topology() finds update->cpu as well as update->new_nid to be 0. In other words, we end up assigning CPU0 (and eventually its siblings) to node 0, incorrectly. Along with the following wrong updating, it causes the sched-domain rebuild code to break and crash the system. Fix this by skipping the topology update in cases where we find that the topology has not actually changed in reality (ie., spurious updates). CC: Benjamin Herrenschmidt <benh@kernel.crashing.org> CC: Paul Mackerras <paulus@samba.org> CC: Nathan Fontenot <nfont@linux.vnet.ibm.com> CC: Stephen Rothwell <sfr@canb.auug.org.au> CC: Andrew Morton <akpm@linux-foundation.org> CC: Robert Jennings <rcj@linux.vnet.ibm.com> CC: Jesse Larrew <jlarrew@linux.vnet.ibm.com> CC: "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com> CC: Alistair Popple <alistair@popple.id.au> Suggested-by: "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com> Signed-off-by: Michael Wang <wangyun@linux.vnet.ibm.com> Reviewed-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com> Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org> |
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