I have tried the same again, this time with openvswitch unconfigured (but still running). ubuntu@node-horsea:~$ sudo ovs-vsctl del-br ovsbr0 ubuntu@node-horsea:~$ sudo ovs-vsctl show 8dfc2067-7b9b-48d7-a50a-df17bbd3cb6c ovs_version: "2.13.5" chcpu disabling/enabling still crashes. ubuntu@node-horsea:~$ sudo chcpu -d 5-11 Killed [ +3.357665] IRQ 56: no longer affine to CPU5 [ +0.000021] IRQ 72: no longer affine to CPU5 [ +0.000009] IRQ 82: no longer affine to CPU5 [ +0.000011] IRQ 96: no longer affine to CPU5 [ +0.000019] IRQ 121: no longer affine to CPU5 [ +0.000012] IRQ 136: no longer affine to CPU5 [ +0.002380] smpboot: CPU 5 is now offline [ +0.000468] BUG: kernel NULL pointer dereference, address: 0000000000000008 [ +0.031357] #PF: supervisor write access in kernel mode [ +0.023816] #PF: error_code(0x0002) - not-present page [ +0.023147] PGD 0 P4D 0 [ +0.011391] Oops: 0002 [#1] SMP PTI [ +0.015688] CPU: 11 PID: 5967 Comm: chcpu Tainted: P W O 5.13.0-27-generic #29~20.04.1-Ubuntu [ +0.043614] Hardware name: HP ProLiant DL360 Gen9/ProLiant DL360 Gen9, BIOS P89 01/22/2018 [ +0.037462] RIP: 0010:blk_mq_hctx_notify_dead+0xc7/0x190 [ +0.023926] Code: 04 49 8d 54 05 08 4c 01 e8 48 8b 48 08 48 39 ca 74 66 48 8b 48 08 48 39 ca 74 21 48 8b 3a 48 8b 4d c8 48 8b 72 08 48 89 7d c8 <4c> 89 77 08 48 89 0e 48 89 71 08 48 89 12 48 89 50 10 41 0f b7 84 [ +0.084916] RSP: 0018:ffffacf3ccddbbb0 EFLAGS: 00010286 [ +0.023534] RAX: ffffccf3bfb7a580 RBX: 0000000000000000 RCX: ffffacf3ccddbbb0 [ +0.033000] RDX: ffffccf3bfb7a588 RSI: 0000000000000000 RDI: 0000000000000000 [ +0.032118] RBP: ffffacf3ccddbbe8 R08: 0000000000000000 R09: ffffacf3ccddbaa8 [ +0.032456] R10: 0000000000000001 R11: 0000000000000001 R12: ffff8da022a50000 [ +0.032331] R13: ffffccf3bfb7a580 R14: ffffacf3ccddbbb0 R15: 0000000000000005 [ +0.032137] FS: 00007f1a4aff9580(0000) GS:ffff8da29fcc0000(0000) knlGS:0000000000000000 [ +0.036480] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [ +0.025981] CR2: 0000000000000008 CR3: 00000005b29d6006 CR4: 00000000001706e0 [ +0.032158] Call Trace: [ +0.010991] ? blk_mq_exit_hctx+0x160/0x160 [ +0.018782] cpuhp_invoke_callback+0x179/0x430 [ +0.020112] cpuhp_invoke_callback_range+0x44/0x80 [ +0.021557] _cpu_down+0x109/0x310 [ +0.015284] cpu_down+0x36/0x60 [ +0.014475] cpu_device_down+0x16/0x20 [ +0.016905] cpu_subsys_offline+0xe/0x10 [ +0.017652] device_offline+0x8e/0xc0 [ +0.016520] online_store+0x4c/0x90 [ +0.015662] dev_attr_store+0x17/0x30 [ +0.016581] sysfs_kf_write+0x3e/0x50 [ +0.016546] kernfs_fop_write_iter+0x138/0x1c0 [ +0.020381] new_sync_write+0x117/0x1b0 [ +0.017389] vfs_write+0x185/0x250 [ +0.015407] ksys_write+0x67/0xe0 [ +0.014961] __x64_sys_write+0x1a/0x20 [ +0.016879] do_syscall_64+0x61/0xb0 [ +0.016095] ? syscall_exit_to_user_mode+0x27/0x50 [ +0.021544] ? __x64_sys_faccessat+0x1c/0x20 [ +0.019347] ? do_syscall_64+0x6e/0xb0 [ +0.016881] entry_SYSCALL_64_after_hwframe+0x44/0xae [ +0.022785] RIP: 0033:0x7f1a4af140a7 [ +0.016084] Code: 64 89 02 48 c7 c0 ff ff ff ff eb bb 0f 1f 80 00 00 00 00 f3 0f 1e fa 64 8b 04 25 18 00 00 00 85 c0 75 10 b8 01 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 51 c3 48 83 ec 28 48 89 54 24 18 48 89 74 24 [ +0.086348] RSP: 002b:00007ffd8b2963d8 EFLAGS: 00000246 ORIG_RAX: 0000000000000001 [ +0.034055] RAX: ffffffffffffffda RBX: 0000000000000040 RCX: 00007f1a4af140a7 [ +0.032183] RDX: 0000000000000001 RSI: 000055feb3b47869 RDI: 0000000000000004 [ +0.032139] RBP: 00007f1a4aff9500 R08: 0000000000000000 R09: 00007ffd8b296380 [ +0.032232] R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000004 [ +0.032338] R13: 00007ffd8b2963e0 R14: 000055feb3b47869 R15: 0000000000000001 [ +0.032799] Modules linked in: ebtable_filter ebtables veth nbd xt_comment zfs(PO) zunicode(PO) zzstd(O) zlua(O) zavl(PO) icp(PO) zcommon(PO) znvpair(PO) spl(O) vhost_vsock vmw_vsock_virtio_transport_common vhost vhost_iotlb vsock xt_MASQUERADE xt_conntrack xt_CHECKSUM ipt_REJECT nf_reject_ipv4 xt_tcpudp ip6table_mangle ip6table_nat iptable_mangle iptable_nat nf_tables ip6table_filter ip6_tables iptable_filter bpfilter bridge stp llc nfnetlink_cttimeout nfnetlink openvswitch nsh nf_conncount nf_nat nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 nls_iso8859_1 dm_multipath scsi_dh_rdac scsi_dh_emc scsi_dh_alua ipmi_ssif intel_rapl_msr intel_rapl_common sb_edac x86_pkg_temp_thermal intel_powerclamp coretemp kvm_intel rpcrdma kvm sunrpc rdma_ucm rapl intel_cstate ib_iser libiscsi scsi_transport_iscsi efi_pstore rdma_cm ib_umad ib_ipoib iw_cm ib_cm hpilo ioatdma acpi_ipmi ipmi_si acpi_power_meter acpi_tad mac_hid sch_fq_codel ipmi_devintf ipmi_msghandler msr ip_tables x_tables autofs4 btrfs [ +0.000048] blake2b_generic zstd_compress raid10 raid456 async_raid6_recov async_memcpy async_pq async_xor async_tx xor raid6_pq libcrc32c raid1 raid0 multipath linear mlx5_ib ib_uverbs ib_core ses enclosure mgag200 i2c_algo_bit drm_kms_helper syscopyarea sysfillrect sysimgblt fb_sys_fops cec rc_core crct10dif_pclmul crc32_pclmul mlx5_core ghash_clmulni_intel nvme ixgbe pci_hyperv_intf drm aesni_intel psample xfrm_algo crypto_simd i2c_i801 xhci_pci hpsa mlxfw dca cryptd tg3 i2c_smbus lpc_ich xhci_pci_renesas tls nvme_core mdio scsi_transport_sas wmi [ +0.618979] CR2: 0000000000000008 [ +0.014940] ---[ end trace acdc0f1424b180b1 ]--- [ +0.026418] RIP: 0010:blk_mq_hctx_notify_dead+0xc7/0x190 [ +0.023965] Code: 04 49 8d 54 05 08 4c 01 e8 48 8b 48 08 48 39 ca 74 66 48 8b 48 08 48 39 ca 74 21 48 8b 3a 48 8b 4d c8 48 8b 72 08 48 89 7d c8 <4c> 89 77 08 48 89 0e 48 89 71 08 48 89 12 48 89 50 10 41 0f b7 84 [ +0.085003] RSP: 0018:ffffacf3ccddbbb0 EFLAGS: 00010286 [ +0.024710] RAX: ffffccf3bfb7a580 RBX: 0000000000000000 RCX: ffffacf3ccddbbb0 [ +0.032751] RDX: ffffccf3bfb7a588 RSI: 0000000000000000 RDI: 0000000000000000 [ +0.032111] RBP: ffffacf3ccddbbe8 R08: 0000000000000000 R09: ffffacf3ccddbaa8 [ +0.032106] R10: 0000000000000001 R11: 0000000000000001 R12: ffff8da022a50000 [ +0.032477] R13: ffffccf3bfb7a580 R14: ffffacf3ccddbbb0 R15: 0000000000000005 [ +0.032263] FS: 00007f1a4aff9580(0000) GS:ffff8da29fcc0000(0000) knlGS:0000000000000000 [ +0.036690] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [ +0.025863] CR2: 0000000000000008 CR3: 00000005b29d6006 CR4: 00000000001706e0 That further simplifies the test as it comes down to "I can not disable CPUs on 5.13.0-27-generic"