Blocked task of memory-hotplug test from ubuntu_kernel_selftests on Bionic ibm-gt

Bionic ibm-gt v4.15 kernel only on amd64 metal nodes rizzo and rumford has a timeout of memory-hotplug from ubuntu_kernel_selftests:
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    14121 14:49:30 ERROR| [stderr] offline_memory_expect_fail 5: unexpected success
    14122 14:56:12 ERROR| [stderr] offline_memory_expect_fail 62: unexpected success
    14123 14:56:12 ERROR| [stderr] offline_memory_expect_fail 63: unexpected success
    14124 15:19:22 INFO | Timer expired (1800 sec.), nuking pid 17231
    14125 15:19:23 INFO | ERROR ubuntu_kernel_selftests.memory-hotplug ubuntu_kernel_selftests.memory-hotplug timestamp=1625066363 localtime=Jun 30 15:19:23
-------------------

In dmesg from next test:
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[ 583.473311] Offlined Pages 32768
[ 583.476247] Offlined Pages 32768
[ 583.479028] Offlined Pages 32768
[ 583.481876] Offlined Pages 32768
[ 1088.798252] INFO: task systemd:1 blocked for more than 120 seconds.
[ 1088.798310] Not tainted 4.15.0-1098-ibm-gt #108-Ubuntu
[ 1088.798360] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[ 1088.798410] systemd D 0 1 0 0x00000000
[ 1088.798413] Call Trace:
[ 1088.798422] __schedule+0x24e/0x890
[ 1088.798426] schedule+0x2c/0x80
[ 1088.798428] schedule_preempt_disabled+0xe/0x10
[ 1088.798431] __mutex_lock.isra.5+0x276/0x4e0
[ 1088.798435] __mutex_lock_slowpath+0x13/0x20
[ 1088.798437] ? __mutex_lock_slowpath+0x13/0x20
[ 1088.798439] mutex_lock+0x2f/0x40
[ 1088.798444] proc_cgroup_show+0x4c/0x2a0
[ 1088.798449] proc_single_show+0x56/0x80
[ 1088.798453] seq_read+0xe5/0x450
[ 1088.798459] __vfs_read+0x1b/0x40
[ 1088.798462] vfs_read+0x8e/0x130
[ 1088.798466] SyS_read+0x5c/0xe0
[ 1088.798470] do_syscall_64+0x73/0x130
[ 1088.798474] entry_SYSCALL_64_after_hwframe+0x41/0xa6
[ 1088.798477] RIP: 0033:0x7fb5b16cf184
[ 1088.798478] RSP: 002b:00007fff21894aa0 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
[ 1088.798481] RAX: ffffffffffffffda RBX: 0000000000000012 RCX: 00007fb5b16cf184
[ 1088.798482] RDX: 0000000000000400 RSI: 000056544b965d80 RDI: 0000000000000012
[ 1088.798484] RBP: 000056544b965d80 R08: 0000000000000000 R09: 00007fb5b12cc9ba
[ 1088.798485] R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000400
[ 1088.798486] R13: 00007fb5b19a72a0 R14: 000056544b8ad570 R15: 00000000000007ff
[ 1088.798497] INFO: task kworker/11:1:179 blocked for more than 120 seconds.
-------------------

This was found on:
2021.06.21: bionic linux-ibm-gt 4.15.0-1095.104 (rizzo)
2021.07.19: bionic linux-ibm-gt 4.15.0-1098.108 (rumford)

Not found on:
2021.06.21: bionic linux-ibm-gt 4.15.0-1096.105 (ricolli)
2021.06.21: bionic linux 4.15.0-150.155 (rumford)