Hi Harish,
excuse me, but you are using a very old and in between outdated kernel with 
4.15.0-29

With bionic/18.04 we are today at 4.15.0.72:
 linux-generic | 4.15.0.20.23  | bionic          | s390x   (initial kernel, 
already superseded) 
 linux-generic | 4.15.0.72.74  | bionic-security | s390x   <==
 linux-generic | 4.15.0.72.74  | bionic-updates  | s390x   <==
 linux-generic | 4.15.0.73.75  | bionic-proposed | s390x
(and even have 4.15.0.73 in proposed).

I also looked up the 3 commit IDs - you've mentioned above - in bionic's
git tree, and they are all in since a couple of kernel updates:

1) "Disable preemption after queueing stopper threads"
Ubuntu-4.15.0-48.51
Ubuntu-4.15.0-49.52
Ubuntu-4.15.0-49.53
Ubuntu-4.15.0-50.54
Ubuntu-4.15.0-51.55
Ubuntu-4.15.0-52.56
Ubuntu-4.15.0-54.58
Ubuntu-4.15.0-55.60
Ubuntu-4.15.0-56.62
Ubuntu-4.15.0-57.63
Ubuntu-4.15.0-58.64
Ubuntu-4.15.0-59.66
Ubuntu-4.15.0-60.67
Ubuntu-4.15.0-62.69
Ubuntu-4.15.0-64.73
Ubuntu-4.15.0-65.74
Ubuntu-4.15.0-66.75
Ubuntu-4.15.0-67.76
Ubuntu-4.15.0-68.77
Ubuntu-4.15.0-69.78
Ubuntu-4.15.0-70.79
Ubuntu-4.15.0-71.80
Ubuntu-4.15.0-72.81
2) "Disable preemption when waking two stopper threads"
Ubuntu-4.15.0-46.49
Ubuntu-4.15.0-47.50
Ubuntu-4.15.0-48.51
Ubuntu-4.15.0-49.52
Ubuntu-4.15.0-49.53
Ubuntu-4.15.0-50.54
Ubuntu-4.15.0-51.55
Ubuntu-4.15.0-52.56
Ubuntu-4.15.0-54.58
Ubuntu-4.15.0-55.60
Ubuntu-4.15.0-56.62
Ubuntu-4.15.0-57.63
Ubuntu-4.15.0-58.64
Ubuntu-4.15.0-59.66
Ubuntu-4.15.0-60.67
Ubuntu-4.15.0-62.69
Ubuntu-4.15.0-64.73
Ubuntu-4.15.0-65.74
Ubuntu-4.15.0-66.75
Ubuntu-4.15.0-67.76
Ubuntu-4.15.0-68.77
Ubuntu-4.15.0-69.78
Ubuntu-4.15.0-70.79
Ubuntu-4.15.0-71.80
Ubuntu-4.15.0-72.81
3) "sched: Fix migrate_swap() vs. active_balance() deadlock"
Ubuntu-4.15.0-37.40
Ubuntu-4.15.0-38.41
Ubuntu-4.15.0-39.42
Ubuntu-4.15.0-40.43
Ubuntu-4.15.0-42.45
Ubuntu-4.15.0-43.46
Ubuntu-4.15.0-44.47
Ubuntu-4.15.0-45.48
Ubuntu-4.15.0-46.49
Ubuntu-4.15.0-47.50
Ubuntu-4.15.0-48.51
Ubuntu-4.15.0-49.52
Ubuntu-4.15.0-49.53
Ubuntu-4.15.0-50.54
Ubuntu-4.15.0-51.55
Ubuntu-4.15.0-52.56
Ubuntu-4.15.0-54.58
Ubuntu-4.15.0-55.60
Ubuntu-4.15.0-56.62
Ubuntu-4.15.0-57.63
Ubuntu-4.15.0-58.64
Ubuntu-4.15.0-59.66
Ubuntu-4.15.0-60.67
Ubuntu-4.15.0-62.69
Ubuntu-4.15.0-64.73
Ubuntu-4.15.0-65.74
Ubuntu-4.15.0-66.75
Ubuntu-4.15.0-67.76
Ubuntu-4.15.0-68.77
Ubuntu-4.15.0-69.78
Ubuntu-4.15.0-70.79
Ubuntu-4.15.0-71.80
Ubuntu-4.15.0-72.81

So please update your system to the very latest bionic kernel (as of today: 
4.15.0-72) and retest.
It's highly likely that the issue you've mentioned is already solved!

** Changed in: linux (Ubuntu)
       Status: New => Incomplete

** Changed in: ubuntu-power-systems
       Status: New => Incomplete

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https://bugs.launchpad.net/bugs/1855679

Title:
  Rcu stalls and Soft-lockups observed on stressing Ubuntu 18 04 1

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