[Bug 1876044] Autopkgtest regression report (linux-oracle-5.4/5.4.0-1019.19~18.04.1)

All autopkgtests for the newly accepted linux-oracle-5.4 (5.4.0-1019.19~18.04.1) for bionic have finished running.
The following regressions have been reported in tests triggered by the package:

zfs-linux/unknown (armhf)


Please visit the excuses page listed below and investigate the failures, proceeding afterwards as per the StableReleaseUpdates policy regarding autopkgtest regressions [1].

https://people.canonical.com/~ubuntu-archive/proposed-
migration/bionic/update_excuses.html#linux-oracle-5.4

[1] https://wiki.ubuntu.com/StableReleaseUpdates#Autopkgtest_Regressions

Thank you!

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https://bugs.launchpad.net/bugs/1876044

Title:
[UBUNTU 20.04] PSI generates overhead on s390x

Status in Ubuntu on IBM z Systems:
Fix Committed
Status in linux package in Ubuntu:
In Progress
Status in linux source package in Focal:
Fix Released

Bug description:
SRU Justification:
==================

[Impact]

* PSI is enabled by default for all architectures in Ubuntu.

* On s390x this leads to performance degradations on popular workloads
like web serving (nginx).

[Fix]

* Leave 'CONFIG_PSI=y', but change 'CONFIG_PSI_DEFAULT_DISABLED=n' to
'CONFIG_PSI_DEFAULT_DISABLED=y'

[Test Case]

* Measure the overhead with 'CONFIG_PSI_DEFAULT_DISABLED=n' and
'CONFIG_PSI_DEFAULT_DISABLED=y' on the same environment with nginx.

[Regression Potential]

* The regression potential can be considered as moderate, since PSI
(Pressure stall information tracking),

* since PSI is just used to collect CPU overcommitted, memory and IO
metrics.

* And it can be enabled again with the kernel argument.
__________

PSI is always enabled in Ubuntu 20.04.
For a test system with 72 guests on 8 cores running a nginx workload this created an overhead of ~1%.

Can we change this back to
CONFIG_PSI=y
CONFIG_PSI_DEFAULT_DISABLED=y

so that by default the overhead is not there but for debugging or if
needed it can still be enabled via kernel parm?

Maybe there has been a reason for this - so feel free to discuss.

---uname output---
Linux t35lp76 5.4.0-26-generic #30-Ubuntu SMP Mon Apr 20 16:57:22 UTC 2020 s390x s390x s390x GNU/Linux

Machine Type = All s390x architecture

---Debugger---
A debugger is not configured

---Steps to Reproduce---
 root@t35lp76:/boot# grep PSI config-5.4.0-26-generic
CONFIG_PSI=y
# CONFIG_PSI_DEFAULT_DISABLED is not set

Stack trace output:
 no

Oops output:
 no

System Dump Info:
  The system is not configured to capture a system dump.

*Additional Instructions for epasch@de,ibm.com:
-Attach sysctl -a output output to the bug.

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