Request submitted to kernel team's mailing list:
https://lists.ubuntu.com/archives/kernel-team/2020-May/thread.html#109592
changing status to 'In Progress'
** Description changed:
+ 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
+ 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.
-
+ 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
-
+
+ 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
+ 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
-
+ no
+
Oops output:
- no
-
+ no
+
System Dump Info:
- The system is not configured to capture a system dump.
-
- *Additional Instructions for epasch@de,ibm.com:
+ 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.
** Changed in: linux (Ubuntu)
Status: New => In Progress
** Changed in: ubuntu-z-systems
Status: Triaged => In Progress
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https://bugs.launchpad.net/bugs/1876044
Title:
[UBUNTU 20.04] PSI generates overhead on s390x
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