This bug is awaiting verification that the kernel in -proposed solves the problem. Please test the kernel and update this bug with the results. If the problem is solved, change the tag 'verification-needed- focal' to 'verification-done-focal'. If the problem still exists, change the tag 'verification-needed-focal' to 'verification-failed-focal'.
If verification is not done by 5 working days from today, this fix will be dropped from the source code, and this bug will be closed. See https://wiki.ubuntu.com/Testing/EnableProposed for documentation how to enable and use -proposed. Thank you! ** Tags added: verification-needed-focal -- You received this bug notification because you are a member of Kernel Packages, which is subscribed to linux in Ubuntu. 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 Committed 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. To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu-z-systems/+bug/1876044/+subscriptions -- Mailing list: https://launchpad.net/~kernel-packages Post to : kernel-packages@lists.launchpad.net Unsubscribe : https://launchpad.net/~kernel-packages More help : https://help.launchpad.net/ListHelp
