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 -- 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: In Progress Status in linux package in Ubuntu: In Progress 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