Public bug reported: [Impact]
With LP: #2023007 we have decided to enable CONFIG_RCU_LAZY in the lowlatency kernel to improve power consumption, but this option can potentially introduce performance regressions in some cases, due to the fact that RCU callbacks are now batched and flushed all at once after a timed delay. It would be definitely safer to have a way to enable/disable lazy RCUs at boot time. In this way we could provide a simple kernel command line option that can be used in all those cases where the lowlatency kernel is required, but we don't want to risk performance regressions. [Test case] In this context providing a single test case is not relevant. After applying the fix any performance benchmark can be used to evaluate if lazy RCU feature should be enabled at boot time or not (according to the specific context where the lowlatency kernel is going to be used/deployed). [Fix] Apply this patch to the *generic* kernel: https://lore.kernel.org/lkml/20231203011252.233748-1-qyou...@layalina.io/T/#u We want to apply this to the generic kernel, not just lowlatency, because in this way *all* derivatives will have the possibility to get this feature, in case some of them want to enable lazy RCUs (even generic itself). Then make sure that lowlatency (or any other kernel with CONFIG_RCU_LAZY=y) also has CONFIG_RCU_LAZY_DEFAULT_OFF not set (so that the previous behavior is not changed). [Regression potential] We may receive reports of small performance regressions vs power consumption regressions, depending on the rcutree.enable_rcu_lazy command line option that is used. In such case we should suggest the user to test both with lazy RCU disabled or enabled. ** Affects: linux (Ubuntu) Importance: Undecided Status: New ** Affects: linux-lowlatency (Ubuntu) Importance: Undecided Status: New ** Affects: linux (Ubuntu Noble) Importance: Undecided Status: New ** Affects: linux-lowlatency (Ubuntu Noble) Importance: Undecided Status: New ** Also affects: linux-lowlatency (Ubuntu Noble) Importance: Undecided Status: New ** Description changed: [Impact] With LP: #2023007 we have decided to enable CONFIG_RCU_LAZY in the lowlatency kernel to improve power consumption, but this option can potentially introduce performance regressions in some cases, due to the fact that RCU callbacks are now batched and flushed all at once after a timed delay. It would be definitely safer to have a way to enable/disable lazy RCUs at boot time. In this way we could provide a simple kernel command line option that can be used in all those cases where the lowlatency kernel is required, but we don't want to risk performance regressions. [Test case] In this context providing a single test case is not relevant. After applying the fix any performance benchmark can be used to evaluate if lazy RCU feature should be enabled at boot time or not (according to the specific context where the lowlatency kernel is going to be used/deployed). [Fix] Apply this patch to the *generic* kernel: https://lore.kernel.org/lkml/20231203011252.233748-1-qyou...@layalina.io/T/#u We want to apply this to the generic kernel, not just lowlatency, because in this way *all* derivatives will have the possibility to get this feature, in case some of them want to enable lazy RCUs (even generic itself). + Then make sure that lowlatency (or any other kernel with + CONFIG_RCU_LAZY=y) also has CONFIG_RCU_LAZY_DEFAULT_OFF not set (so that + the previous behavior is not changed). + [Regression potential] We may receive reports of small performance regressions vs power consumption regressions, depending on the rcutree.enable_rcu_lazy command line option that is used. In such case we should suggest the user to test both with lazy RCU disabled or enabled. ** Also affects: linux (Ubuntu) Importance: Undecided Status: New -- You received this bug notification because you are a member of Kernel Packages, which is subscribed to linux-lowlatency in Ubuntu. https://bugs.launchpad.net/bugs/2045492 Title: make lazy RCU a boot time option Status in linux package in Ubuntu: New Status in linux-lowlatency package in Ubuntu: New Status in linux source package in Noble: New Status in linux-lowlatency source package in Noble: New Bug description: [Impact] With LP: #2023007 we have decided to enable CONFIG_RCU_LAZY in the lowlatency kernel to improve power consumption, but this option can potentially introduce performance regressions in some cases, due to the fact that RCU callbacks are now batched and flushed all at once after a timed delay. It would be definitely safer to have a way to enable/disable lazy RCUs at boot time. In this way we could provide a simple kernel command line option that can be used in all those cases where the lowlatency kernel is required, but we don't want to risk performance regressions. [Test case] In this context providing a single test case is not relevant. After applying the fix any performance benchmark can be used to evaluate if lazy RCU feature should be enabled at boot time or not (according to the specific context where the lowlatency kernel is going to be used/deployed). [Fix] Apply this patch to the *generic* kernel: https://lore.kernel.org/lkml/20231203011252.233748-1-qyou...@layalina.io/T/#u We want to apply this to the generic kernel, not just lowlatency, because in this way *all* derivatives will have the possibility to get this feature, in case some of them want to enable lazy RCUs (even generic itself). Then make sure that lowlatency (or any other kernel with CONFIG_RCU_LAZY=y) also has CONFIG_RCU_LAZY_DEFAULT_OFF not set (so that the previous behavior is not changed). [Regression potential] We may receive reports of small performance regressions vs power consumption regressions, depending on the rcutree.enable_rcu_lazy command line option that is used. In such case we should suggest the user to test both with lazy RCU disabled or enabled. To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/2045492/+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
