** Description changed: BugLink: https://bugs.launchpad.net/bugs/1894591 [Impact] The default clocksource for a KVM VM is kvm-clock, and I happen to need tsc. $ cat /sys/devices/system/clocksource/clocksource0/current_clocksource kvm-clock If I edit /etc/default/grub and append "clocksource=tsc" to GRUB_CMDLINE_LINUX_DEFAULT and reboot, I find the clocksource is still kvm-clock. $ cat /sys/devices/system/clocksource/clocksource0/current_clocksource kvm-clock I can work around this by telling the kernel that the tsc clocksource is reliable, before the watchdog has a chance to see for itself that it is reliable: GRUB_CMDLINE_LINUX_DEFAULT="clocksource=tsc tsc=reliable" $ cat /sys/devices/system/clocksource/clocksource0/current_clocksource tsc If I override the clocksource, the kernel should respect my wishes and I should receive the requested clocksource. [Fix] The fix landed in Linux 4.9 in the below commit: commit 36374583f9084cdab4b5dcf5521a3ce55bebb9fa Author: Kyle Walker <[email protected]> Date: Sat Aug 6 12:07:30 2016 -0400 Subject: clocksource: Defer override invalidation unless clock is unstable Link: https://github.com/torvalds/linux/commit/36374583f9084cdab4b5dcf5521a3ce55bebb9fa The commit ensures the override doesn't get cleared before the watchdog has had an opportunity to check if the clocksource is stable or not. However, if the clocksource is known to be unstable at this point in time, it will clear the override and return to the default. This is a clean cherry-pick to the Xenial 4.4 kernel. [Testcase] Start up a KVM VM, possibly enable invtsc on the QEMU command line. The default clocksource will be kvm-clock: $ cat /sys/devices/system/clocksource/clocksource0/current_clocksource kvm-clock If you set the kernel command line to: GRUB_CMDLINE_LINUX_DEFAULT="clocksource=tsc" If you reboot, you will see the incorrect option of kvm-clock: $ cat /sys/devices/system/clocksource/clocksource0/current_clocksource kvm-clock There is a test kernel available in the below ppa, with the commit applied: https://launchpad.net/~mruffell/+archive/ubuntu/sf291501-test If you install the test kernel, and leave the kernel command line as: GRUB_CMDLINE_LINUX_DEFAULT="clocksource=tsc" You will get the requested clocksource: $ cat /sys/devices/system/clocksource/clocksource0/current_clocksource tsc + You will also get the following in dmesg: + + $ dmesg | grep defer + [ 1.002599] clocksource: Override clocksource tsc is not currently HRT compatible - deferring + [Regression Potential] This commit changes how the kernel treats clocksource overrides. If any users have an override set, but the kernel is clearing the override and returning to the default, when they install a patched kernel, they will change over to their requested override, which may come as a surprise. If there is a regression, it will only affect systems who have clocksource overrides in place, and in the worst case, will revert the system to its default clocksource if the selected clocksource override is found to be unstable. The commit is well tested, and should not cause any regressions.
-- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1894591 Title: clock: overriding the clocksource should select the requested clocksource To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1894591/+subscriptions -- ubuntu-bugs mailing list [email protected] https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
