| Message ID | 20200907041528.12540-1-matthew.ruffell@canonical.com |
|---|---|
| Headers | show |
| Series | clock: overriding the clocksource should select the requested clocksource | expand |
On Mon, Sep 07, 2020 at 04:15:27PM +1200, Matthew Ruffell wrote: > 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 <kwalker@redhat.com> > 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. > > Kyle Walker (1): > clocksource: Defer override invalidation unless clock is unstable > > kernel/time/clocksource.c | 15 ++++++++++++--- > 1 file changed, 12 insertions(+), 3 deletions(-) > > -- > 2.25.1 Applied to Xenial. William Breathitt Gray