** Changed in: linux (Ubuntu Xenial)
Status: In Progress => Fix Committed
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https://bugs.launchpad.net/bugs/1894591
Title:
clock: overriding the clocksource should select the requested
clocksource
Status in linux package in Ubuntu:
Fix Released
Status in linux source package in Xenial:
Fix Committed
Bug description:
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.
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