[SRU][X][PATCH 0/1] clock: overriding the clocksource should select the requested clocksource

[Matthew Ruffell]

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 at 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