[PATCH v2 0/7] Various tsc/clocksource fixes

[Peter Zijlstra <peterz@infradead.org>]

These patches are the result of the regression Diego reported against commit:

  aa83c45762a2 ("x86/tsc: Introduce early tsc clocksource")

His Core2 triggers mark_tsc_unstable() while we're running with tsc-early,
which then results in us failing to unregister the clocksource.

Furthermore, since mark_tsc_unstable() only looks at the regular tsc
clocksource, we don't even mark tsc_early UNSTABLE or de-rate it, resulting in
it actually staying a desirable clocksource.

Fixing that is a little more tricky; albeit far less critical after we
guarantee to unregister tsc-early.

Please consider these for /urgent. I've used both Fixes and stable tags
on a bunch of them because the above commit is already in v4.16 and
would need stable help.

These patches have been tested on both a Core2 and an IVB. On IVB I've both
changed intel_idle to always report the TSC unstable and mucked about with the
TSC msr from userspace to tickle the watchdog paths.

---
Changes since v1

 - fix __clocksource_change_rating() locking
 - removed work->kthread->watchdog hoops
 - fixed up comment