[patch 0/2] timekeeping: NMI safe timekeeper enhancements

Thomas Gleixner
2020-08-14T11:56:26Z

printk intends to store various timestamps (MONOTONIC, REALTIME, BOOTTIME)
to make correlation of dmesg accross different machines easier.

The NMI safe timekeeper allows to retrieve these timestamps from any
context, but it lacks a few things:

1) The nmi safe accessors are not providing time stamps until timekeeping
is initialized during early boot.

This can be mitigated by using sched clock up to the point where time-
keeping becomes available. This has no side effects because clock
monotonic takes sched clock into account at initialization time
anyway. So no random time jumps are possible.

If early sched clock is not available then there is no difference
either, obviously. Both return 0.

2) It requires a new accessor which allows to retrieve all three clock
timestamps in one go.

Trivial excercise. But there are a few twists:

A) Access to boot time can be racy if the sleep time offset on resume
is injected after timekeeping resume. That's the case when the RTC
or whatever is used to calculate sleep time is not availble when
the timekeeping core is resumed.

B) Timestamps are frozen accross the very inner low level
suspend/resume path. Not a big problem, but might affect the
developer debug printks.

A detailed description of these two points is in the changelog of
patch 2.

Thanks,

tglx