Re: TSC improvement

[Taylor R Campbell <campbell+netbsd-tech-kern%NetBSD.org@localhost>]

It's great to see improvements to our calibration of the TSC (and I
tend to agree that cpu_counter should be serializing, so that, e.g.,
cpu_counter(); ...; cpu_counter() reliably measures time taken in the
ellipsis).

At the same time, I wonder whether we should _also_:

1. Modify the tsc timecounter so that it uses a global atomic to
   ensure that there is a global view of time as counted by the tsc.
   This is what the timecounter(9) API per se expects of timecounters,
   and right now tsc (along with various other per-CPU cycle counters)
   fails to guarantee that.

2. Introduce an API for a local timecounter -- a per-CPU timecounter
   that never goes backwards on a single CPU, but:
   (a) measures units of wall clock time, unlike cpu_counter();
   (b) need not be synchronized between CPUs; and
   (c) may be cheaper to read than a global timecounter.
   We could then use that for, e.g., rusage calculations.