Instrumenting context switches

[Frank Ch. Eigler]

> [...]
>         doSwitchTo(gettimeofday_us(), $prev_p, $next_p);
> [...]

It may be that this problem is due to the recent rewrite of
gettimeofday_us.  That code contains bits like preempt_disable() and
_enable(), even though the equivalent (interrupt disabling) should
already be done within probe context.  In particular, I wonder if
changing the latter to preempt_enable_no_resched() might improve the
situation.

- FChE