callouts precision

[Andriy Gapon]

Just want to double-check myself.
It seems that currently, thanks to event timers, we mostly should be able to
schedule a hardware timer to fire at almost arbitrary moment with very fine
precision.
OTOH, our callout subsystem still seems to be completely tick oriented in the
sense that all timeouts are specified and kept in ticks.
As a result, it's impossible to use e.g. nanosleep(2) with a precision better
than HZ.

How deeply ticks are ingrained into callout(9)?  Are they used only as a measure
of time?  Or are there any dependencies on them being integers, like for
indexing, etc?
In other words, how hard it would be to replace ticks with e.g. bintime as an
internal representation of time in callout(9) [leaving interfaces alone for the
start]?  Is it easier to retrofit that code or to replace it with something new?

Thank you.
-- 
Andriy Gapon