questions regarding qdiscs, watchdogs, hrtimers and [NO_] HZ

[Michal Soltys]

After peeking a bit over sch_* code, I have a few basic, possibly weird, 
questions. In the past (and conforming to e.g. tbf's man and comment in 
sch_tbf.c) the watchdogs used in schedulers like tbf, htb, hfsc, ... 
were all limited to 1/HZ resolution. After switching to hrtimers it's 
not the case anymore, still:

- what is the current resolution allowed by hrtimer_start ? During 
simple tests with tbf I could easily schedule at pretty much any rate 
(tested up to 100mbit) with just a packet sized token bucket. And that 
regardless of selected HZ and/or NO_HZ setting. Quite awesome.

- in case of some older hardware - I assume the functionality falls back 
to the old behavior, so - overall accuracy is still 1.024us (judging 
from comments, and contrary to older kernels where jiffies/ 
gettimeofday/ timestamp could be selected) - but allowed watchdog 
resolution remains 1/HZ ? Also - what is NO_HZ's influence (if any) in 
such scenario ?

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