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

[David Miller]

From: Michal Soltys <soltys@xxxxxxxx>
Date: Thu, 25 Sep 2008 00:07:51 +0200

> - 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 ?

Basiclly you'll be able to use whatever resolution the hardware
can provide.  That is the only limit.

NO_HZ has effectively no influence, NO_HZ only kicks in when there
is no work for the CPU to do, so if you have an hrtimer pending
on a qdisc that will get serviced at the intended moment, the cpu
will come out of NO_HZ mode if necessary to do this.
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