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Date: Sat, 04 Dec 2010 13:12:10 +0200
From: Andriy Gapon <avg@freebsd.org>
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To: Jung-uk Kim <jkim@freebsd.org>
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Cc: freebsd-current@freebsd.org
Subject: Re: non-invariant tsc and cputicker
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on 04/12/2010 02:38 Jung-uk Kim said the following:
> If my understanding is correct, your patch uses the dummy timecounter 
> until a real timecounter is chosen.

Perhaps this is one way to look at it.
But I look at it differently - the patch makes cpu_ticks refer to tc_cpu_ticks.
That is, it make _the_ timecounter be used for cpu ticks.
Exact timecounter backend is not important to me.

> When a real timecounter is set, 
> tc_cpu_ticks() changes the frequency naturally.  How are you going to 
> solve this problem?

Do we really care about cpu ticks accounting that early in the boot?

>  What should we do when a user set a new 
> timecounter hardware via "sysctl kern.timecounter.hardware"?

User can expect some instability (*if any*) when he does such a significant
system reconfiguration.
I put "if any", because I think that tc_cpu_ticks() should handle this.
The same way as you don't see time returned by e.g. nanotime() going crazy at
that moment.

> I don't 
> think it is any better than current code.  Am I missing 
> something? :-(

I think that it is much better.
Handling of P-state changes for non-invariant TSC is just incorrect.
kern.timecounter.hardware is not going to be changed as frequently as P-states,
if ever.

-- 
Andriy Gapon