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From: Jung-uk Kim <jkim@FreeBSD.org>
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Date: Fri, 3 Dec 2010 13:05:50 -0500
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Cc: Andriy Gapon <avg@freebsd.org>
Subject: Re: non-invariant tsc and cputicker
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On Friday 03 December 2010 12:26 pm, Andriy Gapon wrote:
> FreeBSD uses cpu_ticks [function pointer] in a few places for a few
> things like process CPU time accounting.  On x86 cpu_ticks always
> points to rdtsc. If TSC is not invariant that leads to incorrect
> accounting of "CPU ticks". The code pretends to try to handle
> changing cpufreq levels, but does that incorrectly.

Arg...  Probably it is my fault. :-(

> I think that we could use a selected timecounter instead of "raw"
> TSC if the latter is not invariant.  In this case cpu_ticks calls
> would be slightly costlier, but always correct.
>
> The change is quite trivial:
> http://people.freebsd.org/~avg/tsc-cputicker.diff
>
> What do you think?

Why don't we just fix it properly?

> P.S. it's probably a good idea to merge i386 and amd64 tsc.c files
> into a common x86 version, which would be the same as i386 version,
> which seems to be generic enough.

Agreed.

Thanks,

Jung-uk Kim