non-invariant tsc and cputicker
avg at freebsd.org
Fri Dec 3 23:47:34 UTC 2010
on 03/12/2010 22:03 Jung-uk Kim said the following:
> On Friday 03 December 2010 01:14 pm, Andriy Gapon wrote:
>> on 03/12/2010 20:05 Jung-uk Kim said the following:
>>> 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:
>>>> What do you think?
>>> Why don't we just fix it properly?
>> Patch? :-)
I fail to see how this corrects the calculations (cpu tick accumulation) in
>> It seems that it is not too trivial to do and is prone to error
>> accumulation given how the ticks are added up.
>> Besides, why using a timecounter would not be a proper fix?
> Well, it is not that simple, unfortunately. Because init_TSC() is
> called very early, your patch will select dummy timecounter as a CPU
> ticker if my memory serves. It is very hard to implement right on
> x86 arch. :-(
I don't think that init_TSC() is called earlier than the code that probes CPU
features. After all, presence of TSC is another CPU feature.
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