Date: Sat, 04 Dec 2010 01:47:30 +0200 From: Andriy Gapon <avg@freebsd.org> To: Jung-uk Kim <jkim@freebsd.org> Cc: freebsd-current@freebsd.org Subject: Re: non-invariant tsc and cputicker Message-ID: <4CF98192.3050909@freebsd.org> In-Reply-To: <201012031504.02532.jkim@FreeBSD.org> References: <4CF92852.20705@freebsd.org> <201012031305.53750.jkim@FreeBSD.org> <4CF93395.3060601@freebsd.org> <201012031504.02532.jkim@FreeBSD.org>
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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: >>>> http://people.freebsd.org/~avg/tsc-cputicker.diff >>>> >>>> What do you think? >>> >>> Why don't we just fix it properly? >> >> Patch? :-) > > Attached. I fail to see how this corrects the calculations (cpu tick accumulation) in !invariant_tsc case. >> 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. -- Andriy Gapon
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