Date: Fri, 03 Dec 2010 20:14:45 +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: <4CF93395.3060601@freebsd.org> In-Reply-To: <201012031305.53750.jkim@FreeBSD.org> References: <4CF92852.20705@freebsd.org> <201012031305.53750.jkim@FreeBSD.org>
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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? :-) 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? >> 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. Cool! -- Andriy Gapon
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