Date: Sat, 04 Dec 2010 13:12:10 +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: <4CFA220A.30405@freebsd.org> In-Reply-To: <201012031938.12684.jkim@FreeBSD.org> References: <4CF92852.20705@freebsd.org> <201012031504.02532.jkim@FreeBSD.org> <4CF98192.3050909@freebsd.org> <201012031938.12684.jkim@FreeBSD.org>
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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
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