linux-kernel - Re: 2.6.24-rc4-git5: Reported regressions from 2.6.23

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Date:	Tue, 11 Dec 2007 09:43:24 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	Guillaume Chazarain <guichaz@...oo.fr>,
	Stefano Brivio <stefano.brivio@...imi.it>,
	Andrew Morton <akpm@...ux-foundation.org>, rjw@...k.pl,
	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org
Subject: Re: 2.6.24-rc4-git5: Reported regressions from 2.6.23


* Arjan van de Ven <arjan@...radead.org> wrote:

> > That sounds like a big problem.
> 
> it'll get way worse going forward. (but even on todays systems, the 
> tsc no longer represents frequency, but is some fixed clock totally 
> unrelated to cpu frequency)

X86_FEATURE_CONSTANT_TSC CPUs (all modern Intel CPUs) should be fine - 
we dont do any TSC frequency fixups for them. The loops_per_jiffy fixup 
looks like this:

                if (!(freq->flags & CPUFREQ_CONST_LOOPS))
                        cpu_data(freq->cpu).loops_per_jiffy =
                                cpufreq_scale(loops_per_jiffy_ref,
                                                ref_freq, freq->new);

i.e. X86_FEATURE_CONSTANT_TSC excluded. The sched_clock() scaling factor 
is modified like this:

                        if (!(freq->flags & CPUFREQ_CONST_LOOPS)) {
                                tsc_khz = cpu_khz;
                                preempt_disable();
                                set_cyc2ns_scale(cpu_khz, smp_processor_id());

so here X86_FEATURE_CONSTANT_TSC is excluded again. So the whole 
frequency scaling issue will become a pure legacy issue only with time.

	Ingo
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