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

[Ingo Molnar]

* Arjan van de Ven <arjan@xxxxxxxxxxxxx> 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 tsc no longer represents frequency, but is some fixed clock totally 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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/