* Arjan van de Ven <[email protected]> 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 tsc no longer represents frequency, but is some fixed clock totally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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