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SubjectRe: [RFC PATCH -v2.1] x86: Kill notsc
On Tue, 17 Nov 2015, Borislav Petkov wrote:
> On Tue, Nov 17, 2015 at 10:11:03AM +0100, Thomas Gleixner wrote:
> > There is an interesting problem:
> >
> > tsc_init()
> > {
> > tsc_khz = x86_platform.calibrate_tsc();
> > if (!tsc_khz) {
> > mark_tsc_unstable("could not calculate TSC khz");
> > ...
> > }
> >
> > In the current code we do NOT use TSC for sched_clock() and that's
> > correct as we have no idea what the TSC frequency is.
> >
> > With your changes that is not longer the case, so you end up with a
> > completely wreckaged sched clock.
>
> Hmm, I see it.
>
> Well, I had a related splat which bombed because cyc2ns_init() had to
> run before we do sched_init()->idle_init() which called sched_clock()
> and there we did the cycles_2_ns() thing and the percpu vars weren't
> initialized yet.
>
> That's why I did this:
>
> +void __init early_tsc_init(void)
> +{
> + int cpu;
> +
> + /*
> + * We need to init the cycles to ns conversion machinery because
> + * init_idle() below will call sched_clock() which needs it.
> + */
> + for_each_possible_cpu(cpu)
> + cyc2ns_init(cpu);
> +}
>
> Maybe I should move tsc_init() before sched_init() so that the
> calibration happens before we use the TSC in sched_clock() for the first
> time...?

How does that help if your TSC frequency is 0 after the calibration?

Thanks,

tglx


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