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    SubjectRe: [RFC PATCH -v2.1] x86: Kill notsc
    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...?

    --
    Regards/Gruss,
    Boris.

    ECO tip #101: Trim your mails when you reply.


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