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    SubjectRe: lost timer check in 2.6.7
    On Wed, Jun 16, 2004 at 01:19:12PM +0200, Andi Kleen wrote:
    > 2.6.7 has
    > + /* ... but give the TSC a fair chance */
    > + if (lost_count > 25)
    > + cpufreq_delayed_get();
    > While looking at porting this code to x86-64 I noticed that this only runs for
    > the first lost timer event.

    Not exactly: lost_count is increased for every "tick" where lost ticks are
    detected, but re-set anytime there is no lost tick.

    > In case of dynamic frequency which varies shouldn't this
    > be more like
    > if ((lost_count % 25) == 0)
    > cpufreq_delayed_get();
    > ? Otherwise this heuristic will only work once.

    So if this heuristic works, no more ticks will be lost (at least in the near
    future), so lost_count will be set to zero again. If, due to some new event,
    ticks are lost again, lost_count will start at zero, reach 25 after some
    time, and cpufreq_delayed_get() will be called again.

    However: inaccuracies are only detected in _one_ direction: ticks being
    missed, not "too many ticks" -- and only if it's a factor of two or
    higher... Probably a better run-time check of the sanity of a timesource is
    needed in future... on the other hand, frequencies shouldn't change behind
    the kernel's back. That's what my other patch sent to the ACPI list a few
    days ago (pstate_cnt) tries to address.

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