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    SubjectRe: [RFC PATCH] x86:Use cpu_khz for loops_per_jiffy calculation
    On Tue, Jun 3, 2008 at 6:20 PM, Arjan van de Ven <arjan@infradead.org> wrote:
    > On Tue, 03 Jun 2008 17:41:09 -0700
    > Alok Kataria <akataria@vmware.com> wrote:
    >
    >> On X86 platform we can use the value of cpu_khz computed during tsc
    >> calibration to calculate the loops_per_jiffy value. Its very
    >> important to keep the error in lpj values to minimum as any error in
    >> that may result in kernel panic in check_timer.
    >> In virtualization environment on a highly overloaded host, the guest
    >> delay calibration may sometimes result in errors beyond the ~50% that
    >> timer_irq_works can handle, resulting in the guest panicking.
    > \
    >
    > can you guarantee that the rate tsc ticks at is the same as the current
    > CPU frequency? Answer: You can't....
    >

    I think at the boot time atleast we can assume that, no ?
    If you are referring to the cpu frequency changing at run time (aka
    dynamic frequency scaling),
    in that case the time_cpufreq_notifier should take care of updating
    the loops_per_jiffy
    value for that corresponding cpu.

    > sadly we do need to calibrate this...
    >
    > In addition, clearly you can have different cpus in a system run at a
    > different rate (both in terms of cpu_khz and, independently, in terms
    > of tsc rate)
    >

    Again yes at run time frequency's may change but they shouldn't at boottime.
    AFAIK, i don't think there are X86 MP systems with asymmetric cpus
    i.e. systems with different
    base frequencies. If thats not true then there sure is a problem.

    Thanks,
    Alok

    >
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