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    Subject[58/80] sched, x86: Avoid unnecessary overflow in sched_clock
    3.0-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Salman Qazi <sqazi@google.com>

    commit 4cecf6d401a01d054afc1e5f605bcbfe553cb9b9 upstream.

    (Added the missing signed-off-by line)

    In hundreds of days, the __cycles_2_ns calculation in sched_clock
    has an overflow. cyc * per_cpu(cyc2ns, cpu) exceeds 64 bits, causing
    the final value to become zero. We can solve this without losing
    any precision.

    We can decompose TSC into quotient and remainder of division by the
    scale factor, and then use this to convert TSC into nanoseconds.

    Signed-off-by: Salman Qazi <sqazi@google.com>
    Acked-by: John Stultz <johnstul@us.ibm.com>
    Reviewed-by: Paul Turner <pjt@google.com>
    Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
    Link: http://lkml.kernel.org/r/20111115221121.7262.88871.stgit@dungbeetle.mtv.corp.google.com
    Signed-off-by: Ingo Molnar <mingo@elte.hu>
    Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

    ---
    arch/x86/include/asm/timer.h | 23 ++++++++++++++++++++++-
    1 file changed, 22 insertions(+), 1 deletion(-)

    --- a/arch/x86/include/asm/timer.h
    +++ b/arch/x86/include/asm/timer.h
    @@ -32,6 +32,22 @@ extern int no_timer_check;
    * (mathieu.desnoyers@polymtl.ca)
    *
    * -johnstul@us.ibm.com "math is hard, lets go shopping!"
    + *
    + * In:
    + *
    + * ns = cycles * cyc2ns_scale / SC
    + *
    + * Although we may still have enough bits to store the value of ns,
    + * in some cases, we may not have enough bits to store cycles * cyc2ns_scale,
    + * leading to an incorrect result.
    + *
    + * To avoid this, we can decompose 'cycles' into quotient and remainder
    + * of division by SC. Then,
    + *
    + * ns = (quot * SC + rem) * cyc2ns_scale / SC
    + * = quot * cyc2ns_scale + (rem * cyc2ns_scale) / SC
    + *
    + * - sqazi@google.com
    */

    DECLARE_PER_CPU(unsigned long, cyc2ns);
    @@ -41,9 +57,14 @@ DECLARE_PER_CPU(unsigned long long, cyc2

    static inline unsigned long long __cycles_2_ns(unsigned long long cyc)
    {
    + unsigned long long quot;
    + unsigned long long rem;
    int cpu = smp_processor_id();
    unsigned long long ns = per_cpu(cyc2ns_offset, cpu);
    - ns += cyc * per_cpu(cyc2ns, cpu) >> CYC2NS_SCALE_FACTOR;
    + quot = (cyc >> CYC2NS_SCALE_FACTOR);
    + rem = cyc & ((1ULL << CYC2NS_SCALE_FACTOR) - 1);
    + ns += quot * per_cpu(cyc2ns, cpu) +
    + ((rem * per_cpu(cyc2ns, cpu)) >> CYC2NS_SCALE_FACTOR);
    return ns;
    }




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