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    SubjectRe: nohz problem with idle time on old hardware
    On Wed, 13 Nov 2013, Matthew Whitehead wrote:

    > I was testing the 3.12 kernel on some _old_ hardware and I uncovered a bug.
    > It arises when nohz=on and goes away with nohz=off. On a crusty dual Pentium-1
    > system that is completely idle, the sar utility reports 0% idle time on cpu0
    > and 100% idle on cpu1. Cpu0 _should_ also be reporting 100% idle, but instead
    > it reports around 75% system time and 25% user time.
    >
    > The problem was diagnosed by Steve Rostedt with help from John
    > Stultz. The old system declares the dual TSCs unstable, and backs
    > down to a timesource of refined-jiffies. Apparently refined-jiffies
    > and jiffies are not a usable timesourcefor nohz, but we don't check
    > for that case because most modern systems have several reliable
    > hardware timesources.

    Wrong.

    > John suggested that we turn off nohz unless a usable hardware timesource is
    > present.

    nohz already depends on two things:

    1) A reliable clocksource which is valid for highres/nohz

    2) A per cpu clockevent device which supports one shot mode.

    and those are evaluated at runtime before we switch into NOHZ mode.

    And neither jiffies nor refined-jiffies qualify as valid clocksource.

    So there is something else wrong.

    Thanks,

    tglx



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