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    SubjectRe: sched_clock() uses are broken
    Nicolas Pitre wrote:
    > On Tue, 2 May 2006, Russell King wrote:
    >
    >
    >>On Tue, May 02, 2006 at 06:43:45PM +0200, Andi Kleen wrote:
    >>
    >>>Russell King <rmk+lkml@arm.linux.org.uk> writes:
    >>>
    >>>>However, this is not the case. On x86 with TSC, it returns a 54 bit
    >>>>number. This means that when t1 < t0, time_passed_ns becomes a very
    >>>>large number which no longer represents the amount of time.
    >>>
    >>>Good point. For a 1Ghz system this would happen every ~0.57 years.
    >>>
    >>>The problem is there is AFAIK no non destructive[1] way to find out how
    >>>many bits the TSC has
    >>>
    >>>Destructive would be to overwrite it with -1 and see how many stick.
    >>>
    >>>
    >>>>All uses in kernel/sched.c seem to be aflicted by this problem.
    >>>>
    >>>>There are several solutions to this - the most obvious being that we
    >>>>need a function which returns the nanosecond difference between two
    >>>>sched_clock() return values, and this function needs to know how to
    >>>>handle the case where sched_clock() has wrapped.
    >>>
    >>>Ok it can be done with a simple test.
    >
    >
    > Better yet the sched_clock() implementation just needs to return a value
    > shifted left so the wrap around always happens on 64 bits and the
    > difference between two consecutive samples is always right.
    >
    >
    >>>>IOW:
    >>>>
    >>>> t0 = sched_clock();
    >>>> /* do something */
    >>>> t1 = sched_clock();
    >>>>
    >>>> time_passed = sched_clock_diff(t1, t0);
    >>>>
    >>>>Comments?
    >>>
    >>>Agreed it's a problem, but probably a small one. At worst you'll get
    >>>a small scheduling hickup every half year, which should be hardly
    >>>that big an issue.
    >
    >
    > ... on x86 that is.
    >
    >
    >>>Might chose to just ignore it with a big fat comment?
    >>
    >>You're right assuming you have a 64-bit TSC, but ARM has at best a
    >>32-bit cycle counter which rolls over about every 179 seconds - with
    >>gives a range of values from sched_clock from 0 to 178956970625 or
    >>0x29AAAAAA81.
    >>
    >>That's rather more of a problem than having it happen every 208 days.
    >
    >
    > Yet that counter isn't necessarily nanosecond based. So rescaling the
    > returned value to nanosecs requires expensive divisions which could be
    > done only once within sched_clock_diff() instead of twice as often in
    > each sched_clock() calls.

    Oh phooey!! Scaling can be done with a mpy and a shift. See the new clock
    code where the TSC (or what ever) is scaled to ns.


    --
    George Anzinger george@wildturkeyranch.net
    HRT (High-res-timers): http://sourceforge.net/projects/high-res-timers/
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