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    SubjectRe: Jiffies wraparound is not treated in the schedstats
    Mauricio Lin wrote:
    > Hi Balbir,
    > Do you know why in the sched_info_arrive() and sched_info_depart()
    > functions the calculation of delta_jiffies does not use the time_after
    > or time_before macro to prevent the miscalculation when jiffies
    > overflow?
    > For instance the delta_jiffies variable is simply calculated as:
    > delta_jiffies = now - t->sched_info.last_queued;
    > Do not you think the more logical way should be
    > if (time_after(now, t->sched_info.last_queued))
    > delta_jiffies = now - t->sched_info.last_queued;
    > else
    > delta_jiffies = (MAX_JIFFIES - t->sched_info.last_queued) + now

    What's MAX_JIFFIES? Is it MAX_ULONG? jiffies is unsigned long
    so you'll have to be careful with unsigned long arithmetic.

    Consider that now is 5 and t->sched_info.last_queued is 10.

    On my system

    perl -e '{printf("%lu\n", -5 + (1<<32) - 1);}'

    perl -e '{printf("%lu\n", -5 );}'

    > I have included more variables to measure some issues of schedule in
    > the kernel (following schedstat idea) and I noticed that jiffies
    > wraparound has led to wrong values, since the user space tool when
    > collecting the values is producing negative values.

    hmm.. jiffies wrapped around in sched_info_depart()? I've never seen
    that happen. Could you post the additions and user space tool to look at?
    What additional features are you planning to measure in the scheduler?

    > Any comments?
    > Can I provide a patch for that?

    Please feel free to provide patches, this is open source!!

    > BR,
    > Mauricio Lin.


    Balbir Singh,
    Linux Technology Center,
    IBM Software Labs
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