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    SubjectRe: Jiffies wraparound is not treated in the schedstats
    Date
    On 2006-11-09, Balbir Singh <balbir@in.ibm.com> wrote:
    > Path: news.gmane.org!not-for-mail
    > From: Balbir Singh <balbir@in.ibm.com>
    > Newsgroups: gmane.linux.kernel
    > Subject: Re: Jiffies wraparound is not treated in the schedstats
    > Date: Thu, 09 Nov 2006 11:59:45 +0530
    > Organization: IBM
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    > Original-To: Mauricio Lin <mauriciolin@gmail.com>
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    >
    > 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);}'
    > 4294967291

    So, according to this
    > perl -e '{printf("%lu\n", -5 );}'
    > 4294967291
    you have 32bit perl (and OS).

    I have same result in
    "This is perl, v5.8.4 built for i386-linux-thread-multi"

    (knoppix 3.2.3 in qemu on intel's amd64).

    That means we are on the same question i asked before about integer
    overflow. In this case "(1<<32) = 1",

    >> 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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