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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);}'
4294967291
perl -e '{printf("%lu\n", -5 );}'
4294967291

> 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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