Mauricio Lin wrote:
> 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
No. Learn about the modulo arithmetic properties of the unsigned types.
> if (time_after(now, t->sched_info.last_queued))
Have you looked at time_after? It just performs a subtraction, after
casting both operands to long. It does not care which one is bigger than
the other. Basically, it performs the same calculation as the
delta_jiffies above, and then checks the sign bit.
> delta_jiffies = now - t->sched_info.last_queued;
> else
> delta_jiffies = (MAX_JIFFIES - t->sched_info.last_queued) + now
I'm looking at a 2.6.17 kernel, and I can't find MAX_JIFFIES, or
JIFFIES_MAX. There is a MAX_JIFFY_OFFSET, which is something else. It's
the biggest delta that you can add to a value X so that time_after(X, X
+ delta) is still true. If you try to schedule something beyond now +
MAX_JIFFY_OFFSET, it will be put back in time.
Also, your calculation is buggy. MAX_JIFFIES would correspond to
ULONG_MAX. What you want is:
(ULONG_MAX + 1 - last_queued + now)
This expression is now mathematically congruent to
-last_queued + now (mod ULONG_MAX + 1)
or
now - last_queued (mod ULONG_MAX + 1).
The arithmetic of the unsigned long type is /already/ carried out modulo
ULONG_MAX + 1, so the ULONG_MAX + 1 term can be dropped from your
expression, leaving it as
now - last_queued
> 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.
What user space tool?
> Any comments?
The user space tool is perhaps buggy, or you are misinterpreting its
output.
> Can I provide a patch for that?
Some day, when you learn how to program.
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