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    SubjectRe: [PATCH] sched: fix/optimise calculation of weight-inverse

    * Stephan Bärwolf <stephan.baerwolf@tu-ilmenau.de> wrote:

    > If the inverse loadweight should be zero, function "calc_delta_mine"
    > calculates the inverse of "lw->weight" (in 32bit integer ops).
    >
    > This calculation is actually a little bit impure (because it is
    > inverting something around "lw-weight"+1), especially when
    > "lw->weight" becomes smaller. (This could explain some aritmetical
    > issues for small shares...)
    >
    > The correct inverse would be 1/lw->weight multiplied by
    > "WMULT_CONST" for fixcomma-scaling it into integers.
    > (So WMULT_CONST/lw->weight ...)
    >
    > For safety it is also important to check if division by zero
    > could happen...
    >
    > The old, impure algorithm took two divisions for inverting lw->weight,
    > the new, more exact one only takes one and an additional unlikely-if.
    >
    > Signed-off-by: Stephan Baerwolf <stephan.baerwolf@tu-ilmenau.de>
    > ---
    > kernel/sched.c | 12 +++++++++---
    > 1 files changed, 9 insertions(+), 3 deletions(-)
    >
    > diff --git a/kernel/sched.c b/kernel/sched.c
    > index 312f8b9..bb55996 100644
    > --- a/kernel/sched.c
    > +++ b/kernel/sched.c
    > @@ -1307,15 +1307,21 @@ calc_delta_mine(unsigned long delta_exec,
    > unsigned long weight,
    > {
    > u64 tmp;
    >
    > + tmp = (u64)delta_exec * weight;
    > +
    > + // actually we would have to trap - division by zero - but we stay
    > and pretend the limit of the operation...
    > + if (unlikely(lw->weight == 0)) {
    > + if (unlikely(tmp == ((u64)0))) return (unsigned long)0;
    > + else return (unsigned long)LONG_MAX;

    Can lw->weight ever be zero here? I dont think so - and if it is then getting a
    kernel crash there is preferred to hiding it.

    Once we do that your patch becomes a lot simpler.

    > + }
    > +
    > if (!lw->inv_weight) {
    > if (BITS_PER_LONG > 32 && unlikely(lw->weight >= WMULT_CONST))
    > lw->inv_weight = 1;
    > else
    > - lw->inv_weight = 1 + (WMULT_CONST-lw->weight/2)
    > - / (lw->weight+1);
    > + lw->inv_weight = WMULT_CONST / lw->weight;

    hm, i definitely think there was a rounding reason for that - but apparently
    i'm an idiot who does not add comments to non-obvious code! :-)

    Peter, do you remember this?

    > }
    >
    > - tmp = (u64)delta_exec * weight;

    I agree that moving this multiplication early in the sequence is better for
    micro-performance regardless of the lw->weight optimization you do: it can be
    executed in parallel.

    Thanks,

    Ingo
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