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    SubjectRe: [PATCH 2/3] sched: enforce per-cpu utilization limits on runtime balancing
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    On Tue, 2010-02-23 at 19:56 +0100, Fabio Checconi wrote:
    > +static u64 from_ratio(unsigned long ratio, u64 period)
    > +{
    > + return (ratio * period) >> 20;
    > +}
    > +
    > +/*
    > + * Try to move *diff units of runtime from src to dst, checking
    > + * that the utilization does not exceed the global limits on the
    > + * destination cpu. Returns true if the migration succeeded, leaving
    > + * in *diff the actual amount of runtime moved, false on failure, which
    > + * means that no more bandwidth can be migrated to rt_rq.
    > + */
    > +static int rt_move_bw(struct rt_rq *src, struct rt_rq *dst,
    > + s64 *diff, u64 rt_period)
    > +{
    > + struct rq *rq = rq_of_rt_rq(dst), *src_rq = rq_of_rt_rq(src);
    > + struct rt_edf_tree *dtree = &rq->rt.rt_edf_tree;
    > + struct rt_edf_tree *stree = &src_rq->rt.rt_edf_tree;
    > + unsigned long bw_to_move;
    > + int ret = 0;
    > +
    > + double_spin_lock(&dtree->rt_bw_lock, &stree->rt_bw_lock);
    > +
    > + if (dtree->rt_free_bw) {
    > + bw_to_move = to_ratio(rt_period, *diff);
    > + if (bw_to_move > dtree->rt_free_bw) {
    > + bw_to_move = dtree->rt_free_bw;
    > + *diff = from_ratio(bw_to_move, rt_period);
    > + }
    > +
    > + stree->rt_free_bw -= bw_to_move;
    > + dtree->rt_free_bw += bw_to_move;
    > + ret = 1;
    > + }
    > +
    > + double_spin_unlock(&dtree->rt_bw_lock, &stree->rt_bw_lock);
    > +
    > + return ret;
    > +}

    The from_ratio() stuff smells like numerical instability for
    ->rt_free_bw, I can't see anything that would, given sufficient balance
    cycles keep the sum of rt_free_bw over the cpus equal to what it started
    out with.



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