Re: [PATCH 2/3] sched: enforce per-cpu utilization limits on runtime balancing

[Peter Zijlstra <peterz@infradead.org>]

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.