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SubjectRe: [PATCH 2/3] sched: enforce per-cpu utilization limits on runtime balancing
> From: Peter Zijlstra <peterz@infradead.org>
> Date: Thu, Feb 25, 2010 09:28:25PM +0100
>
> 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.

You're right... What would you think about the following solution?
It just keep tracks of the bw accounted for every rt_rq when it is
updated, and that should be enough to avoid accumulating the errors.

static inline void rt_update_bw(struct rt_rq *rt_rq, struct rt_edf_tree *tree,
s64 diff, u64 rt_period)
{
unsigned long bw;
rt_rq->rt_runtime += diff;
bw = to_ratio(rt_period, rt_rq->rt_runtime);
tree->rt_free_bw += bw - rt_rq->rt_bw;
rt_rq->rt_bw = bw;
}
static bool 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;
bool ret = false;
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)
*diff = from_ratio(dtree->rt_free_bw, rt_period);
if (*diff) {
rt_update_bw(src, stree, -(*diff), rt_period);
rt_update_bw(dst, dtree, *diff, rt_period);
ret = true;
}
}
double_spin_unlock(&dtree->rt_bw_lock, &stree->rt_bw_lock);

return ret;
}

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