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

[Peter Zijlstra <peterz@infradead.org>]

On Wed, 2010-03-03 at 18:00 +0100, Fabio Checconi wrote:
> > 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;
> }

OK, I think that should work, add a little comment on why we're doing it
this way and all should be well ;-)