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 09:28:23PM +0100
> >
> > On Tue, 2010-02-23 at 19:56 +0100, Fabio Checconi wrote:
> > > +#ifdef CONFIG_SMP
> > > +static inline unsigned long rt_init_free_bw(void)
> > > +{
> > > +       unsigned long used = to_ratio(global_rt_period(), global_rt_runtime());
> > > +
> > > +       return to_ratio(RUNTIME_INF, RUNTIME_INF) - used;
> > > +}
> > > +#endif
> > 
> > > +static void __rt_restart_balancing(void)
> > > +{
> > > +       unsigned long used, global, free;
> > > +       struct rq *rq;
> > > +       int i;
> > > +
> > > +       used = rt_used_bandwidth();
> > > +       global = to_ratio(RUNTIME_INF, RUNTIME_INF);
> > > +
> > > +       free = global - used;
> > 
> > 
> > We take the max as RUNTIME_INF instead of global_rt_* so that we can
> > move runtime around and fully saturate a single cpu (given there is
> > enough free to compensate on other cpus) ?
> 
> The only reason I've used RUNTIME_INF instead of global_rt_* is for the
> !GROUP_SCHED case, where using the global_rt_* values would make balancing
> have no effect at all (the initial value for def_rt_bandwidth already
> uses the maximum bw on each cpu) .  The current throttling implementation
> in this case still tries to concentrate bw on a single cpu, and I wanted
> to replicate the same behaviour.
> 
> Should I go for the global_rt_* values and add some #ifdef unreadability
> to avoid the balancing overhead in the !GROUP_SCHED case?


Nah, but adding a comment to clarify this might help.. I only asked
because it was not immediately obvious.