Re: [PATCH 09/13] sched: Add bandwidth management for sched_dl

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Dec 20, 2013 at 12:37:07PM -0500, Steven Rostedt wrote:
> On Fri, 20 Dec 2013 18:13:43 +0100
> Peter Zijlstra <peterz@infradead.org> wrote:
> 
> 
> > @@ -4985,6 +4942,23 @@ migration_call(struct notifier_block *nf
> >  	unsigned long flags;
> >  	struct rq *rq = cpu_rq(cpu);
> >  
> > +	switch (action) {
> > +	case CPU_DOWN_PREPARE: /* explicitly allow suspend */
> > +		{
> > +			struct dl_bw *dl_b = dl_bw_of(cpu);
> > +			int cpus = dl_bw_cpus(cpu);
> > +			bool overflow;
> > +
> > +			raw_spin_lock_irqsave(&dl_b->lock, flags);
> > +			overflow = __dl_overflow(dl_b, cpus-1, 0, 0);
> > +			raw_spin_unlock_irqrestore(&dl_b->lock, flags);
> > +
> > +			if (overflow)
> > +				return notifier_from_errno(-EBUSY);
> 
> Is it possible to have a race here to create a new deadline task that
> may work with cpus but not cpus-1? That is, if a new deadline task is
> currently being created as a CPU is going offline, this check happens
> first while the creation is spinning on the dl_b->lock, and it sets
> overflow to false, then once the lock is released, the new deadline
> task makes the condition true.
> 
> Should the system call have a get_online_cpus() somewhere?

No, should be all good; the entire admission control is serialized by
that dl_b->lock, and its a raw_spin_lock (as can be seen from the above)
which already very much excludes hotplug.