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

[Steven Rostedt]

On Fri, 20 Dec 2013 18:42:00 +0100
Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:

> 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@xxxxxxxxxxxxx> 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.

I'm saying what stops this?


	CPU 0			CPU 1
	-----			-----
 sched_setattr()
 dl_overflow()
 cpus = __dl_span_weight()

			  cpu_down()
			  raw_spin_lock()
 raw_spin_lock() /* blocks */


			  overflow = __dl_overflow(cpus-1);
			  raw_spin_unlock();

 /* gets lock */
 __dl_overflow(cpus) /* all OK! */



			  /* cpus goes to cpus - 1 making
			     __dl_overflow() not OK anymore */


-- Steve

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