linux-kernel - Re: [PATCH 09/13] sched: Add bandwidth management for sched

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Date:	Fri, 20 Dec 2013 18:42:00 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	tglx@...utronix.de, mingo@...hat.com, oleg@...hat.com,
	fweisbec@...il.com, darren@...art.com, johan.eker@...csson.com,
	p.faure@...tech.ch, linux-kernel@...r.kernel.org,
	claudio@...dence.eu.com, michael@...rulasolutions.com,
	fchecconi@...il.com, tommaso.cucinotta@...up.it,
	juri.lelli@...il.com, nicola.manica@...i.unitn.it,
	luca.abeni@...tn.it, dhaval.giani@...il.com, hgu1972@...il.com,
	paulmck@...ux.vnet.ibm.com, raistlin@...ux.it,
	insop.song@...il.com, liming.wang@...driver.com, jkacur@...hat.com
Subject: Re: [PATCH 09/13] sched: Add bandwidth management for sched_dl

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@...radead.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.
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