Messages in this thread | | | Date | Fri, 20 Dec 2013 13:23:23 -0500 | From | Steven Rostedt <> | Subject | Re: [PATCH 09/13] sched: Add bandwidth management for sched_dl |
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On Fri, 20 Dec 2013 18:42:00 +0100 Peter Zijlstra <peterz@infradead.org> 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@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.
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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