Re: [PATCH RFC] sched/deadline: support dl task migrate during cpu hotplug

[Wanpeng Li]

Hi Peter,
On 14/11/4 äå6:10, Peter Zijlstra wrote:
> On Tue, Nov 04, 2014 at 04:23:45PM +0800, Wanpeng Li wrote:
> > On Tue, Nov 04, 2014 at 09:32:25AM +0100, Peter Zijlstra wrote:
> > > On Tue, Nov 04, 2014 at 07:57:48AM +0800, Wanpeng Li wrote:
> > > > On Mon, Nov 03, 2014 at 11:41:11AM +0100, Peter Zijlstra wrote:
> > > > > On Fri, Oct 31, 2014 at 03:28:17PM +0800, Wanpeng Li wrote:
> > > > > So what is wrong with making dl_task_timer() deal with it? The timer
> > > > > will still fire on the correct time, canceling it and or otherwise
> > > > > messing with the CBS is wrong. Once it fires, all we need to do is
> > > > > migrate it to another cpu (preferably one that is still online of course
> > > > > :-).
> > > > Do you mean what I need to do is push the task to another cpu in dl_task_timer()
> > > > if rq is offline?
> > > That does indeed appear to be the sensible fix to me.
> > >
> > > > In addition, what will happen if dl task can't preempt on
> > > > another cpu?
> > > So if we find that the rq the task was on is no longer available, we
> > > need to select a new rq, the 'right' rq would be the one running the
> > > latest deadline.
> > >
> > > If it cannot preempt the latest (running) deadline, it was not eligible
> > > for running in the first place so no worries, right?
> > I think this will lead to this deadline task cannot running on any rqs any more.
> > If my understanding is not right, when it will be picked?
> So you unconditionally place it on the rq with the latest deadline. If
> it cannot preempt, at least its on an online cpu. It will get scheduled
> whenever its deadline is one of the N earliest, with N the number of
> online CPUs.

If something like this make sense?

(Will test it tomorrow and send out a formal one)
diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index f3d7776..dac33d1 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -553,6 +553,41 @@ again:
push_dl_task(rq);
#endif
}
+
+ /*
+ * So if we find that the rq the task was on is no longer
+ * available, we need to select a new rq, the 'right' rq
+ * would be the one running the latest deadline.
+ *
+ * If it cannot preempt, at least it's on an online cpu. It
+ * will get scheduled whenever its deadline is one of the N
+ * earliest, with N the number of online CPUs.
+ */
+
+ if (!rq->online) {
+ struct rq *latest_rq = NULL;
+ int cpu;
+ u64 dmin = LONG_MAX;
+
+ for_each_cpu(cpu, &p->cpus_allowed)
+ if (cpu_online(cpu) &&
+ cpu_rq(cpu)->dl.earliest_dl.curr < dmin) {
+ latest_rq = cpu_rq(cpu);
+ dmin = latest_rq->dl.earliest_dl.curr;
+ }
+
+ if (!latest_rq)
+ goto unlock;
+
+ raw_spin_lock(&latest_rq->lock);
+
+ deactivate_task(rq, p, 0);
+ set_task_cpu(p, latest_rq->cpu);
+ activate_task(latest_rq, p, 0);
+
+ raw_spin_unlock(&latest_rq->lock);
+ }
+
unlock:
raw_spin_unlock(&rq->lock);

Regards,
Wanpeng Li

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