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

[Wanpeng Li]

On 14/11/4 äå9:30, Wanpeng Li wrote:
> 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);

wow, sorry for mess format.

> Regards,
> Wanpeng Li
>
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