Re: [PATCH v5] sched/deadline: support dl task migration during cpu hotplug

[Wanpeng Li]

Hi Kirill,
On 11/13/14, 6:10 PM, Kirill Tkhai wrote:
> Ð ÐÑ, 13/11/2014 Ð 06:56 +0800, Wanpeng Li ÐÐÑÐÑ:
> > Hi Kirill,
> > On Wed, Nov 12, 2014 at 07:27:06PM +0300, Kirill Tkhai wrote:
> > > Ð ÐÑ, 12/11/2014 Ð 09:06 +0800, Wanpeng Li ÐÐÑÐÑ:
> > > > I observe that dl task can't be migrated to other cpus during cpu hotplug,
> > > > in addition, task may/may not be running again if cpu is added back. The
> > > > root cause which I found is that dl task will be throtted and removed from
> > > > dl rq after comsuming all budget, which leads to stop task can't pick it up
> > > > from dl rq and migrate to other cpus during hotplug.
> > > >
> > > > The method to reproduce:
> > > > schedtool -E -t 50000:100000 -e ./test
> > > > Actually test is just a simple for loop. Then observe which cpu the test
> > > > task is on.
> > > > echo 0 > /sys/devices/system/cpu/cpuN/online
> > > >
> > > > This patch adds the dl task migration during cpu hotplug by finding a most
> > > > suitable later deadline rq after dl timer fire if current rq is offline,
> > > > if fail to find a suitable later deadline rq then fallback to any eligible
> > > > online cpu in order that the deadline task will come back to us, and the
> > > > push/pull mechanism should then move it around properly.
> > > >
> > > > Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxxxxxx>
> > > > ---
> > > > v4 -> v5:
> > > >   * remove raw_spin_unlock(&rq->lock)
> > > >   * cleanup codes, spotted by Peterz
> > > >   * cleanup patch description
> > > > v3 -> v4:
> > > >   * use tsk_cpus_allowed wrapper
> > > >   * fix compile error
> > > > v2 -> v3:
> > > >   * don't get_task_struct
> > > >   * if cannot preempt any rq, fallback to pick any online cpus
> > > >   * use cpu_active_mask as original later_mask if cpu is offline
> > > > v1 -> v2:
> > > >   * push the task to another cpu in dl_task_timer() if rq is offline.
> > > >
> > > >   kernel/sched/deadline.c | 43 +++++++++++++++++++++++++++++++++++++++++--
> > > >   1 file changed, 41 insertions(+), 2 deletions(-)
> > > >
> > > > diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> > > > index f3d7776..7c31906 100644
> > > > --- a/kernel/sched/deadline.c
> > > > +++ b/kernel/sched/deadline.c
> > > > @@ -487,6 +487,7 @@ static int start_dl_timer(struct sched_dl_entity *dl_se, bool boosted)
> > > >   	return hrtimer_active(&dl_se->dl_timer);
> > > >   }
> > > >   
> > > > +static struct rq *find_lock_later_rq(struct task_struct *task, struct rq *rq);
> > > >   /*
> > > >    * This is the bandwidth enforcement timer callback. If here, we know
> > > >    * a task is not on its dl_rq, since the fact that the timer was running
> > > > @@ -538,6 +539,43 @@ again:
> > > >   	update_rq_clock(rq);
> > > >   	dl_se->dl_throttled = 0;
> > > >   	dl_se->dl_yielded = 0;
> > > > +
> > > > +	/*
> > > > +	 * So if we find that the rq the task was on is no longer
> > > > +	 * available, we need to select a new rq.
> > > > +	 */
> > > > +	if (unlikely(!rq->online)) {
> > > > +		struct rq *later_rq = NULL;
> > > > +
> > > > +		later_rq = find_lock_later_rq(p, rq);
> > > > +
> > > > +		if (!later_rq) {
> > > > +			int cpu;
> > > > +
> > > > +			/*
> > > > +			 * If cannot preempt any rq, fallback to pick any
> > > > +			 * online cpu.
> > > > +			 */
> > > > +			cpu = cpumask_any_and(cpu_active_mask,
> > > > +					tsk_cpus_allowed(p));
> > > > +			if (cpu >= nr_cpu_ids) {
> > > > +				pr_warn("fail to find any online cpu and task will never come back\n");
> > > > +				goto unlock;
> > > > +			}
> > > > +			later_rq = cpu_rq(cpu);
> > > later_rq is not locked here, but you activate p on it and you do unlock below.
> > Great catch! How about add double_lock_balance(rq, later_rq); here?
> This sounds good.

I will do this in next version. ;-)

> > Regards,
> > Wanpeng Li
> >
> > > > +		}
> > > > +
> > > > +		deactivate_task(rq, p, 0);
> > > > +		set_task_cpu(p, later_rq->cpu);
> > > > +		activate_task(later_rq, p, 0);
> > > 		^^^^^
> > >
> > > > +
> > > > +		resched_curr(later_rq);
> > > > +
> > > > +		double_unlock_balance(rq, later_rq);
> > > 		^^^^^^
> > >
> > > > +
> > > > +		goto unlock;
> > > > +	}
> > > > +
> > > >   	if (task_on_rq_queued(p)) {
> > > >   		enqueue_task_dl(rq, p, ENQUEUE_REPLENISH);
> > > >   		if (dl_task(rq->curr))
> > > > @@ -1185,8 +1223,9 @@ static int find_later_rq(struct task_struct *task)
> > > >   	 * We have to consider system topology and task affinity
> > > >   	 * first, then we can look for a suitable cpu.
> > > >   	 */
> > > > -	cpumask_copy(later_mask, task_rq(task)->rd->span);
> > > > -	cpumask_and(later_mask, later_mask, cpu_active_mask);
> > > > +	cpumask_copy(later_mask, cpu_active_mask);
> > > > +	if (likely(task_rq(task)->online))
> > > > +		cpumask_and(later_mask, later_mask, task_rq(task)->rd->span);
> > > >   	cpumask_and(later_mask, later_mask, &task->cpus_allowed);
> > > >   	best_cpu = cpudl_find(&task_rq(task)->rd->cpudl,
> > > >   			task, later_mask);
> Also, we should think about the following situation.
>
> DL task is left on dead rq. In your scheme it will be moved by the timer.
> But what will be if somebody changes the class of the task (before timer)?

I think timer will be cancelled in switched_from_dl().

Regards,
Wanpeng Li

> In this case the task still remains on dead rq.
>
> We should handle this situation in some way.
>
> Kirill
>
>
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