Re: [PATCH v4] sched/deadline: support dl task migration during cpu hotplug

[Wanpeng Li]

Hi Kirill,
On 11/11/14, 7:10 PM, Kirill Tkhai wrote:
> Ð ÐÑ, 11/11/2014 Ð 10:30 +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 fix it by push the task to another cpu in dl_task_timer() if
> > rq is offline.
> >
> > Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxxxxxx>
> I'm still thinking we don't have to guarantee any "deadlines" during cpu hotplug...
> But, if speaking about this way:
>
> > ---
> > 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 | 51 ++++++++++++++++++++++++++++++++++++++++++++++---
> >   1 file changed, 48 insertions(+), 3 deletions(-)
> >
> > diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> > index 00324af..e0fbba4 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 +539,46 @@ 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 (!rq->online) {
> > +		struct rq *later_rq = NULL;
> > +
> > +		raw_spin_unlock(&rq->lock);
> > +
> > +		later_rq = find_lock_later_rq(p, rq);
> find_lock_later_rq() expects that rq is locked.
>
> The comment near its head confuses a reader. It locks newly found rq.

Sorry for my bad, what's you think should be changed?

Regards,
Wanpeng Li

> > +
> > +		if (!later_rq) {
> > +			int cpu;
> > +
> > +			/*
> > +			 * If cannot preempt any rq, fallback to pick any
> > +			 * online cpu.
> > +			 */
> > +			for_each_cpu(cpu, tsk_cpus_allowed(p))
> > +				if (cpu_online(cpu))
> > +					later_rq = cpu_rq(cpu);
> > +			if (!later_rq) {
> > +				pr_warn("fail to find any online and task "
> > +				    "will never come back to us\n");
> > +				goto out;
> > +			}
> > +		}
> > +
> > +		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);
> double_unlock_balance() unlocks later_rq only.
>
> > +
> > +		goto out;
> > +	}
> > +
> >   	if (task_on_rq_queued(p)) {
> >   		enqueue_task_dl(rq, p, ENQUEUE_REPLENISH);
> >   		if (dl_task(rq->curr))
> > @@ -555,7 +596,7 @@ again:
> >   	}
> >   unlock:
> >   	raw_spin_unlock(&rq->lock);
> > -
> > +out:
> >   	return HRTIMER_NORESTART;
> >   }
> >   
> > @@ -1185,8 +1226,12 @@ 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);
> > +	if (likely(task_rq(task)->online)) {
> > +		cpumask_copy(later_mask, task_rq(task)->rd->span);
> > +		cpumask_and(later_mask, later_mask, cpu_active_mask);
> > +	} else
> > +		/* for offline cpus we have a singleton rd */
> > +		cpumask_copy(later_mask, cpu_active_mask);
> >   	cpumask_and(later_mask, later_mask, &task->cpus_allowed);
> >   	best_cpu = cpudl_find(&task_rq(task)->rd->cpudl,
> >   			task, later_mask);
>
> --
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