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Message-ID: <1415874091.15631.44.camel@tkhai>
Date: Thu, 13 Nov 2014 13:21:31 +0300
From: Kirill Tkhai <ktkhai@...allels.com>
To: Wanpeng Li <kernellwp@...il.com>
CC: Wanpeng Li <wanpeng.li@...ux.intel.com>,
Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Juri Lelli <juri.lelli@....com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5] sched/deadline: support dl task migration during cpu
hotplug
В Чт, 13/11/2014 в 18:19 +0800, Wanpeng Li пишет:
> Hi Kirill,
> On 11/13/14, 6:10 PM, Kirill Tkhai wrote:
> > В Чт, 13/11/2014 в 06:56 +0800, Wanpeng Li пишет:
> >> Hi Kirill,
> >> On Wed, Nov 12, 2014 at 07:27:06PM +0300, Kirill Tkhai wrote:
> >>> В Ср, 12/11/2014 в 09:06 +0800, Wanpeng Li пишет:
> >>>> 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@...ux.intel.com>
> >>>> ---
> >>>> 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().
Yeah, but nobody will move this task to alive rq.
>
> 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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