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SubjectRe: [PATCH v5] sched/deadline: support dl task migration during cpu hotplug
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@linux.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().

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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