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

From: Juri Lelli
Date: Wed Nov 19 2014 - 08:49:06 EST

Hi,

On 19/11/14 12:30, Wanpeng Li wrote:
> Hi Juri,
> On 11/19/14, 6:13 PM, Juri Lelli wrote:
>> Hi,
>>
>> On 18/11/14 23:18, Wanpeng Li wrote:
>>> Hi Juri,
>>> On 11/12/14, 11:08 PM, Juri Lelli wrote:
>>>> Hi,
>>>>
>>>> On 12/11/14 01:06, Wanpeng Li wrote:
>>>>> 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);
>>>>> + }
>>>>> +
>>>>> + 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);
>>>> So, here you consider the span only when the task_rq is online,
>>>> but there might be others cpus still online belonging to the same
>>>> rd->span. And you have to consider them when migrating. Actually,
>>>> migration must still be restricted to the online cpus of task's
>>>> original rd->span, or I fear you can break clustered scheduling.
>>> Sorry, what's clustered scheduling?
>>>
>> It's a scheduling configuration in which you restrict tasks to run in
>> disjoint subsets of system CPUs. Translated to what we have, it's what
>> you get when you create exclusive cpusets (each one gets a rd) and
>> associate tasks to them.
>>
>> My concern in what above is that you may end up breaking this setup
>> if you don't consider the sd->span when one of the CPUs of your
>> cpuset is off. But, Pang Xunlei patches may solve this, I still have to
>> check :/.
>
> Thanks for your explanation. Could you point out which one of Pang's
> patchset solve this? ;-)
>

https://lkml.org/lkml/2014/11/17/443 may help with this, although I
still have to properly look at it.

Best,

- Juri

> Regards,
> Wanpeng Li
>
>>
>> Thanks,
>>
>> - Juri
>>
>>> Regards,
>>> Wanpeng Li
>>>
>>>> Thanks,
>>>>
>>>> - Juri
>>>>
>>>>> 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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>
>

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