linux-kernel - Re: [PATCH V2] sched/fair: Fix that tasks are not constrained by cfs_b->quota on hotplug core, when hotplug core is offline and then online.

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Date:   Tue, 27 Sep 2016 10:13:30 -0700
From:   bsegall@...gle.com
To:     Jeehong Kim <jhez.kim@...sung.com>
Cc:     mingo@...hat.com, linux-kernel@...r.kernel.org,
        ezjjilong@...il.com, Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH V2] sched/fair: Fix that tasks are not constrained by cfs_b->quota on hotplug core, when hotplug core is offline and then online.

Jeehong Kim <jhez.kim@...sung.com> writes:

> On 2016년 09월 23일 01:53, bsegall@...gle.com wrote:
>> Jeehong Kim <jhez.kim@...sung.com> writes:
>>
>>>> Peter Zijlstra <peterz@...xxxxxxxxxx> writes:
>>>>
>>>>> You forgot to Cc Ben, who gave you feedback on v1, which is rather poor
>>>>> style. Also, I don't see how kernel-janitors is relevant to this patch.
>>>>> This is very much not a janitorial thing.
>>>>>
>>>>> (also, why send it twice?)
>>>>>
>>>>> On Tue, Aug 30, 2016 at 10:12:40PM +0900, Jeehong Kim wrote:
>>>>>> In case that CONFIG_HOTPLUG_CPU and CONFIG_CFS_BANDWIDTH is turned on
>>>>>> and tasks in bandwidth controlled task group run on hotplug core,
>>>>>> the tasks are not controlled by cfs_b->quota when hotplug core is offline
>>>>>> and then online. The remaining tasks in task group consume all of
>>>>>> cfs_b->quota on other cores.
>>>>>>
>>>>>> The cause of this problem is described as below:
>>>>>>
>>>>>> 1. When hotplug core is offline while tasks in task group run
>>>>>> on hotplug core, unregister_fair_sched_group() deletes
>>>>>> leaf_cfs_rq_list of tg->cfs_rq[cpu] from &rq_of(cfs_rq)->leaf_cfs_rq_list.
>>>>>>
>>>>>> 2. Then, when hotplug core is online, update_runtime_enabled()
>>>>> Peter Zijlstra <peterz@...xxxxxxxxxx> writes:
>>>>> You forgot to Cc Ben, who gave you feedback on v1, which is rather poor
>>>>> style. Also, I don't see how kernel-janitors is relevant to this patch.
>>>>> This is very much not a janitorial thing.
>>>>>
>>>>> (also, why send it twice?)
>>>>>
>>>>> On Tue, Aug 30, 2016 at 10:12:40PM +0900, Jeehong Kim wrote:
>>>>>> In case that CONFIG_HOTPLUG_CPU and CONFIG_CFS_BANDWIDTH is turned on
>>>>>> and tasks in bandwidth controlled task group run on hotplug core,
>>>>>> the tasks are not controlled by cfs_b->quota when hotplug core is offline
>>>>>> and then online. The remaining tasks in task group consume all of
>>>>>> cfs_b->quota on other cores.
>>>>>>
>>>>>> The cause of this problem is described as below:
>>>>>>
>>>>>> 1. When hotplug core is offline while tasks in task group run
>>>>>> on hotplug core, unregister_fair_sched_group() deletes
>>>>>> leaf_cfs_rq_list of tg->cfs_rq[cpu] from &rq_of(cfs_rq)->leaf_cfs_rq_list.
>>>>>>
>>>>>> 2. Then, when hotplug core is online, update_runtime_enabled()
>>>>>> registers cfs_b->quota on cfs_rq->runtime_enabled of all leaf cfs_rq
>>>>>> on runqueue. However, because this is before enqueue_entity() adds
>>>>>> &cfs_rq->leaf_cfs_rq_list on &rq_of(cfs_rq)->leaf_cfs_rq_list,
>>>>>> cfs->quota is not register on cfs_rq->runtime_enabled.
>>>>>>
>>>>>> To resolve this problem, this patch makes update_runtime_enabled()
>>>>>> registers cfs_b->quota by using walk_tg_tree_from().
>>>>>
>>>>>> +static int __maybe_unused __update_runtime_enabled(struct task_group *tg, void *data)
>>>>>>  {
>>>>>> +    struct rq *rq = data;
>>>>>> +    struct cfs_rq *cfs_rq = tg->cfs_rq[cpu_of(rq)];
>>>>>> +    struct cfs_bandwidth *cfs_b = &cfs_rq->tg->cfs_bandwidth;
>>>>>>
>>>>>> +    raw_spin_lock(&cfs_b->lock);
>>>>>> +    raw_spin_unlock(&cfs_b->lock);
>>>>>>
>>>>>> +    return 0;
>>>>>> +}
>>>>>> +
>>>>>> +static void __maybe_unused update_runtime_enabled(struct rq *rq)
>>>>>> +{
>>>>>> +    struct cfs_rq *cfs_rq = &rq->cfs;
>>>>>> +
>>>>>> +    /* register cfs_b->quota on the whole tg tree */
>>>>>> +    rcu_read_lock();
>>>>>> +    walk_tg_tree_from(cfs_rq->tg, __update_runtime_enabled, tg_nop, (void *)rq);
>>>>>> +    rcu_read_unlock();
>>>>>>  }
>>>>> Looks ok, performance on hotplug doesn't really matter. Ben, you happy
>>>>> with this?
>>>> I'm not 100% sure about the exact timings and mechanics of hotplug, but
>>>> cfs-bandwidth wise this is ok. We may still have runtime_remaining = 1,
>>>> or we may have < 0 and yet be unthrottled, but either case is ok, even
>>>> if hotplug allows tasks to have migrated here already (I'm not sure,
>>>> looking at the code).
>>>>
>>>> Now that I check again you can just loop over the list of tgs rather
>>>> than the hierarchical walk_tg_tree_from, but there's certainly no harm
>>>> in it.
>>> Ben,
>>>
>>> Is there additional revision which I have to do?
>>> If so, could you let me know about that?
>>>
>>> Regards,
>>> Jeehong Kim
>> Oh, no, this is fine by me.
>>
>>
>>
>
> Ben,
>
> If this is fine to you, could you sign off on this patch?
>
> Regards,
> Jeehong Kim.

Reviewed-By: Ben Segall <bsegall@...gle.com>

(My understanding of signed-off-by is that I don't do that in this case)