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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Message-id: <57E38F55.9020601@samsung.com>
Date:   Thu, 22 Sep 2016 16:59:17 +0900
From:   Jeehong Kim <jhez.kim@...sung.com>
To:     Ben Segall <bsegall@...gle.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.

>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