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]

On 2016ë 09ì 23ì 01:53, bsegall@xxxxxxxxxx wrote:
> Jeehong Kim <jhez.kim@xxxxxxxxxxx> writes:
>
>>> Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx> 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.