linux-kernel - Re: [PATCH v4 1/2] block, bfq: counted root group into 'num_groups_with_pending

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Date:   Wed, 20 Oct 2021 11:43:22 +0200
From:   Paolo Valente <paolo.valente@...aro.org>
To:     "yukuai (C)" <yukuai3@...wei.com>
Cc:     Jens Axboe <axboe@...nel.dk>,
        linux-block <linux-block@...r.kernel.org>,
        linux-kernel@...r.kernel.org, yi.zhang@...wei.com
Subject: Re: [PATCH v4 1/2] block, bfq: counted root group into
 'num_groups_with_pending_reqs'



> Il giorno 20 ott 2021, alle ore 11:38, yukuai (C) <yukuai3@...wei.com> ha scritto:
> 
> On 2021/10/20 17:29, Paolo Valente wrote:
>>> Il giorno 20 ott 2021, alle ore 11:20, yukuai (C) <yukuai3@...wei.com> ha scritto:
>>> 
>>> On 2021/10/20 16:51, Paolo Valente wrote:
>>> 
>>>>> @@ -860,9 +870,25 @@ void bfq_weights_tree_remove(struct bfq_data *bfqd,
>>>>> 			     struct bfq_queue *bfqq)
>>>>> {
>>>>> 	struct bfq_entity *entity = bfqq->entity.parent;
>>>>> +	struct bfq_sched_data *sd;
>>>>> +
>>>>> +	/*
>>>>> +	 * If the bfq queue is in root group, the decrement of
>>>>> +	 * num_groups_with_pending_reqs is performed immediately upon the
>>>>> +	 * deactivation of entity.
>>>>> +	 */
>>>>> +	if (!entity) {
>>>>> +		entity = &bfqd->root_group->entity;
>>>>> +		sd = entity->my_sched_data;
>>>>> +
>>>>> +		if (!sd->in_service_entity)
>>>>> +			bfq_clear_group_with_pending_reqs(bfqd, entity);
>>>>> +
>>>>> +		return;
>>>>> +	}
>>>>> 
>>>>> 	for_each_entity(entity) {
>>>>> -		struct bfq_sched_data *sd = entity->my_sched_data;
>>>>> +		sd = entity->my_sched_data;
>>>>> 
>>>>> 		if (sd->next_in_service || sd->in_service_entity) {
>>>>> 			/*
>>>>> @@ -880,7 +906,8 @@ void bfq_weights_tree_remove(struct bfq_data *bfqd,
>>>>> 		}
>>>>> 
>>>>> 		/*
>>>>> -		 * The decrement of num_groups_with_pending_reqs is
>>>>> +		 * If the bfq queue is not in root group,
>>>>> +		 * the decrement of num_groups_with_pending_reqs is
>>>> I'm sorry if I didn't notice this before, but why do you postpone the
>>>> decrement only for queues not in root group?  If I'm not missing
>>>> anything, the active (i.e., with pending reqs) state of the root group
>>>> is to be computed as that of ay other group.
>>> 
>>> Hi, Paolo
>>> 
>>> I thought if queue is in root group, then bfqq->entity.parent is NULL,
>>> and such case is handled above, which is separate from previous
>>> implementation for queues that are not in root group.
>>> 
>>> Is this the wrong way to handle root group?
>>> 
>> I think that, if we want to count also the root group among the active
>> ones, then the logic for tagging the root group as active must be the
>> same as the other groups. Or am I missing something?
> 
> Hi, Paolo
> 
> Currently, if queue is in root group, bfqq->entity.parent is NULL, and
> this makes it hard to keep the same logic.
> 
> Can we store root_group->my_entity to bfqq->entity.parent if the queue
> is in root group?
> 

Any sensible implementation is ok for me.  Usually, stuff for root
group is in the bfqd.

Thanks,
Paolo

> Thanks,
> Kuai