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 11:29:29 +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 11:20, 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?

Thanks,
Paolo

> Thanks,
> Kuai