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 17:38:49 +0800
From:   "yukuai (C)" <yukuai3@...wei.com>
To:     Paolo Valente <paolo.valente@...aro.org>
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'

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?

Thanks,
Kuai