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Subject: Re: [PATCH -next v10 3/4] block, bfq: refactor the counting of
 'num_groups_with_pending_reqs'
To:     Paolo VALENTE <paolo.valente@unimore.it>
Cc:     Jan Kara <jack@suse.cz>, cgroups@vger.kernel.org,
        linux-block <linux-block@vger.kernel.org>,
        Tejun Heo <tj@kernel.org>, Jens Axboe <axboe@kernel.dk>,
        LKML <linux-kernel@vger.kernel.org>, yi.zhang@huawei.com,
        "yukuai (C)" <yukuai3@huawei.com>
References: <20220610021701.2347602-1-yukuai3@huawei.com>
 <20220610021701.2347602-4-yukuai3@huawei.com>
 <27F2DF19-7CC6-42C5-8CEB-43583EB4AE46@linaro.org>
 <abdbb5db-e280-62f8-0670-536fcb8ec4d9@huaweicloud.com>
 <C2CF100A-9A7C-4300-9A70-1295BC939C66@unimore.it>
 <9b2d667f-6636-9347-08a1-8bd0aa2346f2@huaweicloud.com>
From:   Yu Kuai <yukuai1@huaweicloud.com>
Message-ID: <2f94f241-445f-1beb-c4a8-73f6efce5af2@huaweicloud.com>
Date:   Wed, 27 Jul 2022 20:11:14 +0800
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Hi, Paolo

Are you still interested in this patchset?

在 2022/07/20 19:38, Yu Kuai 写道:
> Hi
>
> 在 2022/07/20 19:24, Paolo VALENTE 写道:
>>
>>
>>> Il giorno 12 lug 2022, alle ore 15:30, Yu Kuai 
>>> <yukuai1@huaweicloud.com <mailto:yukuai1@huaweicloud.com>> ha scritto:
>>>
>>> Hi!
>>>
>>> I'm copying my reply with new mail address, because Paolo seems
>>> didn't receive my reply.
>>>
>>> 在 2022/06/23 23:32, Paolo Valente 写道:
>>>> Sorry for the delay.
>>>>> Il giorno 10 giu 2022, alle ore 04:17, Yu Kuai <yukuai3@huawei.com 
>>>>> <mailto:yukuai3@huawei.com>> ha scritto:
>>>>>
>>>>> Currently, bfq can't handle sync io concurrently as long as they
>>>>> are not issued from root group. This is because
>>>>> 'bfqd->num_groups_with_pending_reqs > 0' is always true in
>>>>> bfq_asymmetric_scenario().
>>>>>
>>>>> The way that bfqg is counted into 'num_groups_with_pending_reqs':
>>>>>
>>>>> Before this patch:
>>>>> 1) root group will never be counted.
>>>>> 2) Count if bfqg or it's child bfqgs have pending requests.
>>>>> 3) Don't count if bfqg and it's child bfqgs complete all the 
>>>>> requests.
>>>>>
>>>>> After this patch:
>>>>> 1) root group is counted.
>>>>> 2) Count if bfqg have pending requests.
>>>>> 3) Don't count if bfqg complete all the requests.
>>>>>
>>>>> With this change, the occasion that only one group is activated 
>>>>> can be
>>>>> detected, and next patch will support concurrent sync io in the
>>>>> occasion.
>>>>>
>>>>> Signed-off-by: Yu Kuai <yukuai3@huawei.com 
>>>>> <mailto:yukuai3@huawei.com>>
>>>>> Reviewed-by: Jan Kara <jack@suse.cz <mailto:jack@suse.cz>>
>>>>> ---
>>>>> block/bfq-iosched.c | 42 ------------------------------------------
>>>>> block/bfq-iosched.h | 18 +++++++++---------
>>>>> block/bfq-wf2q.c    | 19 ++++---------------
>>>>> 3 files changed, 13 insertions(+), 66 deletions(-)
>>>>>
>>>>> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
>>>>> index 0ec21018daba..03b04892440c 100644
>>>>> --- a/block/bfq-iosched.c
>>>>> +++ b/block/bfq-iosched.c
>>>>> @@ -970,48 +970,6 @@ void __bfq_weights_tree_remove(struct 
>>>>> bfq_data *bfqd,
>>>>> void bfq_weights_tree_remove(struct bfq_data *bfqd,
>>>>>     struct bfq_queue *bfqq)
>>>>> {
>>>>> -struct bfq_entity *entity = bfqq->entity.parent;
>>>>> -
>>>>> -for_each_entity(entity) {
>>>>> -struct bfq_sched_data *sd = entity->my_sched_data;
>>>>> -
>>>>> -if (sd->next_in_service || sd->in_service_entity) {
>>>>> -/*
>>>>> -* entity is still active, because either
>>>>> -* next_in_service or in_service_entity is not
>>>>> -* NULL (see the comments on the definition of
>>>>> -* next_in_service for details on why
>>>>> -* in_service_entity must be checked too).
>>>>> -*
>>>>> -* As a consequence, its parent entities are
>>>>> -* active as well, and thus this loop must
>>>>> -* stop here.
>>>>> -*/
>>>>> -break;
>>>>> -}
>>>>> -
>>>>> -/*
>>>>> -* The decrement of num_groups_with_pending_reqs is
>>>>> -* not performed immediately upon the deactivation of
>>>>> -* entity, but it is delayed to when it also happens
>>>>> -* that the first leaf descendant bfqq of entity gets
>>>>> -* all its pending requests completed. The following
>>>>> -* instructions perform this delayed decrement, if
>>>>> -* needed. See the comments on
>>>>> -* num_groups_with_pending_reqs for details.
>>>>> -*/
>>>>> -if (entity->in_groups_with_pending_reqs) {
>>>>> -entity->in_groups_with_pending_reqs = false;
>>>>> -bfqd->num_groups_with_pending_reqs--;
>>>>> -}
>>>>> -}
>>>> With this part removed, I'm missing how you handle the following
>>>> sequence of events:
>>>> 1.  a queue Q becomes non busy but still has dispatched requests, so
>>>> it must not be removed from the counter of queues with pending reqs
>>>> yet
>>>> 2.  the last request of Q is completed with Q being still idle (non
>>>> busy).  At this point Q must be removed from the counter.  It seems to
>>>> me that this case is not handled any longer
>>> Hi, Paolo
>>>
>>> 1) At first, patch 1 support to track if bfqq has pending requests, 
>>> it's
>>> done by setting the flag 'entity->in_groups_with_pending_reqs' when the
>>> first request is inserted to bfqq, and it's cleared when the last
>>> request is completed(based on weights_tree insertion and removal).
>>>
>>
>> In patch 1 I don't see the flag cleared for the request-completion 
>> event :(
>>
>> The piece of code involved is this:
>>
>> static void bfq_completed_request(struct bfq_queue *bfqq, struct 
>> bfq_data *bfqd)
>> {
>> u64 now_ns;
>> u32 delta_us;
>>
>> bfq_update_hw_tag(bfqd);
>>
>> bfqd->rq_in_driver[bfqq->actuator_idx]--;
>> bfqd->tot_rq_in_driver--;
>> bfqq->dispatched--;
>>
>> if (!bfqq->dispatched && !bfq_bfqq_busy(bfqq)) {
>> /*
>> * Set budget_timeout (which we overload to store the
>> * time at which the queue remains with no backlog and
>> * no outstanding request; used by the weight-raising
>> * mechanism).
>> */
>> bfqq->budget_timeout = jiffies;
>>
>> bfq_weights_tree_remove(bfqd, bfqq);
>> }
>> ...
>>
>> Am I missing something?
>
> I add a new api bfq_del_bfqq_in_groups_with_pending_reqs() in patch 1
> to clear the flag, and it's called both from bfq_del_bfqq_busy() and
> bfq_completed_request(). I think you may miss the later:
>
> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
> index 0d46cb728bbf..0ec21018daba 100644
> --- a/block/bfq-iosched.c
> +++ b/block/bfq-iosched.c
> @@ -6263,6 +6263,7 @@ static void bfq_completed_request(struct 
> bfq_queue *bfqq, struct bfq_data *bfqd)
>           */
>          bfqq->budget_timeout = jiffies;
>
> +        bfq_del_bfqq_in_groups_with_pending_reqs(bfqq);
>          bfq_weights_tree_remove(bfqd, bfqq);
>      }
>
> Thanks,
> Kuai
>>
>> Thanks,
>> Paolo