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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>,
        Yu Kuai <yukuai1@huaweicloud.com>
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
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>
From:   Yu Kuai <yukuai1@huaweicloud.com>
Message-ID: <9b2d667f-6636-9347-08a1-8bd0aa2346f2@huaweicloud.com>
Date:   Wed, 20 Jul 2022 19:38:06 +0800
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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