Message-Id: <5A64AAAC-6795-4188-919A-EE6352E32C7C@linaro.org>
Date:   Sat, 28 May 2022 10:41:20 +0200
From:   Paolo Valente <paolo.valente@...aro.org>
To:     Yu Kuai <yukuai3@...wei.com>
Cc:     Jan Kara <jack@...e.cz>, Tejun Heo <tj@...nel.org>,
        Jens Axboe <axboe@...nel.dk>, cgroups@...r.kernel.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        yi.zhang@...wei.com
Subject: Re: [PATCH -next v6 2/3] block, bfq: refactor the counting of
 'num_groups_with_pending_reqs'



> Il giorno 28 mag 2022, alle ore 10:39, Yu Kuai <yukuai3@...wei.com> ha scritto:
> 
> 在 2022/05/28 16:27, Paolo Valente 写道:
>>> Il giorno 23 mag 2022, alle ore 15:18, Yu Kuai <yukuai3@...wei.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 at least one bfqq that is marked busy.
>>> 3) Don't count if bfqg doesn't have any busy bfqqs.
>>> 
>>> The main reason to use busy state of bfqq instead of 'pending requests'
>>> is that bfqq can stay busy after dispatching the last request if idling
>>> is needed for service guarantees.
>>> 
>>> 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.
>>> 
>>> This patch also rename 'num_groups_with_pending_reqs' to
>>> 'num_groups_with_busy_queues'.
>>> 
>>> Signed-off-by: Yu Kuai <yukuai3@...wei.com>
>>> Reviewed-by: Jan Kara <jack@...e.cz>
>>> ---
>>> block/bfq-iosched.c | 46 ++-----------------------------------
>>> block/bfq-iosched.h | 55 ++++++---------------------------------------
>>> block/bfq-wf2q.c    | 19 ++++------------
>>> 3 files changed, 13 insertions(+), 107 deletions(-)
>>> 
>>> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
>>> index e47c75f1fa0f..609b4e894684 100644
>>> --- a/block/bfq-iosched.c
>>> +++ b/block/bfq-iosched.c
>>> @@ -844,7 +844,7 @@ static bool bfq_asymmetric_scenario(struct bfq_data *bfqd,
>>> 
>>> 	return varied_queue_weights || multiple_classes_busy
>>> #ifdef CONFIG_BFQ_GROUP_IOSCHED
>>> -	       || bfqd->num_groups_with_pending_reqs > 0
>>> +	       || bfqd->num_groups_with_busy_queues > 0
>>> #endif
>>> 		;
>>> }
>>> @@ -962,48 +962,6 @@ 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--;
>>> -		}
>>> -	}
>>> -
>>> -	/*
>>> -	 * Next function is invoked last, because it causes bfqq to be
>>> -	 * freed if the following holds: bfqq is not in service and
>>> -	 * has no dispatched request. DO NOT use bfqq after the next
>>> -	 * function invocation.
>>> -	 */
>>> 	__bfq_weights_tree_remove(bfqd, bfqq,
>>> 				  &bfqd->queue_weights_tree);
>>> }
>>> @@ -7107,7 +7065,7 @@ static int bfq_init_queue(struct request_queue *q, struct elevator_type *e)
>>> 	bfqd->idle_slice_timer.function = bfq_idle_slice_timer;
>>> 
>>> 	bfqd->queue_weights_tree = RB_ROOT_CACHED;
>>> -	bfqd->num_groups_with_pending_reqs = 0;
>>> +	bfqd->num_groups_with_busy_queues = 0;
>>> 
>>> 	INIT_LIST_HEAD(&bfqd->active_list);
>>> 	INIT_LIST_HEAD(&bfqd->idle_list);
>>> diff --git a/block/bfq-iosched.h b/block/bfq-iosched.h
>>> index 3847f4ab77ac..b71a088a7f1d 100644
>>> --- a/block/bfq-iosched.h
>>> +++ b/block/bfq-iosched.h
>>> @@ -197,9 +197,6 @@ struct bfq_entity {
>>> 	/* flag, set to request a weight, ioprio or ioprio_class change  */
>>> 	int prio_changed;
>>> 
>>> -	/* flag, set if the entity is counted in groups_with_pending_reqs */
>>> -	bool in_groups_with_pending_reqs;
>>> -
>>> 	/* last child queue of entity created (for non-leaf entities) */
>>> 	struct bfq_queue *last_bfqq_created;
>>> };
>>> @@ -495,52 +492,14 @@ struct bfq_data {
>>> 	struct rb_root_cached queue_weights_tree;
>>> 
>>> 	/*
>>> -	 * Number of groups with at least one descendant process that
>>> -	 * has at least one request waiting for completion. Note that
>>> -	 * this accounts for also requests already dispatched, but not
>>> -	 * yet completed. Therefore this number of groups may differ
>>> -	 * (be larger) than the number of active groups, as a group is
>>> -	 * considered active only if its corresponding entity has
>>> -	 * descendant queues with at least one request queued. This
>>> -	 * number is used to decide whether a scenario is symmetric.
>>> -	 * For a detailed explanation see comments on the computation
>>> -	 * of the variable asymmetric_scenario in the function
>>> -	 * bfq_better_to_idle().
>>> -	 *
>>> -	 * However, it is hard to compute this number exactly, for
>>> -	 * groups with multiple descendant processes. Consider a group
>>> -	 * that is inactive, i.e., that has no descendant process with
>>> -	 * pending I/O inside BFQ queues. Then suppose that
>>> -	 * num_groups_with_pending_reqs is still accounting for this
>>> -	 * group, because the group has descendant processes with some
>>> -	 * I/O request still in flight. num_groups_with_pending_reqs
>>> -	 * should be decremented when the in-flight request of the
>>> -	 * last descendant process is finally completed (assuming that
>>> -	 * nothing else has changed for the group in the meantime, in
>>> -	 * terms of composition of the group and active/inactive state of child
>>> -	 * groups and processes). To accomplish this, an additional
>>> -	 * pending-request counter must be added to entities, and must
>>> -	 * be updated correctly. To avoid this additional field and operations,
>>> -	 * we resort to the following tradeoff between simplicity and
>>> -	 * accuracy: for an inactive group that is still counted in
>>> -	 * num_groups_with_pending_reqs, we decrement
>>> -	 * num_groups_with_pending_reqs when the first descendant
>>> -	 * process of the group remains with no request waiting for
>>> -	 * completion.
>>> -	 *
>>> -	 * Even this simpler decrement strategy requires a little
>>> -	 * carefulness: to avoid multiple decrements, we flag a group,
>>> -	 * more precisely an entity representing a group, as still
>>> -	 * counted in num_groups_with_pending_reqs when it becomes
>>> -	 * inactive. Then, when the first descendant queue of the
>>> -	 * entity remains with no request waiting for completion,
>>> -	 * num_groups_with_pending_reqs is decremented, and this flag
>>> -	 * is reset. After this flag is reset for the entity,
>>> -	 * num_groups_with_pending_reqs won't be decremented any
>>> -	 * longer in case a new descendant queue of the entity remains
>>> -	 * with no request waiting for completion.
>>> +	 * Number of groups with at least one bfqq that is marked busy,
>>> +	 * and this number is used to decide whether a scenario is symmetric.
>>> +	 * Note that bfqq is busy doesn't mean that the bfqq contains requests.
>>> +	 * If idling is needed for service guarantees, bfqq will stay busy
>>> +	 * after dispatching the last request, see details in
>>> +	 * __bfq_bfqq_expire().
>>> 	 */
>>> -	unsigned int num_groups_with_pending_reqs;
>>> +	unsigned int num_groups_with_busy_queues;
>>> 
>>> 	/*
>>> 	 * Per-class (RT, BE, IDLE) number of bfq_queues containing
>>> diff --git a/block/bfq-wf2q.c b/block/bfq-wf2q.c
>>> index d9ff33e0be38..42464e6ff40c 100644
>>> --- a/block/bfq-wf2q.c
>>> +++ b/block/bfq-wf2q.c
>>> @@ -220,12 +220,14 @@ static bool bfq_no_longer_next_in_service(struct bfq_entity *entity)
>>> 
>>> static void bfq_inc_busy_queues(struct bfq_queue *bfqq)
>>> {
>>> -	bfqq_group(bfqq)->busy_queues++;
>>> +	if (!(bfqq_group(bfqq)->busy_queues++))
>>> +		bfqq->bfqd->num_groups_with_busy_queues++;
>>> }
>>> 
>>> static void bfq_dec_busy_queues(struct bfq_queue *bfqq)
>>> {
>>> -	bfqq_group(bfqq)->busy_queues--;
>>> +	if (!(--bfqq_group(bfqq)->busy_queues))
>> Are you sure this is correct?  You want to decrement
>> num_groups_with_busy_queues if busy_queues switches from 1 to 0.  But
>> if busy_queues == 1, then !(busy_queues) is false.
> 
> Hi, Paolo
> 
> I'm sure this is correct.
> 

Yeah, sorry, I didn't notice you switched to prefix increment here.

Paolo

> if busy_queues == 1, then !(--busy_queues) is true; while
> !(busy_queues--) is false.
> 
> Thanks,
> Kuai
>> Paolo
>>> +		bfqq->bfqd->num_groups_with_busy_queues--;
>>> }
>>> 
>>> #else /* CONFIG_BFQ_GROUP_IOSCHED */
>>> @@ -1002,19 +1004,6 @@ static void __bfq_activate_entity(struct bfq_entity *entity,
>>> 		entity->on_st_or_in_serv = true;
>>> 	}
>>> 
>>> -#ifdef CONFIG_BFQ_GROUP_IOSCHED
>>> -	if (!bfq_entity_to_bfqq(entity)) { /* bfq_group */
>>> -		struct bfq_group *bfqg =
>>> -			container_of(entity, struct bfq_group, entity);
>>> -		struct bfq_data *bfqd = bfqg->bfqd;
>>> -
>>> -		if (!entity->in_groups_with_pending_reqs) {
>>> -			entity->in_groups_with_pending_reqs = true;
>>> -			bfqd->num_groups_with_pending_reqs++;
>>> -		}
>>> -	}
>>> -#endif
>>> -
>>> 	bfq_update_fin_time_enqueue(entity, st, backshifted);
>>> }
>>> 
>>> -- 
>>> 2.31.1
>>> 
>> .

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