Date:   Mon, 30 May 2022 10:10:00 +0200
From:   Paolo Valente <paolo.valente@...aro.org>
To:     Yu Kuai <yukuai3@...wei.com>
Cc:     Tejun Heo <tj@...nel.org>, Jens Axboe <axboe@...nel.dk>,
        cgroups@...r.kernel.org, linux-block <linux-block@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, yi.zhang@...wei.com
Subject: Re: [PATCH -next v7 2/3] block, bfq: refactor the counting of
 'num_groups_with_pending_reqs'



> Il giorno 28 mag 2022, alle ore 11:50, 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.

Unfortunately, I see a last problem here. I see a double change:
(1) a bfqg is now counted only as a function of the state of its child
    queues, and not of also its child bfqgs
(2) the state considered for counting a bfqg moves from having pending
    requests to having busy queues

I'm ok with with (1), which is a good catch (you are lady explained
the idea to me some time ago IIRC).

Yet I fear that (2) is not ok.  A bfqq can become non busy even if it
still has in-flight I/O, i.e.  I/O being served in the drive.  The
weight of such a bfqq must still be considered in the weights_tree,
and the group containing such a queue must still be counted when
checking whether the scenario is asymmetric.  Otherwise service
guarantees are broken.  The reason is that, if a scenario is deemed as
symmetric because in-flight I/O is not taken into account, then idling
will not be performed to protect some bfqq, and in-flight I/O may
steal bandwidth to that bfqq in an uncontrolled way.

I verified this also experimentally a few years ago, when I added this
weights_tree stuff.  That's the rationale behind the part of
bfq_weights_tree_remove that this patch eliminates.  IOW,
for a bfqq and its parent bfqg to be out of the count for symmetry,
all bfqq's requests must also be completed.

Thanks,
Paolo

> 
> 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 0d46cb728bbf..eb1da1bd5eb4 100644
> --- a/block/bfq-iosched.c
> +++ b/block/bfq-iosched.c
> @@ -852,7 +852,7 @@ 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
> 		;
> }
> @@ -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--;
> -		}
> -	}
> -
> -	/*
> -	 * 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);
> }
> @@ -7118,7 +7076,7 @@ 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 d92adbdd70ee..6c6cd984d769 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;
> };
> @@ -496,52 +493,14 @@ 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 b97e33688335..48ca7922035c 100644
> --- a/block/bfq-wf2q.c
> +++ b/block/bfq-wf2q.c
> @@ -221,13 +221,15 @@ static bool bfq_no_longer_next_in_service(struct bfq_entity *entity)
> static void bfq_inc_busy_queues(struct bfq_queue *bfqq)
> {
> 	bfqq->bfqd->busy_queues[bfqq->ioprio_class - 1]++;
> -	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->bfqd->busy_queues[bfqq->ioprio_class - 1]--;
> -	bfqq_group(bfqq)->busy_queues--;
> +	if (!(--bfqq_group(bfqq)->busy_queues))
> +		bfqq->bfqd->num_groups_with_busy_queues--;
> }
> 
> #else /* CONFIG_BFQ_GROUP_IOSCHED */
> @@ -1006,19 +1008,6 @@ 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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