Date:   Wed, 13 Apr 2022 13:28:16 +0200
From:   Jan Kara <jack@...e.cz>
To:     Yu Kuai <yukuai3@...wei.com>
Cc:     tj@...nel.org, axboe@...nel.dk, paolo.valente@...aro.org,
        jack@...e.cz, cgroups@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, yi.zhang@...wei.com
Subject: Re: [PATCH -next 10/11] block, bfq: decrease
 'num_groups_with_pending_reqs' earlier

On Sat 05-03-22 17:12:04, Yu Kuai wrote:
> Currently 'num_groups_with_pending_reqs' won't be decreased when
> the group doesn't have any pending requests, while some child group
> still have pending requests. The decrement is delayed to when all the
> child groups doesn't have any pending requests.
> 
> For example:
> 1) t1 issue sync io on root group, t2 and t3 issue sync io on the same
> child group. num_groups_with_pending_reqs is 2 now.
> 2) t1 stopped, num_groups_with_pending_reqs is still 2. io from t2 and
> t3 still can't be handled concurrently.
> 
> Fix the problem by decreasing 'num_groups_with_pending_reqs'
> immediately upon the weights_tree removal of last bfqq of the group.
> 
> Signed-off-by: Yu Kuai <yukuai3@...wei.com>

So I'd find the logic easier to follow if you completely removed
entity->in_groups_with_pending_reqs and did updates of
bfqd->num_groups_with_pending_reqs like:

	if (!bfqg->num_entities_with_pending_reqs++)
		bfqd->num_groups_with_pending_reqs++;

and similarly on the remove side. And there would we literally two places
(addition & removal from weight tree) that would need to touch these
counters. Pretty obvious and all can be done in patch 9.

								Honza

> ---
>  block/bfq-iosched.c | 56 +++++++++++++++------------------------------
>  block/bfq-iosched.h | 16 ++++++-------
>  2 files changed, 27 insertions(+), 45 deletions(-)
> 
> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
> index f221e9cab4d0..119b64c9c1d9 100644
> --- a/block/bfq-iosched.c
> +++ b/block/bfq-iosched.c
> @@ -970,6 +970,24 @@ void __bfq_weights_tree_remove(struct bfq_data *bfqd,
>  	bfq_put_queue(bfqq);
>  }
>  
> +static void decrease_groups_with_pending_reqs(struct bfq_data *bfqd,
> +					      struct bfq_queue *bfqq)
> +{
> +#ifdef CONFIG_BFQ_GROUP_IOSCHED
> +	struct bfq_entity *entity = bfqq->entity.parent;
> +
> +	/*
> +	 * The decrement of num_groups_with_pending_reqs is performed
> +	 * immediately when the last bfqq completes all the requests.
> +	 */
> +	if (!bfqq_group(bfqq)->num_entities_with_pending_reqs &&
> +	    entity->in_groups_with_pending_reqs) {
> +		entity->in_groups_with_pending_reqs = false;
> +		bfqd->num_groups_with_pending_reqs--;
> +	}
> +#endif
> +}
> +
>  /*
>   * Invoke __bfq_weights_tree_remove on bfqq and decrement the number
>   * of active groups for each queue's inactive parent entity.
> @@ -977,8 +995,6 @@ 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;
> -
>  	/*
>  	 * grab a ref to prevent bfqq to be freed in
>  	 * __bfq_weights_tree_remove
> @@ -991,41 +1007,7 @@ void bfq_weights_tree_remove(struct bfq_data *bfqd,
>  	 */
>  	__bfq_weights_tree_remove(bfqd, bfqq,
>  				  &bfqd->queue_weights_tree);
> -
> -	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--;
> -		}
> -	}
> -
> +	decrease_groups_with_pending_reqs(bfqd, bfqq);
>  	bfq_put_queue(bfqq);
>  }
>  
> diff --git a/block/bfq-iosched.h b/block/bfq-iosched.h
> index 5d904851519c..9ec72bd24fc2 100644
> --- a/block/bfq-iosched.h
> +++ b/block/bfq-iosched.h
> @@ -495,7 +495,7 @@ struct bfq_data {
>  	struct rb_root_cached queue_weights_tree;
>  
>  	/*
> -	 * Number of groups with at least one descendant process that
> +	 * Number of groups with at least one 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
> @@ -508,14 +508,14 @@ struct bfq_data {
>  	 * 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
> +	 * groups with multiple processes. Consider a group
> +	 * that is inactive, i.e., that has no 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
> +	 * group, because the group has 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
> +	 * last 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
> @@ -524,7 +524,7 @@ struct bfq_data {
>  	 * 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
> +	 * num_groups_with_pending_reqs when the last
>  	 * process of the group remains with no request waiting for
>  	 * completion.
>  	 *
> @@ -532,12 +532,12 @@ struct bfq_data {
>  	 * 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
> +	 * inactive. Then, when the last 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
> +	 * longer in case a new queue of the entity remains
>  	 * with no request waiting for completion.
>  	 */
>  	unsigned int num_groups_with_pending_reqs;
> -- 
> 2.31.1
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR

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