Re: [PATCH v2 2/4] block, bfq: do not idle if only one cgroup is activated

[Paolo Valente]

> Il giorno 6 ago 2021, alle ore 04:08, Yu Kuai <yukuai3@xxxxxxxxxx> ha scritto:
> 
> If only one group is activated, there is no need to guarantee the same
> share of the throughput of queues in the same group.
> 
> If CONFIG_BFQ_GROUP_IOSCHED is enabled, there is no need to check
> 'varied_queue_weights' and 'multiple_classes_busy':
> 1) num_groups_with_pending_reqs = 0, idle is not needed
> 2) num_groups_with_pending_reqs = 1
>   - if root group have any pending requests, idle is needed
>   - if root group is idle, idle is not needed
> 3) num_groups_with_pending_reqs > 1, idle is needed
> 
> Test procedure:
> run "fio -numjobs=1 -ioengine=psync -bs=4k -direct=1 -rw=randread..."
> multiple times in the same cgroup(not root).
> 
> Test result: total bandwidth(Mib/s)
> | total jobs | before this patch | after this patch      |
> | ---------- | ----------------- | --------------------- |
> | 1          | 33.8              | 33.8                  |
> | 2          | 33.8              | 65.4 (32.7 each job)  |
> | 4          | 33.8              | 106.8 (26.7 each job) |
> | 8          | 33.8              | 126.4 (15.8 each job) |
> 
> By the way, if I test with "fio -numjobs=1/2/4/8 ...", test result is
> the same with or without this patch.
> 
> Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
> ---
> block/bfq-iosched.c | 35 ++++++++++++++++++++++++++---------
> 1 file changed, 26 insertions(+), 9 deletions(-)
> 
> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
> index 7c6b412f9a9c..a780205a1be4 100644
> --- a/block/bfq-iosched.c
> +++ b/block/bfq-iosched.c
> @@ -709,7 +709,9 @@ bfq_pos_tree_add_move(struct bfq_data *bfqd, struct bfq_queue *bfqq)
>  * much easier to maintain the needed state:
>  * 1) all active queues have the same weight,
>  * 2) all active queues belong to the same I/O-priority class,
> - * 3) there are no active groups.
> + * 3) there are one active group at most(incluing root_group).

there are -> there is
incluing -> including
add a space before left parenthesis

> + * If the last condition is false, there is no need to guarantee the,

remove comma

> + * same share of the throughput of queues in the same group.

Actually, I would not add this extra comment on the last condition at all.

>  * In particular, the last condition is always true if hierarchical
>  * support or the cgroups interface are not enabled, thus no state
>  * needs to be maintained in this case.
> @@ -717,7 +719,26 @@ bfq_pos_tree_add_move(struct bfq_data *bfqd, struct bfq_queue *bfqq)
> static bool bfq_asymmetric_scenario(struct bfq_data *bfqd,
> 				   struct bfq_queue *bfqq)
> {
> -	bool smallest_weight = bfqq &&
> +	bool smallest_weight;
> +	bool varied_queue_weights;
> +	bool multiple_classes_busy;
> +
> +#ifdef CONFIG_BFQ_GROUP_IOSCHED
> +	if (bfqd->num_groups_with_pending_reqs > 1)
> +		return true;
> +
> +	if (bfqd->num_groups_with_pending_reqs &&
> +	    bfqd->num_queues_with_pending_reqs_in_root)
> +		return true;
> +
> +	/*
> +	 * Reach here means only one group(incluing root group) has pending
> +	 * requests, thus it's safe to return.
> +	 */
> +	return false;
> +#endif
> +
> +	smallest_weight = bfqq &&
> 		bfqq->weight_counter &&
> 		bfqq->weight_counter ==
> 		container_of(
> @@ -729,21 +750,17 @@ static bool bfq_asymmetric_scenario(struct bfq_data *bfqd,
> 	 * For queue weights to differ, queue_weights_tree must contain
> 	 * at least two nodes.
> 	 */
> -	bool varied_queue_weights = !smallest_weight &&
> +	varied_queue_weights = !smallest_weight &&
> 		!RB_EMPTY_ROOT(&bfqd->queue_weights_tree.rb_root) &&
> 		(bfqd->queue_weights_tree.rb_root.rb_node->rb_left ||
> 		 bfqd->queue_weights_tree.rb_root.rb_node->rb_right);
> 
> -	bool multiple_classes_busy =
> +	multiple_classes_busy =
> 		(bfqd->busy_queues[0] && bfqd->busy_queues[1]) ||
> 		(bfqd->busy_queues[0] && bfqd->busy_queues[2]) ||
> 		(bfqd->busy_queues[1] && bfqd->busy_queues[2]);
> 
> -	return varied_queue_weights || multiple_classes_busy
> -#ifdef CONFIG_BFQ_GROUP_IOSCHED
> -	       || bfqd->num_groups_with_pending_reqs > 0

Why do you make these extensive changes, while you can leave all the
function unchanged and just modify the above condition to something
like

|| bfqd->num_groups_with_pending_reqs > 1
|| (bfqd->num_groups_with_pending_reqs && bfqd->num_queues_with_pending_reqs_in_root)

In addition, I still wonder whether you can simply add also the root
group to bfqd->num_groups_with_pending_reqs (when the root group is
active).  This would make the design much cleaner.

Thanks,
Paolo

> -#endif
> -		;
> +	return varied_queue_weights || multiple_classes_busy;
> }
> 
> /*
> -- 
> 2.31.1
>