Re: [PATCH -next v4 2/3] block, bfq: refactor the counting of 'num_groups_with_pending_reqs'

[Jan Kara]

On Thu 28-04-22 19:19:06, Yu Kuai wrote:
> 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@xxxxxxxxxx>
> ---
>  block/bfq-iosched.c | 46 ++-----------------------------------
>  block/bfq-iosched.h | 55 ++++++---------------------------------------
>  block/bfq-wf2q.c    | 19 ++++------------
>  3 files changed, 13 insertions(+), 107 deletions(-)

I love the diffstat :)

> +	 * Number of groups with at leaset one bfqq that is marked busy,
				    ^^^^^ least

Otherwise the patch looks good so feel free to add:

Reviewed-by: Jan Kara <jack@xxxxxxx>

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR