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

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Date:   Thu, 28 Apr 2022 13:23:34 +0200
From:   Jan Kara <jack@...e.cz>
To:     Yu Kuai <yukuai3@...wei.com>
Cc:     jack@...e.cz, tj@...nel.org, axboe@...nel.dk,
        paolo.valente@...aro.org, cgroups@...r.kernel.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        yi.zhang@...wei.com
Subject: Re: [PATCH -next v4 2/3] block, bfq: refactor the counting of
 'num_groups_with_pending_reqs'

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@...wei.com>
> ---
>  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@...e.cz>

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR