linux-kernel - Re: [PATCH -next v2 2/5] block, bfq: add fake weight

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Message-ID: <20220425161650.xzyijgkb5yzviea3@quack3.lan>
Date:   Mon, 25 Apr 2022 18:16:50 +0200
From:   Jan Kara <jack@...e.cz>
To:     "yukuai (C)" <yukuai3@...wei.com>
Cc:     Jan Kara <jack@...e.cz>, paolo.valente@...aro.org, axboe@...nel.dk,
        tj@...nel.org, linux-block@...r.kernel.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        yi.zhang@...wei.com
Subject: Re: [PATCH -next v2 2/5] block, bfq: add fake weight_counter for
 weight-raised queue

Hello!

On Mon 25-04-22 21:34:16, yukuai (C) wrote:
> 在 2022/04/25 17:48, Jan Kara 写道:
> > On Sat 16-04-22 17:37:50, Yu Kuai wrote:
> > > Weight-raised queue is not inserted to weights_tree, which makes it
> > > impossible to track how many queues have pending requests through
> > > weights_tree insertion and removel. This patch add fake weight_counter
> > > for weight-raised queue to do that.
> > > 
> > > Signed-off-by: Yu Kuai <yukuai3@...wei.com>
> > 
> > This is a bit hacky. I was looking into a better place where to hook to
> > count entities in a bfq_group with requests and I think bfq_add_bfqq_busy()
> > and bfq_del_bfqq_busy() are ideal for this. It also makes better sense
> > conceptually than hooking into weights tree handling.
> 
> bfq_del_bfqq_busy() will be called when all the reqs in the bfqq are
> dispatched, however there might still some reqs are't completed yet.
> 
> Here what we want to track is how many bfqqs have pending reqs,
> specifically if the bfqq have reqs are't complted.
> 
> Thus I think bfq_del_bfqq_busy() is not the right place to do that.

Yes, I'm aware there will be a difference. But note that bfqq can stay busy
with only dispatched requests because the logic in __bfq_bfqq_expire() will
not call bfq_del_bfqq_busy() if idling is needed for service guarantees. So
I think using bfq_add/del_bfqq_busy() would work OK. 

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