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

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Message-ID: <20220425162608.feya66a5amdnsr4e@quack3.lan>
Date:   Mon, 25 Apr 2022 18:26:08 +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

On Mon 25-04-22 21:55:46, yukuai (C) wrote:
> 在 2022/04/25 21:34, yukuai (C) 写道:
> > 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.
> > > 
> > Hi,
> > 
> > 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.
> 
> BTW, there is a counter 'dispatched' in bfqq, how about we rename it
> to 'inflight', and inc when adding req to bfqq, dec the same as
> 'dispatched' ?
> 
> This way we can count bfqq when adding 'inflight' from 0 to 1, and
> stop when decreasing 'inflight' from 1 to 0.

Well, but 'dispatched' is used in quite a few places and it would require
quite some thinking to decide which impact using 'inflight' has there...
But we also have 'bfqq->entity.allocated' which is number of requests in
some state associated with bfqq and we could use that. But as I wrote in my
previous email, I'm not convinced it is really necessary...

								Honza

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