linux-kernel - Re: [PATCH -next 10/11] block, bfq: decrease 'num_groups_with_pending

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Date:   Tue, 19 Apr 2022 11:49:55 +0200
From:   Jan Kara <jack@...e.cz>
To:     "yukuai (C)" <yukuai3@...wei.com>
Cc:     Jan Kara <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 10/11] block, bfq: decrease
 'num_groups_with_pending_reqs' earlier

On Fri 15-04-22 09:10:06, yukuai (C) wrote:
> 在 2022/04/13 19:40, yukuai (C) 写道:
> > 在 2022/04/13 19:28, Jan Kara 写道:
> > > On Sat 05-03-22 17:12:04, Yu Kuai wrote:
> > > > Currently 'num_groups_with_pending_reqs' won't be decreased when
> > > > the group doesn't have any pending requests, while some child group
> > > > still have pending requests. The decrement is delayed to when all the
> > > > child groups doesn't have any pending requests.
> > > > 
> > > > For example:
> > > > 1) t1 issue sync io on root group, t2 and t3 issue sync io on the same
> > > > child group. num_groups_with_pending_reqs is 2 now.
> > > > 2) t1 stopped, num_groups_with_pending_reqs is still 2. io from t2 and
> > > > t3 still can't be handled concurrently.
> > > > 
> > > > Fix the problem by decreasing 'num_groups_with_pending_reqs'
> > > > immediately upon the weights_tree removal of last bfqq of the group.
> > > > 
> > > > Signed-off-by: Yu Kuai <yukuai3@...wei.com>
> > > 
> > > So I'd find the logic easier to follow if you completely removed
> > > entity->in_groups_with_pending_reqs and did updates of
> > > bfqd->num_groups_with_pending_reqs like:
> > > 
> > >     if (!bfqg->num_entities_with_pending_reqs++)
> > >         bfqd->num_groups_with_pending_reqs++;
> > > 
> > Hi,
> > 
> > Indeed, this is an excellent idle, and much better than the way I did.
> > 
> > Thanks,
> > Kuai
> > 
> > > and similarly on the remove side. And there would we literally two places
> > > (addition & removal from weight tree) that would need to touch these
> > > counters. Pretty obvious and all can be done in patch 9.
> 
> I think with this change, we can count root_group while activating bfqqs
> that are under root_group, thus there is no need to modify
> for_each_entity(or fake bfq_sched_data) any more.

Sure, if you can make this work, it would be easier :)

> The special case is that weight racing bfqqs are not inserted into
> weights tree, and I think this can be handled by adding a fake
> bfq_weight_counter for such bfqqs.

Do you mean "weight raised bfqqs"? Yes, you are right they would need
special treatment - maybe bfq_weights_tree_add() is not the best function
to use for this and we should rather use insertion / removal from the
service tree for maintaining num_entities_with_pending_reqs counter?
I can even see we already have bfqg->active_entities so maybe we could just
somehow tweak that accounting and use it for our purposes?

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