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

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Date:   Wed, 13 Apr 2022 19:40:32 +0800
From:   "yukuai (C)" <yukuai3@...wei.com>
To:     Jan Kara <jack@...e.cz>
CC:     <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

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.
> 
> 								Honza
>