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

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Date:   Fri, 15 Apr 2022 09:10:06 +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 19:40, 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.
>>
>>                                 Honza
Hi, Jan

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.

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.

What do you think ?

Kuai