linux-kernel - Re: [PATCH RFC 0/9] support concurrent sync io for bfq on a specail occasion

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Date:   Fri, 10 Dec 2021 11:23:39 +0100
From:   Paolo Valente <paolo.valente@...aro.org>
To:     "yukuai (C)" <yukuai3@...wei.com>
Cc:     Tejun Heo <tj@...nel.org>, Jens Axboe <axboe@...nel.dk>,
        cgroups@...r.kernel.org, linux-block <linux-block@...r.kernel.org>,
        linux-kernel@...r.kernel.org, yi.zhang@...wei.com
Subject: Re: [PATCH RFC 0/9] support concurrent sync io for bfq on a specail
 occasion



> Il giorno 10 dic 2021, alle ore 10:50, yukuai (C) <yukuai3@...wei.com> ha scritto:
> 
> 在 2021/12/10 17:20, Paolo Valente 写道:
>>> Il giorno 27 nov 2021, alle ore 11:11, Yu Kuai <yukuai3@...wei.com> ha scritto:
>>> 
>>> Bfq can't handle sync io concurrently as long as the io are not issued
>>> from root group currently.
>>> 
>>> Previous patch set:
>>> https://lore.kernel.org/lkml/20211014014556.3597008-2-yukuai3@huawei.com/t/
>>> 
>>> During implemting the method mentioned by the above patch set, I found
>>> more problems that will block implemting concurrent sync io. The
>>> modifications of this patch set are as follows:
>>> 
>>> 1) count root group into 'num_groups_with_pending_reqs';
>>> 2) don't idle if 'num_groups_with_pending_reqs' is 1;
>>> 3) If the group doesn't have pending requests while it's child groups
>>> have pending requests, don't count the group.
>> Why don't yo count the parent group? It seems to me that we should count it.
> Hi, Paolo
> 
> For example, we only issue io in child group c2(root->c1->c2),
> 'num_groups_with_pending_reqs' will end up greater than 1, thus it's
> impossible to handle sync io concurrently. Thus I don't count root and
> c1, only count c2.

Right!

Please explain this clearly in comments.


>>> 4) Once the group doesn't have pending requests, decrease
>>> 'num_groups_with_pending_reqs' immediately. Don't delay to when all
>>> it's child groups don't have pending requests.
>>> 
>> I guess this action is related to 3).
> Yes, if c1, c2 are both active, and then c1 don't have any pending reqs,
> I want to decrease num_groups_with_pending_reqs to 1 immediately.

I'll check this point directly on the patch that does this decrement,
because something is not clear to me.

Thanks,
Paolo

>  So
> that sync io on c2 can be handled concurrently.
> 


> Thanks,
> Kuai
> 
>> Thanks,
>> Paolo
>>> Noted that I just tested basic functionality of this patchset, and I
>>> think it's better to see if anyone have suggestions or better
>>> solutions.
>>> 
>>> Yu Kuai (9):
>>>  block, bfq: add new apis to iterate bfq entities
>>>  block, bfq: apply news apis where root group is not expected
>>>  block, bfq: handle the case when for_each_entity() access root group
>>>  block, bfq: count root group into 'num_groups_with_pending_reqs'
>>>  block, bfq: do not idle if only one cgroup is activated
>>>  block, bfq: only count group that the bfq_queue belongs to
>>>  block, bfq: record how many queues have pending requests in bfq_group
>>>  block, bfq: move forward __bfq_weights_tree_remove()
>>>  block, bfq: decrease 'num_groups_with_pending_reqs' earlier
>>> 
>>> block/bfq-cgroup.c  |  3 +-
>>> block/bfq-iosched.c | 92 +++++++++++++++++++++++----------------------
>>> block/bfq-iosched.h | 41 +++++++++++++-------
>>> block/bfq-wf2q.c    | 44 +++++++++++++++-------
>>> 4 files changed, 106 insertions(+), 74 deletions(-)
>>> 
>>> -- 
>>> 2.31.1
>>> 
>> .