linux-kernel - Re: [PATCH v7 1/9] blk-throttle: fix that io throttle can only work for single bio

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Date:   Thu, 18 Aug 2022 09:23:39 +0800
From:   Yu Kuai <yukuai1@...weicloud.com>
To:     Tejun Heo <tj@...nel.org>, Yu Kuai <yukuai1@...weicloud.com>
Cc:     mkoutny@...e.com, axboe@...nel.dk, ming.lei@...hat.com,
        cgroups@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, yi.zhang@...wei.com,
        "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH v7 1/9] blk-throttle: fix that io throttle can only work
 for single bio

Hi, Tejun!

在 2022/08/18 1:50, Tejun Heo 写道:
> Hello,
> 
> On Wed, Aug 17, 2022 at 09:13:38AM +0800, Yu Kuai wrote:
>>> So, as a fix for the immediate problem, I guess this might do but this feels
>>> really fragile. How can we be certain that re-entering only happens because
>>> of splitting? What if future core development changes that? It seems to be
>>> solving the problem in the wrong place. Shouldn't we flag the bio indicating
>>> that it's split when we're splitting the bio so that we only limit them for
>>> iops in the first place?
>>
>> Splited bio is tracked in __bio_clone:
> 
> As the word is used in commit messages and comments, the past perfect form
> of the verb "split" is "split". It looks like "splitted" is used in rare
> cases but dictionary says it's an archaic form.

Ok, thanks for pointing it out, I'll change that in next iteration.
> 
>> if (bio_flagged(bio_src, BIO_THROTTLED))
>> 	bio_set_flag(bio, BIO_THROTTLED);
>>
>> And currenty, the iops limit and bps limit are treated differently,
>> however there are only one flag 'BIO_THROTTLED' and they can't be
>> distinguished.
>>
>> Perhaps I can use two flags, for example BIO_IOPS_THROTTLED and
>> BIO_BPS_THROTTLED, this way only iops limit can be handled and bps
>> limit can be skipped for splited bio.
>>
>> What do you think?
> 
> I think the code would be a lot more intuitive and less fragile if we used
> two flags but the bits in the bi_flags field are a scarce resource
> unfortunately. Even then, I think the right thing to do here is using two
> flags.

Yes, the field 'bio->bi_flags' is unsigned short, and there are only two
bits left. I'll use the new sulution which will acquire a new bit.

Thanks,
Kuai
> 
> Thanks.
>