linux-kernel - Re: [PATCH RFC 3/3] lib/sbitmap: fix shallow

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Message-ID: <0e7225ea-72de-76c8-9154-0afc197a7e22@huaweicloud.com>
Date: Tue, 10 Dec 2024 09:28:46 +0800
From: Yu Kuai <yukuai1@...weicloud.com>
To: Bart Van Assche <bvanassche@....org>, Yu Kuai <yukuai1@...weicloud.com>,
 axboe@...nel.dk, akpm@...ux-foundation.org, yang.yang@...o.com,
 ming.lei@...hat.com, osandov@...com, paolo.valente@...aro.org
Cc: linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
 yi.zhang@...wei.com, yangerkun@...wei.com, "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH RFC 3/3] lib/sbitmap: fix shallow_depth tag allocation

Hi,

在 2024/12/10 2:11, Bart Van Assche 写道:
> On 12/9/24 7:55 PM, Yu Kuai wrote:
>> From: Yu Kuai <yukuai3@...wei.com>
>>
>> Currently, shallow_depth is used by bfq, kyber and mq-deadline, they both
>> pass in the value for the whole sbitmap, while sbitmap treate the value
>> for just one word. Which means, shallow_depth never work as expected,
>> and there really is no such functional tests to covert it.
> 
> treate -> treats
> 
> The above text is incorrect. I have verified that shallow_depth 
> restricts the queue depth of asynchronous requests for mq-deadline if it
> is reduced from its default value. The function dd_word_to_depth() in
> mq-deadline converts the value written into the sysfs attribute into a
> value that is appropriate for the sbitmap implementation.

Hi,

please notice that patch 1 is reverted in this set, which means
dd_word_to_depth() is not existed. :)

And for patch 1, dd_word_to_depth() doesn't(and can't) handle the last
word, for the case that avaliable bits in the last word is less than
1 << sb->shift. So, in fact it's not convert to the appropriate value.

> 
> That being said, it seems like a good idea to me to modify how the
> sbitmap code interprets 'shallow_depth'.

Good to know!

Thanks,
Kuai

> 
> Thanks,
> 
> Bart.
> .
>