linux-kernel - Re: [PATCH] Revert "bfq: Fix computation of shallow depth"

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Date:   Tue, 2 Feb 2021 19:39:51 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Lin Feng <linf@...gsu.com>, Jan Kara <jack@...e.cz>
Cc:     paolo.valente@...aro.org, linux-kernel@...r.kernel.org,
        linux-block@...r.kernel.org
Subject: Re: [PATCH] Revert "bfq: Fix computation of shallow depth"

On 2/2/21 7:36 PM, Lin Feng wrote:
> Hi all,
> 
> On 2/2/21 22:20, Jens Axboe wrote:
>> On 2/2/21 5:28 AM, Jan Kara wrote:
>>> Hello!
>>>
>>> On Fri 29-01-21 19:18:08, Lin Feng wrote:
>>>> This reverts commit 6d4d273588378c65915acaf7b2ee74e9dd9c130a.
>>>>
>>>> bfq.limit_depth passes word_depths[] as shallow_depth down to sbitmap core
>>>> sbitmap_get_shallow, which uses just the number to limit the scan depth of
>>>> each bitmap word, formula:
>>>> scan_percentage_for_each_word = shallow_depth / (1 << sbimap->shift) * 100%
>>>
>>> Looking at sbitmap_get_shallow() again more carefully, I agree that I
>>> misunderstood how shallow_depth argument gets used and the original code
>>> was correct and I broke it. Thanks for spotting this!
>>>
>>> What I didn't notice is that shallow_depth indeed gets used for each bitmap
>>> word separately and not for bitmap as a whole. I'd say this could use some
>>> more documentation but that's unrelated to your revert. So feel free to add:
>>>
>>> Reviewed-by: Jan Kara <jack@...e.cz>
>>
>> I don't have the original patch (neither directly nor in the archive), so
>> I had to hand-apply it. In any case, applied for 5.11, thanks.
>>
> 
> Take a look at linux-block.git tree, the hand-applied commit for this patch
> is broken, the following changing line is left out:
> -	bfqd->word_depths[0][1] = max((bt->sb.depth * 3) >> 2, 1U);
> +	bfqd->word_depths[0][1] = max(((1U << bt->sb.shift) * 3) >> 2, 1U);
> 
> Sorry for making troubles to you, I will resend this patch with tiny commit
> log typo fix(sbimap -> sbitmap) and attaching Jan's Reviewed-by, also thanks
> his time for reviewing.
> 
> Hope this time lkml server will not block my patch.

Thanks for checking - just send me an incremental and I'll fold it in.

-- 
Jens Axboe