Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Subject: Re: [PATCH] Revert "bfq: Fix computation of shallow depth"
To:     Jan Kara <jack@suse.cz>, Lin Feng <linf@wangsu.com>
Cc:     paolo.valente@linaro.org, linux-kernel@vger.kernel.org,
        linux-block@vger.kernel.org
References: <20210129111808.45796-1-linf@wangsu.com>
 <20210202122836.GC17147@quack2.suse.cz>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <f92a55b4-0743-a700-8296-e739f4fcf093@kernel.dk>
Date:   Tue, 2 Feb 2021 07:20:16 -0700
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101
 Thunderbird/68.10.0
MIME-Version: 1.0
In-Reply-To: <20210202122836.GC17147@quack2.suse.cz>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Precedence: bulk

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@suse.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.

-- 
Jens Axboe