linux-kernel - Re: [PATCH -next RFC 1/6] blk-mq: add a new flag 'BLK_MQ_F_NO_TAG

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Message-ID: <f83ea9d7-bc89-6855-f9a6-c42f4647d383@huawei.com>
Date:   Wed, 30 Mar 2022 09:18:07 +0800
From:   "yukuai (C)" <yukuai3@...wei.com>
To:     Jens Axboe <axboe@...nel.dk>, <andriy.shevchenko@...ux.intel.com>,
        <john.garry@...wei.com>, <ming.lei@...hat.com>
CC:     <linux-block@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <yi.zhang@...wei.com>
Subject: Re: [PATCH -next RFC 1/6] blk-mq: add a new flag
 'BLK_MQ_F_NO_TAG_PREEMPTION'

On 2022/03/29 20:44, Jens Axboe wrote:
> On 3/29/22 3:40 AM, Yu Kuai wrote:
>> Tag preemption is the default behaviour, specifically blk_mq_get_tag()
>> will try to get tag unconditionally, which means a new io can preempt tag
>> even if there are lots of ios that are waiting for tags.
>>
>> This patch introduce a new flag, prepare to disable such behaviour, in
>> order to optimize io performance for large random io for HHD.
> 
> Not sure why we need a flag for this behavior. Does it ever make sense
> to allow preempting waiters, jumping the queue?
> 

Hi,

I was thinking using the flag to control the new behavior, in order to
reduce the impact on general path.

If wake up path is handled properly, I think it's ok to disable
preempting tags.

Thanks
Kuai