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: <190625d8-ed84-f657-6058-2d151f6d4caa@kernel.dk>
Date:   Tue, 29 Mar 2022 06:44:28 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Yu Kuai <yukuai3@...wei.com>, 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 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?

-- 
Jens Axboe