linux-kernel - Re: [PATCH 2/5] block: add request->io_data

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Message-ID: <20200409014406.GA370295@localhost.localdomain>
Date:   Thu, 9 Apr 2020 09:44:06 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     axboe@...nel.dk, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel-team@...r.kernel.org,
        cgroups@...r.kernel.org, newella@...com, josef@...icpanda.com
Subject: Re: [PATCH 2/5] block: add request->io_data_len

Hi Tejun,

On Wed, Apr 08, 2020 at 04:14:47PM -0400, Tejun Heo wrote:
> Currently, at the time of completeion, there's no way of knowing how big a
> request was. blk-iocost will need this information to account for IO size when
> calculating expected latencies.
> 
> This patch adds rq->io_data_len which remembers blk_rq_bytes() at the time the
> request gets issued. The field is enabled iff CONFIG_BLK_IO_DATA_LEN is set and
> doesn't increase the size of the struct even when enabled.

Almost all __blk_mq_end_request() follow blk_update_request(), so the
completed bytes can be passed to __blk_mq_end_request(), then we can
avoid to introduce this field.

Also there is just 20 callers of __blk_mq_end_request(), looks this kind
of change shouldn't be too big.


Thanks, 
Ming