linux-kernel - Re: [PATCH v2 1/1] blk-mq: fill header with kernel-doc

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Message-ID: <86de2c88-5812-4a87-b5d8-1b7b1808d013@acm.org>
Date:   Tue, 8 Oct 2019 13:01:38 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Gabriel Krisman Bertazi <krisman@...labora.com>
Cc:     André Almeida <andrealmeid@...labora.com>,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        axboe@...nel.dk, kernel@...labora.com
Subject: Re: [PATCH v2 1/1] blk-mq: fill header with kernel-doc

On 10/8/19 11:46 AM, Gabriel Krisman Bertazi wrote:
> Hmm, sorry, but I'm confused.  I'm sure I'm missing something simple,
> since I haven't touched this in a while, so maybe you can quickly point
> me in the right direction?
> 
> I see blk_mq_requeue_request() being used by device drivers to retry
> requests that failed, but if I read the code correctly, the flushed
> queue seems to be moved to hctx->dispatch when the device
> driver returned BLK_STS_RESOURCE or BLK_STS_DEV_RESOURCE to
> blk_mq_dispatch_rq_list(). I thought BLK_STS_RESOURCE was returned by
> the driver on .queue_rq() to signal there was no more resources on the
> hardware to service further requests.

Hi Gabriel,

The simplified version of how requests are requeued as follows:
* A block driver calls blk_mq_requeue_request().
* blk_mq_requeue_request() calls blk_mq_add_to_requeue_list()
* blk_mq_add_to_requeue_list() executes the following code:
     list_add_tail(&rq->queuelist, &q->requeue_list)
* A block driver or the block layer core calls
   blk_mq_kick_requeue_list() or blk_mq_delay_kick_requeue_list(). Both
   functions trigger a call of blk_mq_requeue_work().
* blk_mq_requeue_work() processes q->requeue_list.

Bart.