linux-kernel - Re: [PATCH 5/8] block: add code to track actual device queue depth

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Date:   Wed, 2 Nov 2016 15:59:08 +0100
From:   Christoph Hellwig <hch@....de>
To:     Jens Axboe <axboe@...com>
Cc:     axboe@...nel.dk, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org,
        jack@...e.cz, hch@....de
Subject: Re: [PATCH 5/8] block: add code to track actual device queue depth

On Tue, Nov 01, 2016 at 03:08:48PM -0600, Jens Axboe wrote:
> For blk-mq, ->nr_requests does track queue depth, at least at init
> time. But for the older queue paths, it's simply a soft setting.
> On top of that, it's generally larger than the hardware setting
> on purpose, to allow backup of requests for merging.
> 
> Fill a hole in struct request with a 'queue_depth' member, that

That would be struct request_queue..

>  /**
> + * blk_set_queue_depth - tell the block layer about the device queue depth
> + * @q:		the request queue for the device
> + * @depth:		queue depth
> + *
> + */
> +void blk_set_queue_depth(struct request_queue *q, unsigned int depth)
> +{
> +	q->queue_depth = depth;
> +}
> +EXPORT_SYMBOL(blk_set_queue_depth);

Do we really need this wrapper?

> +++ b/drivers/scsi/scsi.c
> @@ -621,6 +621,9 @@ int scsi_change_queue_depth(struct scsi_device *sdev, int depth)
>  		wmb();
>  	}
>  
> +	if (sdev->request_queue)
> +		blk_set_queue_depth(sdev->request_queue, depth);
> +
>  	return sdev->queue_depth;

Can we kill the scsi_device queue_depth member and just always use
the request_queue one?