linux-kernel - Re: [PATCH RFC 2/7] blk-mq: delay tag fair sharing until fail to get driver tag

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Date:   Mon, 19 Jun 2023 07:55:22 +0200
From:   Hannes Reinecke <hare@...e.de>
To:     Yu Kuai <yukuai1@...weicloud.com>, bvanassche@....org,
        axboe@...nel.dk
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        yukuai3@...wei.com, yi.zhang@...wei.com, yangerkun@...wei.com
Subject: Re: [PATCH RFC 2/7] blk-mq: delay tag fair sharing until fail to get
 driver tag

On 6/18/23 18:07, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@...wei.com>
> 
> Start tag fair sharing when a device start to issue io will waste
> resources, same number of tags will be assigned to each disk/hctx,
> and such tags can't be used for other disk/hctx, which means a disk/hctx
> can't use more than assinged tags even if there are still lots of tags
> that is assinged to other disks are unused.
> 
> Add a new api blk_mq_driver_tag_busy(), it will be called when get
> driver tag failed, and move tag sharing from blk_mq_tag_busy() to
> blk_mq_driver_tag_busy().
> 
> This approch will work well if total tags are not exhausted, and follow
> up patches will try to refactor how tag is shared to handle this case.
> 
> Signed-off-by: Yu Kuai <yukuai3@...wei.com>
> ---
>   block/blk-mq-debugfs.c |  4 ++-
>   block/blk-mq-tag.c     | 60 ++++++++++++++++++++++++++++++++++--------
>   block/blk-mq.c         |  4 ++-
>   block/blk-mq.h         | 13 ++++++---
>   include/linux/blk-mq.h |  6 +++--
>   include/linux/blkdev.h |  1 +
>   6 files changed, 70 insertions(+), 18 deletions(-)
> 
> diff --git a/block/blk-mq-debugfs.c b/block/blk-mq-debugfs.c
> index 431aaa3eb181..de5a911b07c2 100644
> --- a/block/blk-mq-debugfs.c
> +++ b/block/blk-mq-debugfs.c
> @@ -400,8 +400,10 @@ static void blk_mq_debugfs_tags_show(struct seq_file *m,
>   {
>   	seq_printf(m, "nr_tags=%u\n", tags->nr_tags);
>   	seq_printf(m, "nr_reserved_tags=%u\n", tags->nr_reserved_tags);
> -	seq_printf(m, "active_queues=%d\n",
> +	seq_printf(m, "active_queues=%u\n",
>   		   READ_ONCE(tags->ctl.active_queues));
> +	seq_printf(m, "share_queues=%u\n",
> +		   READ_ONCE(tags->ctl.share_queues));
>   
>   	seq_puts(m, "\nbitmap_tags:\n");
>   	sbitmap_queue_show(&tags->bitmap_tags, m);
> diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
> index fe41a0d34fc0..1c2bde917195 100644
> --- a/block/blk-mq-tag.c
> +++ b/block/blk-mq-tag.c
> @@ -29,6 +29,32 @@ static void blk_mq_update_wake_batch(struct blk_mq_tags *tags,
>   			users);
>   }
>   
> +void __blk_mq_driver_tag_busy(struct blk_mq_hw_ctx *hctx)
> +{
> +	struct blk_mq_tags *tags = hctx->tags;
> +
> +	/*
> +	 * calling test_bit() prior to test_and_set_bit() is intentional,
> +	 * it avoids dirtying the cacheline if the queue is already active.
> +	 */
> +	if (blk_mq_is_shared_tags(hctx->flags)) {
> +		struct request_queue *q = hctx->queue;
> +
> +		if (test_bit(QUEUE_FLAG_HCTX_BUSY, &q->queue_flags) ||
> +		    test_and_set_bit(QUEUE_FLAG_HCTX_BUSY, &q->queue_flags))
> +			return;
> +	} else {
> +		if (test_bit(BLK_MQ_S_DTAG_BUSY, &hctx->state) ||
> +		    test_and_set_bit(BLK_MQ_S_DTAG_BUSY, &hctx->state))
> +			return;
> +	}
> +
> +	spin_lock_irq(&tags->lock);
> +	WRITE_ONCE(tags->ctl.share_queues, tags->ctl.active_queues);
> +	blk_mq_update_wake_batch(tags, tags->ctl.share_queues);
> +	spin_unlock_irq(&tags->lock);
> +}
> +
>   /*
>    * If a previously inactive queue goes active, bump the active user count.
>    * We need to do this before try to allocate driver tag, then even if fail
> @@ -37,7 +63,6 @@ static void blk_mq_update_wake_batch(struct blk_mq_tags *tags,
>    */
>   void __blk_mq_tag_busy(struct blk_mq_hw_ctx *hctx)
>   {
> -	unsigned int users;
>   	struct blk_mq_tags *tags = hctx->tags;
>   
>   	/*
> @@ -57,9 +82,7 @@ void __blk_mq_tag_busy(struct blk_mq_hw_ctx *hctx)
>   	}
>   
>   	spin_lock_irq(&tags->lock);
> -	users = tags->ctl.active_queues + 1;
> -	WRITE_ONCE(tags->ctl.active_queues, users);
> -	blk_mq_update_wake_batch(tags, users);
> +	WRITE_ONCE(tags->ctl.active_queues, tags->ctl.active_queues + 1);

Why did you remove the call to blk_mq_update_wake_batch() here?

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                Kernel Storage Architect
hare@...e.de                              +49 911 74053 688
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