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From: Hannes Reinecke <hare@suse.de>
To: Bart Van Assche <bvanassche@acm.org>, Jens Axboe <axboe@kernel.dk>
Cc: linux-block@vger.kernel.org, Christoph Hellwig <hch@lst.de>,
	Jaegeuk Kim <jaegeuk@kernel.org>,
	Damien Le Moal <damien.lemoal@wdc.com>,
	Ming Lei <ming.lei@redhat.com>,
	Johannes Thumshirn <johannes.thumshirn@wdc.com>,
	Himanshu Madhani <himanshu.madhani@oracle.com>
Subject: Re: [PATCH 11/14] block/mq-deadline: Reserve 25% of scheduler tags for synchronous requests
Date: Thu, 10 Jun 2021 08:26:18 +0200	[thread overview]
Message-ID: <81ee9a14-77e1-f3a4-af28-ba06ba9f7413@suse.de> (raw)
In-Reply-To: <20210608230703.19510-12-bvanassche@acm.org>

On 6/9/21 1:07 AM, Bart Van Assche wrote:
> For interactive workloads it is important that synchronous requests are
> not delayed. Hence reserve 25% of scheduler tags for synchronous requests.
> This patch still allows asynchronous requests to fill the hardware queues
> since blk_mq_init_sched() makes sure that the number of scheduler requests
> is the double of the hardware queue depth. From blk_mq_init_sched():
> 
> 	q->nr_requests = 2 * min_t(unsigned int, q->tag_set->queue_depth,
> 				   BLKDEV_MAX_RQ);
> 
> Cc: Damien Le Moal <damien.lemoal@wdc.com>
> Cc: Hannes Reinecke <hare@suse.de>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Ming Lei <ming.lei@redhat.com>
> Cc: Johannes Thumshirn <johannes.thumshirn@wdc.com>
> Cc: Himanshu Madhani <himanshu.madhani@oracle.com>
> Signed-off-by: Bart Van Assche <bvanassche@acm.org>
> ---
>  block/mq-deadline.c | 52 +++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 52 insertions(+)
> 
> diff --git a/block/mq-deadline.c b/block/mq-deadline.c
> index 1d1bb7a41d2a..a7d0584437d1 100644
> --- a/block/mq-deadline.c
> +++ b/block/mq-deadline.c
> @@ -67,6 +67,7 @@ struct deadline_data {
>  	int fifo_batch;
>  	int writes_starved;
>  	int front_merges;
> +	u32 async_depth;
>  
>  	spinlock_t lock;
>  	spinlock_t zone_lock;
> @@ -397,6 +398,44 @@ static struct request *dd_dispatch_request(struct blk_mq_hw_ctx *hctx)
>  	return rq;
>  }
>  
> +/*
> + * Called by __blk_mq_alloc_request(). The shallow_depth value set by this
> + * function is used by __blk_mq_get_tag().
> + */
> +static void dd_limit_depth(unsigned int op, struct blk_mq_alloc_data *data)
> +{
> +	struct deadline_data *dd = data->q->elevator->elevator_data;
> +
> +	/* Do not throttle synchronous reads. */
> +	if (op_is_sync(op) && !op_is_write(op))
> +		return;
> +
> +	/*
> +	 * Throttle asynchronous requests and writes such that these requests
> +	 * do not block the allocation of synchronous requests.
> +	 */
> +	data->shallow_depth = dd->async_depth;
> +}
> +
> +/* Called by blk_mq_update_nr_requests(). */
> +static void dd_depth_updated(struct blk_mq_hw_ctx *hctx)
> +{
> +	struct request_queue *q = hctx->queue;
> +	struct deadline_data *dd = q->elevator->elevator_data;
> +	struct blk_mq_tags *tags = hctx->sched_tags;
> +
> +	dd->async_depth = max(1UL, 3 * q->nr_requests / 4);
> +
> +	sbitmap_queue_min_shallow_depth(tags->bitmap_tags, dd->async_depth);
> +}
> +
> +/* Called by blk_mq_init_hctx() and blk_mq_init_sched(). */
> +static int dd_init_hctx(struct blk_mq_hw_ctx *hctx, unsigned int hctx_idx)
> +{
> +	dd_depth_updated(hctx);
> +	return 0;
> +}
> +
>  static void dd_exit_sched(struct elevator_queue *e)
>  {
>  	struct deadline_data *dd = e->elevator_data;
> @@ -733,6 +772,15 @@ static int deadline_starved_show(void *data, struct seq_file *m)
>  	return 0;
>  }
>  
> +static int dd_async_depth_show(void *data, struct seq_file *m)
> +{
> +	struct request_queue *q = data;
> +	struct deadline_data *dd = q->elevator->elevator_data;
> +
> +	seq_printf(m, "%u\n", dd->async_depth);
> +	return 0;
> +}
> +
>  static void *deadline_dispatch_start(struct seq_file *m, loff_t *pos)
>  	__acquires(&dd->lock)
>  {
> @@ -775,6 +823,7 @@ static const struct blk_mq_debugfs_attr deadline_queue_debugfs_attrs[] = {
>  	DEADLINE_QUEUE_DDIR_ATTRS(write),
>  	{"batching", 0400, deadline_batching_show},
>  	{"starved", 0400, deadline_starved_show},
> +	{"async_depth", 0400, dd_async_depth_show},
>  	{"dispatch", 0400, .seq_ops = &deadline_dispatch_seq_ops},
>  	{},
>  };
> @@ -783,6 +832,8 @@ static const struct blk_mq_debugfs_attr deadline_queue_debugfs_attrs[] = {
>  
>  static struct elevator_type mq_deadline = {
>  	.ops = {
> +		.depth_updated		= dd_depth_updated,
> +		.limit_depth		= dd_limit_depth,
>  		.insert_requests	= dd_insert_requests,
>  		.dispatch_request	= dd_dispatch_request,
>  		.prepare_request	= dd_prepare_request,
> @@ -796,6 +847,7 @@ static struct elevator_type mq_deadline = {
>  		.has_work		= dd_has_work,
>  		.init_sched		= dd_init_sched,
>  		.exit_sched		= dd_exit_sched,
> +		.init_hctx		= dd_init_hctx,
>  	},
>  
>  #ifdef CONFIG_BLK_DEBUG_FS
> 
Can't say I like heuristics. Might be needed, but there will always be
use-cases where the heuristics fail. Can't we make this value
configurable via sysfs?

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		        Kernel Storage Architect
hare@suse.de			               +49 911 74053 688
SUSE Software Solutions Germany GmbH, 90409 Nürnberg
GF: F. Imendörffer, HRB 36809 (AG Nürnberg)

  reply	other threads:[~2021-06-10  6:26 UTC|newest]

Thread overview: 45+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-08 23:06 [PATCH 00/14] Improve I/O priority support Bart Van Assche
2021-06-08 23:06 ` [PATCH 01/14] block/Kconfig: Make the BLK_WBT and BLK_WBT_MQ entries consecutive Bart Van Assche
2021-06-09  4:14   ` Damien Le Moal
2021-06-09  7:03   ` Johannes Thumshirn
2021-06-10  6:02   ` Hannes Reinecke
2021-06-08 23:06 ` [PATCH 02/14] block/blk-cgroup: Swap the blk_throtl_init() and blk_iolatency_init() calls Bart Van Assche
2021-06-09  4:19   ` Damien Le Moal
2021-06-09  7:05   ` Johannes Thumshirn
2021-06-10  6:02   ` Hannes Reinecke
2021-06-08 23:06 ` [PATCH 03/14] block/blk-rq-qos: Move a function from a header file into a C file Bart Van Assche
2021-06-09  4:22   ` Damien Le Moal
2021-06-10  6:03   ` Hannes Reinecke
2021-06-08 23:06 ` [PATCH 04/14] block: Introduce the ioprio rq-qos policy Bart Van Assche
2021-06-09  4:40   ` Damien Le Moal
2021-06-09 16:48     ` Bart Van Assche
2021-06-10  0:29       ` Damien Le Moal
2021-06-10  6:20   ` Hannes Reinecke
2021-06-10 17:14     ` Bart Van Assche
2021-06-08 23:06 ` [PATCH 05/14] block/mq-deadline: Add several comments Bart Van Assche
2021-06-10  6:21   ` Hannes Reinecke
2021-06-08 23:06 ` [PATCH 06/14] block/mq-deadline: Add two lockdep_assert_held() statements Bart Van Assche
2021-06-10  6:21   ` Hannes Reinecke
2021-06-08 23:06 ` [PATCH 07/14] block/mq-deadline: Remove two local variables Bart Van Assche
2021-06-10  6:22   ` Hannes Reinecke
2021-06-08 23:06 ` [PATCH 08/14] block/mq-deadline: Rename dd_init_queue() and dd_exit_queue() Bart Van Assche
2021-06-08 23:06 ` [PATCH 09/14] block/mq-deadline: Improve compile-time argument checking Bart Van Assche
2021-06-08 23:06 ` [PATCH 10/14] block/mq-deadline: Improve the sysfs show and store macros Bart Van Assche
2021-06-09  4:46   ` Damien Le Moal
2021-06-09 16:54     ` Bart Van Assche
2021-06-10  0:31       ` Damien Le Moal
2021-06-09  7:11   ` Johannes Thumshirn
2021-06-09 16:59     ` Bart Van Assche
2021-06-10  6:23   ` Hannes Reinecke
2021-06-08 23:07 ` [PATCH 11/14] block/mq-deadline: Reserve 25% of scheduler tags for synchronous requests Bart Van Assche
2021-06-10  6:26   ` Hannes Reinecke [this message]
2021-06-08 23:07 ` [PATCH 12/14] block/mq-deadline: Add I/O priority support Bart Van Assche
2021-06-09  5:03   ` Damien Le Moal
2021-06-09 17:25     ` Bart Van Assche
2021-06-10  0:39       ` Damien Le Moal
2021-06-10  6:35   ` Hannes Reinecke
2021-06-08 23:07 ` [PATCH 13/14] block/mq-deadline: Add cgroup support Bart Van Assche
2021-06-10  6:37   ` Hannes Reinecke
2021-06-08 23:07 ` [PATCH 14/14] block/mq-deadline: Prioritize high-priority requests Bart Van Assche
2021-06-09  5:10   ` Damien Le Moal
2021-06-10  6:43   ` Hannes Reinecke

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