Re: [PATCH 7/8] mq-deadline: add blk-mq adaptation of the deadline IO scheduler

[Paolo Valente]

> Il giorno 17 dic 2016, alle ore 01:12, Jens Axboe <axboe@xxxxxx> ha scritto:
> 
> This is basically identical to deadline-iosched, except it registers
> as a MQ capable scheduler. This is still a single queue design.
> 
> Signed-off-by: Jens Axboe <axboe@xxxxxx>
> ---
> block/Kconfig.iosched |   6 +
> block/Makefile        |   1 +
> block/mq-deadline.c   | 649 ++++++++++++++++++++++++++++++++++++++++++++++++++
> 3 files changed, 656 insertions(+)
> create mode 100644 block/mq-deadline.c
> 
> diff --git a/block/Kconfig.iosched b/block/Kconfig.iosched
> index 421bef9c4c48..490ef2850fae 100644
> --- a/block/Kconfig.iosched
> +++ b/block/Kconfig.iosched
> @@ -32,6 +32,12 @@ config IOSCHED_CFQ
> 
> 	  This is the default I/O scheduler.
> 
> +config MQ_IOSCHED_DEADLINE
> +	tristate "MQ deadline I/O scheduler"
> +	default y
> +	---help---
> +	  MQ version of the deadline IO scheduler.
> +
> config CFQ_GROUP_IOSCHED
> 	bool "CFQ Group Scheduling support"
> 	depends on IOSCHED_CFQ && BLK_CGROUP
> diff --git a/block/Makefile b/block/Makefile
> index 2eee9e1bb6db..3ee0abd7205a 100644
> --- a/block/Makefile
> +++ b/block/Makefile
> @@ -18,6 +18,7 @@ obj-$(CONFIG_BLK_DEV_THROTTLING)	+= blk-throttle.o
> obj-$(CONFIG_IOSCHED_NOOP)	+= noop-iosched.o
> obj-$(CONFIG_IOSCHED_DEADLINE)	+= deadline-iosched.o
> obj-$(CONFIG_IOSCHED_CFQ)	+= cfq-iosched.o
> +obj-$(CONFIG_MQ_IOSCHED_DEADLINE)	+= mq-deadline.o
> 
> obj-$(CONFIG_BLOCK_COMPAT)	+= compat_ioctl.o
> obj-$(CONFIG_BLK_CMDLINE_PARSER)	+= cmdline-parser.o
> diff --git a/block/mq-deadline.c b/block/mq-deadline.c
> new file mode 100644
> index 000000000000..3cb9de21ab21
> --- /dev/null
> +++ b/block/mq-deadline.c
> @@ -0,0 +1,649 @@
> +/*
> + *  MQ Deadline i/o scheduler - adaptation of the legacy deadline scheduler,
> + *  for the blk-mq scheduling framework
> + *
> + *  Copyright (C) 2016 Jens Axboe <axboe@xxxxxxxxx>
> + */
> +#include <linux/kernel.h>
> +#include <linux/fs.h>
> +#include <linux/blkdev.h>
> +#include <linux/blk-mq.h>
> +#include <linux/elevator.h>
> +#include <linux/bio.h>
> +#include <linux/module.h>
> +#include <linux/slab.h>
> +#include <linux/init.h>
> +#include <linux/compiler.h>
> +#include <linux/rbtree.h>
> +#include <linux/sbitmap.h>
> +
> +#include "blk.h"
> +#include "blk-mq.h"
> +#include "blk-mq-tag.h"
> +#include "blk-mq-sched.h"
> +
> +static unsigned int queue_depth = 256;
> +module_param(queue_depth, uint, 0644);
> +MODULE_PARM_DESC(queue_depth, "Use this value as the scheduler queue depth");
> +
> +/*
> + * See Documentation/block/deadline-iosched.txt
> + */
> +static const int read_expire = HZ / 2;  /* max time before a read is submitted. */
> +static const int write_expire = 5 * HZ; /* ditto for writes, these limits are SOFT! */
> +static const int writes_starved = 2;    /* max times reads can starve a write */
> +static const int fifo_batch = 16;       /* # of sequential requests treated as one
> +				     by the above parameters. For throughput. */
> +
> +struct deadline_data {
> +	/*
> +	 * run time data
> +	 */
> +
> +	/*
> +	 * requests (deadline_rq s) are present on both sort_list and fifo_list
> +	 */
> +	struct rb_root sort_list[2];
> +	struct list_head fifo_list[2];
> +
> +	/*
> +	 * next in sort order. read, write or both are NULL
> +	 */
> +	struct request *next_rq[2];
> +	unsigned int batching;		/* number of sequential requests made */
> +	unsigned int starved;		/* times reads have starved writes */
> +
> +	/*
> +	 * settings that change how the i/o scheduler behaves
> +	 */
> +	int fifo_expire[2];
> +	int fifo_batch;
> +	int writes_starved;
> +	int front_merges;
> +
> +	spinlock_t lock;
> +	struct list_head dispatch;
> +	struct blk_mq_tags *tags;
> +	atomic_t wait_index;
> +};
> +
> +static inline struct rb_root *
> +deadline_rb_root(struct deadline_data *dd, struct request *rq)
> +{
> +	return &dd->sort_list[rq_data_dir(rq)];
> +}
> +
> +/*
> + * get the request after `rq' in sector-sorted order
> + */
> +static inline struct request *
> +deadline_latter_request(struct request *rq)
> +{
> +	struct rb_node *node = rb_next(&rq->rb_node);
> +
> +	if (node)
> +		return rb_entry_rq(node);
> +
> +	return NULL;
> +}
> +
> +static void
> +deadline_add_rq_rb(struct deadline_data *dd, struct request *rq)
> +{
> +	struct rb_root *root = deadline_rb_root(dd, rq);
> +
> +	elv_rb_add(root, rq);
> +}
> +
> +static inline void
> +deadline_del_rq_rb(struct deadline_data *dd, struct request *rq)
> +{
> +	const int data_dir = rq_data_dir(rq);
> +
> +	if (dd->next_rq[data_dir] == rq)
> +		dd->next_rq[data_dir] = deadline_latter_request(rq);
> +
> +	elv_rb_del(deadline_rb_root(dd, rq), rq);
> +}
> +
> +/*
> + * remove rq from rbtree and fifo.
> + */
> +static void deadline_remove_request(struct request_queue *q, struct request *rq)
> +{
> +	struct deadline_data *dd = q->elevator->elevator_data;
> +
> +	list_del_init(&rq->queuelist);
> +
> +	/*
> +	 * We might not be on the rbtree, if we are doing an insert merge
> +	 */
> +	if (!RB_EMPTY_NODE(&rq->rb_node))
> +		deadline_del_rq_rb(dd, rq);
> +
> +	elv_rqhash_del(q, rq);
> +	if (q->last_merge == rq)
> +		q->last_merge = NULL;
> +}
> +
> +static void dd_request_merged(struct request_queue *q, struct request *req,
> +			      int type)
> +{
> +	struct deadline_data *dd = q->elevator->elevator_data;
> +
> +	/*
> +	 * if the merge was a front merge, we need to reposition request
> +	 */
> +	if (type == ELEVATOR_FRONT_MERGE) {
> +		elv_rb_del(deadline_rb_root(dd, req), req);
> +		deadline_add_rq_rb(dd, req);
> +	}
> +}
> +
> +static void dd_merged_requests(struct request_queue *q, struct request *req,
> +			       struct request *next)
> +{
> +	/*
> +	 * if next expires before rq, assign its expire time to rq
> +	 * and move into next position (next will be deleted) in fifo
> +	 */
> +	if (!list_empty(&req->queuelist) && !list_empty(&next->queuelist)) {
> +		if (time_before((unsigned long)next->fifo_time,
> +				(unsigned long)req->fifo_time)) {
> +			list_move(&req->queuelist, &next->queuelist);
> +			req->fifo_time = next->fifo_time;
> +		}
> +	}
> +
> +	/*
> +	 * kill knowledge of next, this one is a goner
> +	 */
> +	deadline_remove_request(q, next);
> +}
> +
> +/*
> + * move an entry to dispatch queue
> + */
> +static void
> +deadline_move_request(struct deadline_data *dd, struct request *rq)
> +{
> +	const int data_dir = rq_data_dir(rq);
> +
> +	dd->next_rq[READ] = NULL;
> +	dd->next_rq[WRITE] = NULL;
> +	dd->next_rq[data_dir] = deadline_latter_request(rq);
> +
> +	/*
> +	 * take it off the sort and fifo list
> +	 */
> +	deadline_remove_request(rq->q, rq);
> +}
> +
> +/*
> + * deadline_check_fifo returns 0 if there are no expired requests on the fifo,
> + * 1 otherwise. Requires !list_empty(&dd->fifo_list[data_dir])
> + */
> +static inline int deadline_check_fifo(struct deadline_data *dd, int ddir)
> +{
> +	struct request *rq = rq_entry_fifo(dd->fifo_list[ddir].next);
> +
> +	/*
> +	 * rq is expired!
> +	 */
> +	if (time_after_eq(jiffies, (unsigned long)rq->fifo_time))
> +		return 1;
> +
> +	return 0;
> +}
> +
> +/*
> + * deadline_dispatch_requests selects the best request according to
> + * read/write expire, fifo_batch, etc
> + */
> +static struct request *__dd_dispatch_request(struct blk_mq_hw_ctx *hctx)
> +{
> +	struct deadline_data *dd = hctx->queue->elevator->elevator_data;
> +	struct request *rq;
> +	bool reads, writes;
> +	int data_dir;
> +
> +	spin_lock(&dd->lock);
> +
> +	if (!list_empty(&dd->dispatch)) {
> +		rq = list_first_entry(&dd->dispatch, struct request, queuelist);
> +		list_del_init(&rq->queuelist);
> +		goto done;
> +	}
> +
> +	reads = !list_empty(&dd->fifo_list[READ]);
> +	writes = !list_empty(&dd->fifo_list[WRITE]);
> +
> +	/*
> +	 * batches are currently reads XOR writes
> +	 */
> +	if (dd->next_rq[WRITE])
> +		rq = dd->next_rq[WRITE];
> +	else
> +		rq = dd->next_rq[READ];
> +
> +	if (rq && dd->batching < dd->fifo_batch)
> +		/* we have a next request are still entitled to batch */
> +		goto dispatch_request;
> +
> +	/*
> +	 * at this point we are not running a batch. select the appropriate
> +	 * data direction (read / write)
> +	 */
> +
> +	if (reads) {
> +		BUG_ON(RB_EMPTY_ROOT(&dd->sort_list[READ]));
> +
> +		if (writes && (dd->starved++ >= dd->writes_starved))
> +			goto dispatch_writes;
> +
> +		data_dir = READ;
> +
> +		goto dispatch_find_request;
> +	}
> +
> +	/*
> +	 * there are either no reads or writes have been starved
> +	 */
> +
> +	if (writes) {
> +dispatch_writes:
> +		BUG_ON(RB_EMPTY_ROOT(&dd->sort_list[WRITE]));
> +
> +		dd->starved = 0;
> +
> +		data_dir = WRITE;
> +
> +		goto dispatch_find_request;
> +	}
> +
> +	spin_unlock(&dd->lock);
> +	return NULL;
> +
> +dispatch_find_request:
> +	/*
> +	 * we are not running a batch, find best request for selected data_dir
> +	 */
> +	if (deadline_check_fifo(dd, data_dir) || !dd->next_rq[data_dir]) {
> +		/*
> +		 * A deadline has expired, the last request was in the other
> +		 * direction, or we have run out of higher-sectored requests.
> +		 * Start again from the request with the earliest expiry time.
> +		 */
> +		rq = rq_entry_fifo(dd->fifo_list[data_dir].next);
> +	} else {
> +		/*
> +		 * The last req was the same dir and we have a next request in
> +		 * sort order. No expired requests so continue on from here.
> +		 */
> +		rq = dd->next_rq[data_dir];
> +	}
> +
> +	dd->batching = 0;
> +
> +dispatch_request:
> +	/*
> +	 * rq is the selected appropriate request.
> +	 */
> +	dd->batching++;
> +	deadline_move_request(dd, rq);
> +done:
> +	rq->rq_flags |= RQF_STARTED;
> +	spin_unlock(&dd->lock);
> +	return rq;
> +}
> +
> +static void dd_dispatch_requests(struct blk_mq_hw_ctx *hctx,
> +				 struct list_head *rq_list)
> +{
> +	blk_mq_sched_dispatch_shadow_requests(hctx, rq_list, __dd_dispatch_request);
> +}
> +
> +static void dd_exit_queue(struct elevator_queue *e)
> +{
> +	struct deadline_data *dd = e->elevator_data;
> +
> +	BUG_ON(!list_empty(&dd->fifo_list[READ]));
> +	BUG_ON(!list_empty(&dd->fifo_list[WRITE]));
> +
> +	blk_mq_sched_free_requests(dd->tags);
> +	kfree(dd);
> +}
> +
> +/*
> + * initialize elevator private data (deadline_data).
> + */
> +static int dd_init_queue(struct request_queue *q, struct elevator_type *e)
> +{
> +	struct deadline_data *dd;
> +	struct elevator_queue *eq;
> +
> +	eq = elevator_alloc(q, e);
> +	if (!eq)
> +		return -ENOMEM;
> +
> +	dd = kzalloc_node(sizeof(*dd), GFP_KERNEL, q->node);
> +	if (!dd) {
> +		kobject_put(&eq->kobj);
> +		return -ENOMEM;
> +	}
> +	eq->elevator_data = dd;
> +
> +	dd->tags = blk_mq_sched_alloc_requests(queue_depth, q->node);
> +	if (!dd->tags) {
> +		kfree(dd);
> +		kobject_put(&eq->kobj);
> +		return -ENOMEM;
> +	}
> +
> +	INIT_LIST_HEAD(&dd->fifo_list[READ]);
> +	INIT_LIST_HEAD(&dd->fifo_list[WRITE]);
> +	dd->sort_list[READ] = RB_ROOT;
> +	dd->sort_list[WRITE] = RB_ROOT;
> +	dd->fifo_expire[READ] = read_expire;
> +	dd->fifo_expire[WRITE] = write_expire;
> +	dd->writes_starved = writes_starved;
> +	dd->front_merges = 1;
> +	dd->fifo_batch = fifo_batch;
> +	spin_lock_init(&dd->lock);
> +	INIT_LIST_HEAD(&dd->dispatch);
> +	atomic_set(&dd->wait_index, 0);
> +
> +	q->elevator = eq;
> +	return 0;
> +}
> +
> +static int dd_request_merge(struct request_queue *q, struct request **rq,
> +			    struct bio *bio)
> +{
> +	struct deadline_data *dd = q->elevator->elevator_data;
> +	sector_t sector = bio_end_sector(bio);
> +	struct request *__rq;
> +
> +	if (!dd->front_merges)
> +		return ELEVATOR_NO_MERGE;
> +
> +	__rq = elv_rb_find(&dd->sort_list[bio_data_dir(bio)], sector);
> +	if (__rq) {
> +		BUG_ON(sector != blk_rq_pos(__rq));
> +
> +		if (elv_bio_merge_ok(__rq, bio)) {
> +			*rq = __rq;
> +			return ELEVATOR_FRONT_MERGE;
> +		}
> +	}
> +
> +	return ELEVATOR_NO_MERGE;
> +}
> +
> +static bool dd_bio_merge(struct blk_mq_hw_ctx *hctx, struct bio *bio)
> +{
> +	struct request_queue *q = hctx->queue;
> +	struct deadline_data *dd = q->elevator->elevator_data;
> +	int ret;
> +
> +	spin_lock(&dd->lock);
> +	ret = blk_mq_sched_try_merge(q, bio);
> +	spin_unlock(&dd->lock);
> +
> +	return ret;
> +}
> +
> +/*
> + * add rq to rbtree and fifo
> + */
> +static void dd_insert_request(struct blk_mq_hw_ctx *hctx, struct request *rq,
> +			      bool at_head)
> +{
> +	struct request_queue *q = hctx->queue;
> +	struct deadline_data *dd = q->elevator->elevator_data;
> +	const int data_dir = rq_data_dir(rq);
> +
> +	if (blk_mq_sched_try_insert_merge(q, rq))
> +		return;
> +

A related doubt: shouldn't blk_mq_sched_try_insert_merge be invoked with the scheduler lock held too, as blk_mq_sched_try_merge, to protect (at least) q->last_merge?

In bfq this function is invoked with the lock held.

Thanks,
Paolo

> +	blk_mq_sched_request_inserted(rq);
> +
> +	/*
> +	 * If we're trying to insert a real request, just send it directly
> +	 * to the hardware dispatch list. This only happens for a requeue,
> +	 * or FUA/FLUSH requests.
> +	 */
> +	if (!blk_mq_sched_rq_is_shadow(rq)) {
> +		spin_lock(&hctx->lock);
> +		list_add_tail(&rq->queuelist, &hctx->dispatch);
> +		spin_unlock(&hctx->lock);
> +		return;
> +	}
> +
> +	if (at_head || rq->cmd_type != REQ_TYPE_FS) {
> +		if (at_head)
> +			list_add(&rq->queuelist, &dd->dispatch);
> +		else
> +			list_add_tail(&rq->queuelist, &dd->dispatch);
> +	} else {
> +		deadline_add_rq_rb(dd, rq);
> +
> +		if (rq_mergeable(rq)) {
> +			elv_rqhash_add(q, rq);
> +			if (!q->last_merge)
> +				q->last_merge = rq;
> +		}
> +
> +		/*
> +		 * set expire time and add to fifo list
> +		 */
> +		rq->fifo_time = jiffies + dd->fifo_expire[data_dir];
> +		list_add_tail(&rq->queuelist, &dd->fifo_list[data_dir]);
> +	}
> +}
> +
> +static void dd_insert_requests(struct blk_mq_hw_ctx *hctx,
> +			       struct list_head *list, bool at_head)
> +{
> +	struct request_queue *q = hctx->queue;
> +	struct deadline_data *dd = q->elevator->elevator_data;
> +
> +	spin_lock(&dd->lock);
> +	while (!list_empty(list)) {
> +		struct request *rq;
> +
> +		rq = list_first_entry(list, struct request, queuelist);
> +		list_del_init(&rq->queuelist);
> +		dd_insert_request(hctx, rq, at_head);
> +	}
> +	spin_unlock(&dd->lock);
> +}
> +
> +static struct request *dd_get_request(struct request_queue *q, unsigned int op,
> +				      struct blk_mq_alloc_data *data)
> +{
> +	struct deadline_data *dd = q->elevator->elevator_data;
> +	struct request *rq;
> +
> +	/*
> +	 * The flush machinery intercepts before we insert the request. As
> +	 * a work-around, just hand it back a real request.
> +	 */
> +	if (unlikely(op & (REQ_PREFLUSH | REQ_FUA)))
> +		rq = __blk_mq_alloc_request(data, op);
> +	else {
> +		rq = blk_mq_sched_alloc_shadow_request(q, data, dd->tags, &dd->wait_index);
> +		if (rq)
> +			blk_mq_rq_ctx_init(q, data->ctx, rq, op);
> +	}
> +
> +	return rq;
> +}
> +
> +static bool dd_put_request(struct request *rq)
> +{
> +	/*
> +	 * If it's a real request, we just have to free it. For a shadow
> +	 * request, we should only free it if we haven't started it. A
> +	 * started request is mapped to a real one, and the real one will
> +	 * free it. We can get here with request merges, since we then
> +	 * free the request before we start/issue it.
> +	 */
> +	if (!blk_mq_sched_rq_is_shadow(rq))
> +		return false;
> +
> +	if (!(rq->rq_flags & RQF_STARTED)) {
> +		struct request_queue *q = rq->q;
> +		struct deadline_data *dd = q->elevator->elevator_data;
> +
> +		/*
> +		 * IO completion would normally do this, but if we merge
> +		 * and free before we issue the request, drop both the
> +		 * tag and queue ref
> +		 */
> +		blk_mq_sched_free_shadow_request(dd->tags, rq);
> +		blk_queue_exit(q);
> +	}
> +
> +	return true;
> +}
> +
> +static void dd_completed_request(struct blk_mq_hw_ctx *hctx, struct request *rq)
> +{
> +	struct request *sched_rq = rq->end_io_data;
> +
> +	/*
> +	 * sched_rq can be NULL, if we haven't setup the shadow yet
> +	 * because we failed getting one.
> +	 */
> +	if (sched_rq) {
> +		struct deadline_data *dd = hctx->queue->elevator->elevator_data;
> +
> +		blk_mq_sched_free_shadow_request(dd->tags, sched_rq);
> +		blk_mq_start_stopped_hw_queue(hctx, true);
> +	}
> +}
> +
> +static bool dd_has_work(struct blk_mq_hw_ctx *hctx)
> +{
> +	struct deadline_data *dd = hctx->queue->elevator->elevator_data;
> +
> +	return !list_empty_careful(&dd->dispatch) ||
> +		!list_empty_careful(&dd->fifo_list[0]) ||
> +		!list_empty_careful(&dd->fifo_list[1]);
> +}
> +
> +/*
> + * sysfs parts below
> + */
> +static ssize_t
> +deadline_var_show(int var, char *page)
> +{
> +	return sprintf(page, "%d\n", var);
> +}
> +
> +static ssize_t
> +deadline_var_store(int *var, const char *page, size_t count)
> +{
> +	char *p = (char *) page;
> +
> +	*var = simple_strtol(p, &p, 10);
> +	return count;
> +}
> +
> +#define SHOW_FUNCTION(__FUNC, __VAR, __CONV)				\
> +static ssize_t __FUNC(struct elevator_queue *e, char *page)		\
> +{									\
> +	struct deadline_data *dd = e->elevator_data;			\
> +	int __data = __VAR;						\
> +	if (__CONV)							\
> +		__data = jiffies_to_msecs(__data);			\
> +	return deadline_var_show(__data, (page));			\
> +}
> +SHOW_FUNCTION(deadline_read_expire_show, dd->fifo_expire[READ], 1);
> +SHOW_FUNCTION(deadline_write_expire_show, dd->fifo_expire[WRITE], 1);
> +SHOW_FUNCTION(deadline_writes_starved_show, dd->writes_starved, 0);
> +SHOW_FUNCTION(deadline_front_merges_show, dd->front_merges, 0);
> +SHOW_FUNCTION(deadline_fifo_batch_show, dd->fifo_batch, 0);
> +#undef SHOW_FUNCTION
> +
> +#define STORE_FUNCTION(__FUNC, __PTR, MIN, MAX, __CONV)			\
> +static ssize_t __FUNC(struct elevator_queue *e, const char *page, size_t count)	\
> +{									\
> +	struct deadline_data *dd = e->elevator_data;			\
> +	int __data;							\
> +	int ret = deadline_var_store(&__data, (page), count);		\
> +	if (__data < (MIN))						\
> +		__data = (MIN);						\
> +	else if (__data > (MAX))					\
> +		__data = (MAX);						\
> +	if (__CONV)							\
> +		*(__PTR) = msecs_to_jiffies(__data);			\
> +	else								\
> +		*(__PTR) = __data;					\
> +	return ret;							\
> +}
> +STORE_FUNCTION(deadline_read_expire_store, &dd->fifo_expire[READ], 0, INT_MAX, 1);
> +STORE_FUNCTION(deadline_write_expire_store, &dd->fifo_expire[WRITE], 0, INT_MAX, 1);
> +STORE_FUNCTION(deadline_writes_starved_store, &dd->writes_starved, INT_MIN, INT_MAX, 0);
> +STORE_FUNCTION(deadline_front_merges_store, &dd->front_merges, 0, 1, 0);
> +STORE_FUNCTION(deadline_fifo_batch_store, &dd->fifo_batch, 0, INT_MAX, 0);
> +#undef STORE_FUNCTION
> +
> +#define DD_ATTR(name) \
> +	__ATTR(name, S_IRUGO|S_IWUSR, deadline_##name##_show, \
> +				      deadline_##name##_store)
> +
> +static struct elv_fs_entry deadline_attrs[] = {
> +	DD_ATTR(read_expire),
> +	DD_ATTR(write_expire),
> +	DD_ATTR(writes_starved),
> +	DD_ATTR(front_merges),
> +	DD_ATTR(fifo_batch),
> +	__ATTR_NULL
> +};
> +
> +static struct elevator_type mq_deadline = {
> +	.ops.mq = {
> +		.get_request		= dd_get_request,
> +		.put_request		= dd_put_request,
> +		.insert_requests	= dd_insert_requests,
> +		.dispatch_requests	= dd_dispatch_requests,
> +		.completed_request	= dd_completed_request,
> +		.next_request		= elv_rb_latter_request,
> +		.former_request		= elv_rb_former_request,
> +		.bio_merge		= dd_bio_merge,
> +		.request_merge		= dd_request_merge,
> +		.requests_merged	= dd_merged_requests,
> +		.request_merged		= dd_request_merged,
> +		.has_work		= dd_has_work,
> +		.init_sched		= dd_init_queue,
> +		.exit_sched		= dd_exit_queue,
> +	},
> +
> +	.uses_mq	= true,
> +	.elevator_attrs = deadline_attrs,
> +	.elevator_name = "mq-deadline",
> +	.elevator_owner = THIS_MODULE,
> +};
> +
> +static int __init deadline_init(void)
> +{
> +	if (!queue_depth) {
> +		pr_err("mq-deadline: queue depth must be > 0\n");
> +		return -EINVAL;
> +	}
> +	return elv_register(&mq_deadline);
> +}
> +
> +static void __exit deadline_exit(void)
> +{
> +	elv_unregister(&mq_deadline);
> +}
> +
> +module_init(deadline_init);
> +module_exit(deadline_exit);
> +
> +MODULE_AUTHOR("Jens Axboe");
> +MODULE_LICENSE("GPL");
> +MODULE_DESCRIPTION("MQ deadline IO scheduler");
> -- 
> 2.7.4
> 
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