[PATCH v4 1/2] block: Adding ROW scheduling algorithm

[Tanya Brokhman]

This patch adds the implementation of a new scheduling algorithm - ROW.
The policy of this algorithm is to prioritize READ requests over WRITE
as much as possible without starving the WRITE requests.
The requests are kept in queues according to their priority. The dispatch
is done in a Round Robin manner with a different slice for each queue.
READ request queues get bigger dispatch quantum than the write requests.

Signed-off-by: Tatyana Brokhman <tlinder@xxxxxxxxxxxxxx>
---
v4: 
  - Fix bug in idling mechanism:
    -- Fix the delay passed to scedule_delayed_work
    -- Change the idle delay and idling-trigger frequency to be HZ dependent
  - Destroy idle_workqueue on queue exit
  - Update the documentation
v3: 
  - Fix bug in forced dispatch: Don't idle if forced dispatch
v2: 
  No changes. Resend.
v1: Initial version

diff --git a/Documentation/block/row-iosched.txt b/Documentation/block/row-iosched.txt
new file mode 100644
index 0000000..0d794ee
--- /dev/null
+++ b/Documentation/block/row-iosched.txt
@@ -0,0 +1,134 @@
+Introduction
+============
+
+The ROW scheduling algorithm will be used in mobile devices as default
+block layer IO scheduling algorithm. ROW stands for "READ Over WRITE"
+which is the main requests dispatch policy of this algorithm.
+
+The ROW IO scheduler was developed with the mobile devices needs in
+mind. In mobile devices we favor user experience upon everything else,
+thus we want to give READ IO requests as much priority as possible.
+The main idea of the ROW scheduling policy is just that:
+- If there are READ requests in pipe - dispatch them, while write
+starvation is considered.
+
+Software description
+====================
+The elevator defines a registering mechanism for different IO scheduler
+to implement. This makes implementing a new algorithm quite straight
+forward and requires almost no changes to block/elevator framework. A
+new IO scheduler just has to implement a set of callback functions
+defined by the elevator.
+These callbacks cover all the required IO operations such as
+adding/removing request to/from the scheduler, merging two requests,
+dispatching a request etc.
+
+Design
+======
+
+The requests are kept in queues according to their priority. The
+dispatching of requests is done in a Round Robin manner with a
+different slice for each queue. The dispatch quantum for a specific
+queue is set according to the queues priority. READ queues are
+given bigger dispatch quantum than the WRITE queues, within a dispatch
+cycle.
+
+At the moment there are 6 types of queues the requests are
+distributed to:
+-	High priority READ queue
+-	High priority Synchronous WRITE queue
+-	Regular priority READ queue
+-	Regular priority Synchronous WRITE queue
+-	Regular priority WRITE queue
+-	Low priority READ queue
+
+The marking of request as high/low priority will be done by the
+application adding the request and not the scheduler. See TODO section.
+If the request is not marked in any way (high/low) the scheduler
+assigns it to one of the regular priority queues:
+read/write/sync write.
+
+If in a certain dispatch cycle one of the queues was empty and didn't
+use its quantum that queue will be marked as "un-served". If we're in
+a middle of a dispatch cycle dispatching from queue Y and a request
+arrives for queue X that was un-served in the previous cycle, if X's
+priority is higher than Y's, queue X will be preempted in the favor of
+queue Y.
+
+For READ request queues ROW IO scheduler allows idling within a
+dispatch quantum in order to give the application a chance to insert
+more requests. Idling means adding some extra time for serving a
+certain queue even if the queue is empty. The idling is enabled if
+the ROW IO scheduler identifies the application is inserting requests
+in a high frequency.
+Not all queues can idle. ROW scheduler exposes an enablement struct
+for idling.
+For idling on READ queues, the ROW IO scheduler uses timer mechanism.
+When the timer expires we schedule a delayed work that will signal the
+device driver to fetch another request for dispatch.
+
+ROW scheduler will support additional services for block devices that
+supports Urgent Requests. That is, the scheduler may inform the
+device driver upon urgent requests using a newly defined callback.
+In addition it will support rescheduling of requests that were
+interrupted. For example if the device driver issues a long write
+request and a sudden urgent request is received by the scheduler.
+The scheduler will inform the device driver about the urgent request,
+so the device driver can stop the current write request and serve the
+urgent request. In such a case the device driver may also insert back
+to the scheduler the remainder of the interrupted write request, such
+that the scheduler may continue sending urgent requests without the
+need to interrupt the ongoing write again and again. The write
+remainder will be sent later on according to the scheduler policy.
+
+SMP/multi-core
+==============
+At the moment the code is accessed from 2 contexts:
+- Application context (from block/elevator layer): adding the requests.
+- device driver thread: dispatching the requests and notifying on
+  completion.
+
+One lock is used to synchronize between the two. This lock is provided
+by the block device driver along with the dispatch queue.
+
+Config options
+==============
+1. hp_read_quantum: dispatch quantum for the high priority READ queue
+   (default is 100 requests)
+2. rp_read_quantum: dispatch quantum for the regular priority READ
+   queue (default is 100 requests)
+3. hp_swrite_quantum: dispatch quantum for the high priority
+   Synchronous WRITE queue (default is 2 requests)
+4. rp_swrite_quantum: dispatch quantum for the regular priority
+   Synchronous WRITE queue (default is 1 requests)
+5. rp_write_quantum: dispatch quantum for the regular priority WRITE
+   queue (default is 1 requests)
+6. lp_read_quantum: dispatch quantum for the low priority READ queue
+   (default is 1 requests)
+7. lp_swrite_quantum: dispatch quantum for the low priority Synchronous
+   WRITE queue (default is 1 requests)
+8. read_idle: how long to idle on read queue in Msec (in case idling
+   is enabled on that queue). (default is 5 Msec)
+9. read_idle_freq: frequency of inserting READ requests that will
+   trigger idling. This is the time in Msec between inserting two READ
+   requests. (default is 8 Msec)
+
+Note: Dispatch quantum is number of requests that will be dispatched
+from a certain queue in a dispatch cycle.
+
+To do
+=====
+The ROW algorithm takes the scheduling policy one step further, making
+it a bit more "user-needs oriented", by allowing the application to
+hint on the urgency of its requests. For example: even among the READ
+requests several requests may be more urgent for completion than other.
+The former will go to the High priority READ queue, that is given the
+bigger dispatch quantum than any other queue.
+
+Still need to design the way applications will "hint" on the urgency of
+their requests. May be done by ioctl(). We need to look into concrete
+use-cases in order to determine the best solution for this.
+This will be implemented as a second phase.
+
+Design and implement additional services for block devices that
+supports High Priority Requests.
diff --git a/block/Kconfig.iosched b/block/Kconfig.iosched
index 421bef9..5a747e2 100644
--- a/block/Kconfig.iosched
+++ b/block/Kconfig.iosched
@@ -21,6 +21,16 @@ config IOSCHED_DEADLINE
 	  a new point in the service tree and doing a batch of IO from there
 	  in case of expiry.
 
+config IOSCHED_ROW
+	tristate "ROW I/O scheduler"
+	---help---
+	  The ROW I/O scheduler gives priority to READ requests over the
+	  WRITE requests when dispatching, without starving WRITE requests.
+	  Requests are kept in priority queues. Dispatching is done in a RR
+	  manner when the dispatch quantum for each queue is calculated
+	  according to queue priority.
+	  Most suitable for mobile devices.
+
 config IOSCHED_CFQ
 	tristate "CFQ I/O scheduler"
 	default y
@@ -49,6 +59,16 @@ choice
 	config DEFAULT_DEADLINE
 		bool "Deadline" if IOSCHED_DEADLINE=y
 
+	config DEFAULT_ROW
+		bool "ROW" if IOSCHED_ROW=y
+		help
+		  The ROW I/O scheduler gives priority to READ requests
+		  over the WRITE requests when dispatching, without starving
+		  WRITE requests. Requests are kept in priority queues.
+		  Dispatching is done in a RR manner when the dispatch quantum
+		  for each queue is defined according to queue priority.
+		  Most suitable for mobile devices.
+
 	config DEFAULT_CFQ
 		bool "CFQ" if IOSCHED_CFQ=y
 
@@ -60,6 +80,7 @@ endchoice
 config DEFAULT_IOSCHED
 	string
 	default "deadline" if DEFAULT_DEADLINE
+	default "row" if DEFAULT_ROW
 	default "cfq" if DEFAULT_CFQ
 	default "noop" if DEFAULT_NOOP
 
diff --git a/block/Makefile b/block/Makefile
index 39b76ba..dd80dc2 100644
--- a/block/Makefile
+++ b/block/Makefile
@@ -14,6 +14,7 @@ obj-$(CONFIG_BLK_CGROUP)	+= blk-cgroup.o
 obj-$(CONFIG_BLK_DEV_THROTTLING)	+= blk-throttle.o
 obj-$(CONFIG_IOSCHED_NOOP)	+= noop-iosched.o
 obj-$(CONFIG_IOSCHED_DEADLINE)	+= deadline-iosched.o
+obj-$(CONFIG_IOSCHED_ROW)	+= row-iosched.o
 obj-$(CONFIG_IOSCHED_CFQ)	+= cfq-iosched.o
 
 obj-$(CONFIG_BLOCK_COMPAT)	+= compat_ioctl.o
diff --git a/block/row-iosched.c b/block/row-iosched.c
new file mode 100644
index 0000000..1f50180
--- /dev/null
+++ b/block/row-iosched.c
@@ -0,0 +1,694 @@
+/*
+ * ROW (Read Over Write) I/O scheduler.
+ *
+ * Copyright (c) 2012, The Linux Foundation. All rights reserved.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 and
+ * only version 2 as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ */
+
+/* See Documentation/block/row-iosched.txt */
+
+#include <linux/kernel.h>
+#include <linux/fs.h>
+#include <linux/blkdev.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/blktrace_api.h>
+#include <linux/jiffies.h>
+
+/*
+ * enum row_queue_prio - Priorities of the ROW queues
+ *
+ * This enum defines the priorities (and the number of queues)
+ * the requests will be disptributed to. The higher priority -
+ * the bigger is the dispatch quantum given to that queue.
+ * ROWQ_PRIO_HIGH_READ - is the higher priority queue.
+ *
+ */
+enum row_queue_prio {
+	ROWQ_PRIO_HIGH_READ = 0,
+	ROWQ_PRIO_REG_READ,
+	ROWQ_PRIO_HIGH_SWRITE,
+	ROWQ_PRIO_REG_SWRITE,
+	ROWQ_PRIO_REG_WRITE,
+	ROWQ_PRIO_LOW_READ,
+	ROWQ_PRIO_LOW_SWRITE,
+	ROWQ_MAX_PRIO,
+};
+
+/* Flags indicating whether idling is enabled on the queue */
+static const bool queue_idling_enabled[] = {
+	true,	/* ROWQ_PRIO_HIGH_READ */
+	true,	/* ROWQ_PRIO_REG_READ */
+	false,	/* ROWQ_PRIO_HIGH_SWRITE */
+	false,	/* ROWQ_PRIO_REG_SWRITE */
+	false,	/* ROWQ_PRIO_REG_WRITE */
+	false,	/* ROWQ_PRIO_LOW_READ */
+	false,	/* ROWQ_PRIO_LOW_SWRITE */
+};
+
+/* Default values for row queues quantums in each dispatch cycle */
+static const int queue_quantum[] = {
+	100,	/* ROWQ_PRIO_HIGH_READ */
+	100,	/* ROWQ_PRIO_REG_READ */
+	2,	/* ROWQ_PRIO_HIGH_SWRITE */
+	1,	/* ROWQ_PRIO_REG_SWRITE */
+	1,	/* ROWQ_PRIO_REG_WRITE */
+	1,	/* ROWQ_PRIO_LOW_READ */
+	1	/* ROWQ_PRIO_LOW_SWRITE */
+};
+
+/* Default values for idling on read queues */
+#define ROW_IDLE_TIME_MSEC 5	/* msec */
+#define ROW_READ_FREQ_MSEC 20	/* msec */
+
+/**
+ * struct rowq_idling_data -  parameters for idling on the queue
+ * @last_insert_time:	time the last request was inserted
+ *			to the queue
+ * @begin_idling:	flag indicating wether we should idle
+ *
+ */
+struct rowq_idling_data {
+	ktime_t			last_insert_time;
+	bool			begin_idling;
+};
+
+/**
+ * struct row_queue - requests grouping structure
+ * @rdata:		parent row_data structure
+ * @fifo:		fifo of requests
+ * @prio:		queue priority (enum row_queue_prio)
+ * @nr_dispatched:	number of requests already dispatched in
+ *			the current dispatch cycle
+ * @slice:		number of requests to dispatch in a cycle
+ * @idle_data:		data for idling on queues
+ *
+ */
+struct row_queue {
+	struct row_data		*rdata;
+	struct list_head	fifo;
+	enum row_queue_prio	prio;
+
+	unsigned int		nr_dispatched;
+	unsigned int		slice;
+
+	/* used only for READ queues */
+	struct rowq_idling_data	idle_data;
+};
+
+/**
+ * struct idling_data - data for idling on empty rqueue
+ * @idle_time:		idling duration (jiffies)
+ * @freq:		min time between two requests that
+ *			triger idling (msec)
+ * @idle_work:		pointer to struct delayed_work
+ *
+ */
+struct idling_data {
+	unsigned long			idle_time;
+	u32				freq;
+
+	struct workqueue_struct	*idle_workqueue;
+	struct delayed_work		idle_work;
+};
+
+/**
+ * struct row_queue - Per block device rqueue structure
+ * @dispatch_queue:	dispatch rqueue
+ * @row_queues:		array of priority request queues with
+ *			dispatch quantum per rqueue
+ * @curr_queue:		index in the row_queues array of the
+ *			currently serviced rqueue
+ * @read_idle:		data for idling after READ request
+ * @nr_reqs: nr_reqs[0] holds the number of all READ requests in
+ *			scheduler, nr_reqs[1] holds the number of all WRITE
+ *			requests in scheduler
+ * @cycle_flags:	used for marking unserved queueus
+ *
+ */
+struct row_data {
+	struct request_queue		*dispatch_queue;
+
+	struct {
+		struct row_queue	rqueue;
+		int			disp_quantum;
+	} row_queues[ROWQ_MAX_PRIO];
+
+	enum row_queue_prio		curr_queue;
+
+	struct idling_data		read_idle;
+	unsigned int			nr_reqs[2];
+
+	unsigned int			cycle_flags;
+};
+
+#define RQ_ROWQ(rq) ((struct row_queue *) ((rq)->elv.priv[0]))
+
+#define row_log(q, fmt, args...)   \
+	blk_add_trace_msg(q, "%s():" fmt , __func__, ##args)
+#define row_log_rowq(rdata, rowq_id, fmt, args...)		\
+	blk_add_trace_msg(rdata->dispatch_queue, "rowq%d " fmt, \
+		rowq_id, ##args)
+
+static inline void row_mark_rowq_unserved(struct row_data *rd,
+					 enum row_queue_prio qnum)
+{
+	rd->cycle_flags |= (1 << qnum);
+}
+
+static inline void row_clear_rowq_unserved(struct row_data *rd,
+					  enum row_queue_prio qnum)
+{
+	rd->cycle_flags &= ~(1 << qnum);
+}
+
+static inline int row_rowq_unserved(struct row_data *rd,
+				   enum row_queue_prio qnum)
+{
+	return rd->cycle_flags & (1 << qnum);
+}
+
+/******************** Static helper functions ***********************/
+/*
+ * kick_queue() - Wake up device driver queue thread
+ * @work:	pointer to struct work_struct
+ *
+ * This is a idling delayed work function. It's purpose is to wake up the
+ * device driver in order for it to start fetching requests.
+ *
+ */
+static void kick_queue(struct work_struct *work)
+{
+	struct delayed_work *idle_work = to_delayed_work(work);
+	struct idling_data *read_data =
+		container_of(idle_work, struct idling_data, idle_work);
+	struct row_data *rd =
+		container_of(read_data, struct row_data, read_idle);
+
+	row_log_rowq(rd, rd->curr_queue, "Performing delayed work");
+	/* Mark idling process as done */
+	rd->row_queues[rd->curr_queue].rqueue.idle_data.begin_idling = false;
+
+	if (!(rd->nr_reqs[0] + rd->nr_reqs[1]))
+		row_log(rd->dispatch_queue, "No requests in scheduler");
+	else {
+		spin_lock_irq(rd->dispatch_queue->queue_lock);
+		__blk_run_queue(rd->dispatch_queue);
+		spin_unlock_irq(rd->dispatch_queue->queue_lock);
+	}
+}
+
+/*
+ * row_restart_disp_cycle() - Restart the dispatch cycle
+ * @rd:	pointer to struct row_data
+ *
+ * This function restarts the dispatch cycle by:
+ * - Setting current queue to ROWQ_PRIO_HIGH_READ
+ * - For each queue: reset the number of requests dispatched in
+ *   the cycle
+ */
+static inline void row_restart_disp_cycle(struct row_data *rd)
+{
+	int i;
+
+	for (i = 0; i < ROWQ_MAX_PRIO; i++)
+		rd->row_queues[i].rqueue.nr_dispatched = 0;
+
+	rd->curr_queue = ROWQ_PRIO_HIGH_READ;
+	row_log(rd->dispatch_queue, "Restarting cycle");
+}
+
+static inline void row_get_next_queue(struct row_data *rd)
+{
+	rd->curr_queue++;
+	if (rd->curr_queue == ROWQ_MAX_PRIO)
+		row_restart_disp_cycle(rd);
+}
+
+/******************* Elevator callback functions *********************/
+
+/*
+ * row_add_request() - Add request to the scheduler
+ * @q:	requests queue
+ * @rq:	request to add
+ *
+ */
+static void row_add_request(struct request_queue *q,
+			    struct request *rq)
+{
+	struct row_data *rd = (struct row_data *)q->elevator->elevator_data;
+	struct row_queue *rqueue = RQ_ROWQ(rq);
+
+	list_add_tail(&rq->queuelist, &rqueue->fifo);
+	rd->nr_reqs[rq_data_dir(rq)]++;
+	rq_set_fifo_time(rq, jiffies); /* for statistics*/
+
+	if (queue_idling_enabled[rqueue->prio]) {
+		if (delayed_work_pending(&rd->read_idle.idle_work))
+			(void)cancel_delayed_work(
+				&rd->read_idle.idle_work);
+		if (ktime_to_ms(ktime_sub(ktime_get(),
+				rqueue->idle_data.last_insert_time)) <
+				rd->read_idle.freq) {
+			rqueue->idle_data.begin_idling = true;
+			row_log_rowq(rd, rqueue->prio, "Enable idling");
+		} else {
+			rqueue->idle_data.begin_idling = false;
+			row_log_rowq(rd, rqueue->prio, "Disable idling");
+		}
+
+		rqueue->idle_data.last_insert_time = ktime_get();
+	}
+	row_log_rowq(rd, rqueue->prio, "added request");
+}
+
+/*
+ * row_remove_request() -  Remove given request from scheduler
+ * @q:	requests queue
+ * @rq:	request to remove
+ *
+ */
+static void row_remove_request(struct request_queue *q,
+			       struct request *rq)
+{
+	struct row_data *rd = (struct row_data *)q->elevator->elevator_data;
+
+	rq_fifo_clear(rq);
+	rd->nr_reqs[rq_data_dir(rq)]--;
+}
+
+/*
+ * row_dispatch_insert() - move request to dispatch queue
+ * @rd:	pointer to struct row_data
+ *
+ * This function moves the next request to dispatch from
+ * rd->curr_queue to the dispatch queue
+ *
+ */
+static void row_dispatch_insert(struct row_data *rd)
+{
+	struct request *rq;
+
+	rq = rq_entry_fifo(rd->row_queues[rd->curr_queue].rqueue.fifo.next);
+	row_remove_request(rd->dispatch_queue, rq);
+	elv_dispatch_add_tail(rd->dispatch_queue, rq);
+	rd->row_queues[rd->curr_queue].rqueue.nr_dispatched++;
+	row_clear_rowq_unserved(rd, rd->curr_queue);
+	row_log_rowq(rd, rd->curr_queue, " Dispatched request nr_disp = %d",
+		     rd->row_queues[rd->curr_queue].rqueue.nr_dispatched);
+}
+
+/*
+ * row_choose_queue() -  choose the next queue to dispatch from
+ * @rd:	pointer to struct row_data
+ *
+ * Updates rd->curr_queue. Returns 1 if there are requests to
+ * dispatch, 0 if there are no requests in scheduler
+ *
+ */
+static int row_choose_queue(struct row_data *rd)
+{
+	int prev_curr_queue = rd->curr_queue;
+
+	if (!(rd->nr_reqs[0] + rd->nr_reqs[1])) {
+		row_log(rd->dispatch_queue, "No more requests in scheduler");
+		return 0;
+	}
+
+	row_get_next_queue(rd);
+
+	/*
+	 * Loop over all queues to find the next queue that is not empty.
+	 * Stop when you get back to curr_queue
+	 */
+	while (list_empty(&rd->row_queues[rd->curr_queue].rqueue.fifo)
+	       && rd->curr_queue != prev_curr_queue) {
+		/* Mark rqueue as unserved */
+		row_mark_rowq_unserved(rd, rd->curr_queue);
+		row_get_next_queue(rd);
+	}
+
+	return 1;
+}
+
+/*
+ * row_dispatch_requests() - selects the next request to dispatch
+ * @q:		requests queue
+ * @force:	ignored
+ *
+ * Return 0 if no requests were moved to the dispatch queue.
+ *	  1 otherwise
+ *
+ */
+static int row_dispatch_requests(struct request_queue *q, int force)
+{
+	struct row_data *rd = (struct row_data *)q->elevator->elevator_data;
+	int ret = 0, currq, i;
+
+	currq = rd->curr_queue;
+
+	/*
+	 * Find the first unserved queue (with higher priority then currq)
+	 * that is not empty
+	 */
+	for (i = 0; i < currq; i++) {
+		if (row_rowq_unserved(rd, i) &&
+		    !list_empty(&rd->row_queues[i].rqueue.fifo)) {
+			row_log_rowq(rd, currq,
+				" Preemting for unserved rowq%d", i);
+			rd->curr_queue = i;
+			row_dispatch_insert(rd);
+			ret = 1;
+			goto done;
+		}
+	}
+
+	if (rd->row_queues[currq].rqueue.nr_dispatched >=
+	    rd->row_queues[currq].disp_quantum) {
+		rd->row_queues[currq].rqueue.nr_dispatched = 0;
+		row_log_rowq(rd, currq, "Expiring rqueue");
+		ret = row_choose_queue(rd);
+		if (ret)
+			row_dispatch_insert(rd);
+		goto done;
+	}
+
+	/* Dispatch from curr_queue */
+	if (list_empty(&rd->row_queues[currq].rqueue.fifo)) {
+		/* check idling */
+		if (delayed_work_pending(&rd->read_idle.idle_work)) {
+			if (force) {
+				(void)cancel_delayed_work(
+				&rd->read_idle.idle_work);
+				row_log_rowq(rd, currq,
+					"Canceled delayed work - forced dispatch");
+			} else {
+				row_log_rowq(rd, currq,
+						 "Delayed work pending. Exiting");
+				goto done;
+			}
+		}
+
+		if (!force && queue_idling_enabled[currq] &&
+		    rd->row_queues[currq].rqueue.idle_data.begin_idling) {
+			if (!queue_delayed_work(rd->read_idle.idle_workqueue,
+						&rd->read_idle.idle_work,
+						rd->read_idle.idle_time)) {
+				row_log_rowq(rd, currq,
+					     "Work already on queue!");
+				pr_err("ROW_BUG: Work already on queue!");
+			} else
+				row_log_rowq(rd, currq,
+				     "Scheduled delayed work. exiting");
+			goto done;
+		} else {
+			row_log_rowq(rd, currq,
+				     "Currq empty. Choose next queue");
+			ret = row_choose_queue(rd);
+			if (!ret)
+				goto done;
+		}
+	}
+
+	ret = 1;
+	row_dispatch_insert(rd);
+
+done:
+	return ret;
+}
+
+/*
+ * row_init_queue() - Init scheduler data structures
+ * @q:	requests queue
+ *
+ * Return pointer to struct row_data to be saved in elevator for
+ * this dispatch queue
+ *
+ */
+static void *row_init_queue(struct request_queue *q)
+{
+
+	struct row_data *rdata;
+	int i;
+
+	rdata = kmalloc_node(sizeof(*rdata),
+			     GFP_KERNEL | __GFP_ZERO, q->node);
+	if (!rdata)
+		return NULL;
+
+	for (i = 0; i < ROWQ_MAX_PRIO; i++) {
+		INIT_LIST_HEAD(&rdata->row_queues[i].rqueue.fifo);
+		rdata->row_queues[i].disp_quantum = queue_quantum[i];
+		rdata->row_queues[i].rqueue.rdata = rdata;
+		rdata->row_queues[i].rqueue.prio = i;
+		rdata->row_queues[i].rqueue.idle_data.begin_idling = false;
+		rdata->row_queues[i].rqueue.idle_data.last_insert_time =
+			ktime_set(0, 0);
+	}
+
+	/*
+	 * Currently idling is enabled only for READ queues. If we want to
+	 * enable it for write queues also, note that idling frequency will
+	 * be the same in both cases
+	 */
+	rdata->read_idle.idle_time = msecs_to_jiffies(ROW_IDLE_TIME_MSEC);
+	/* Maybe 0 on some platforms */
+	if (!rdata->read_idle.idle_time)
+		rdata->read_idle.idle_time = 1;
+	rdata->read_idle.freq = ROW_READ_FREQ_MSEC;
+	rdata->read_idle.idle_workqueue = alloc_workqueue("row_idle_work",
+					    WQ_MEM_RECLAIM | WQ_HIGHPRI, 0);
+	if (!rdata->read_idle.idle_workqueue)
+		panic("Failed to create idle workqueue\n");
+	INIT_DELAYED_WORK(&rdata->read_idle.idle_work, kick_queue);
+
+	rdata->curr_queue = ROWQ_PRIO_HIGH_READ;
+	rdata->dispatch_queue = q;
+
+	rdata->nr_reqs[READ] = rdata->nr_reqs[WRITE] = 0;
+
+	return rdata;
+}
+
+/*
+ * row_exit_queue() - called on unloading the RAW scheduler
+ * @e:	poiner to struct elevator_queue
+ *
+ */
+static void row_exit_queue(struct elevator_queue *e)
+{
+	struct row_data *rd = (struct row_data *)e->elevator_data;
+	int i;
+
+	for (i = 0; i < ROWQ_MAX_PRIO; i++)
+		BUG_ON(!list_empty(&rd->row_queues[i].rqueue.fifo));
+	(void)cancel_delayed_work_sync(&rd->read_idle.idle_work);
+	BUG_ON(delayed_work_pending(&rd->read_idle.idle_work));
+	destroy_workqueue(rd->read_idle.idle_workqueue);
+	kfree(rd);
+}
+
+/*
+ * row_merged_requests() - Called when 2 requests are merged
+ * @q:		requests queue
+ * @rq:		request the two requests were merged into
+ * @next:	request that was merged
+ */
+static void row_merged_requests(struct request_queue *q, struct request *rq,
+				 struct request *next)
+{
+	struct row_queue   *rqueue = RQ_ROWQ(next);
+
+	list_del_init(&next->queuelist);
+
+	rqueue->rdata->nr_reqs[rq_data_dir(rq)]--;
+}
+
+/*
+ * get_queue_type() - Get queue type for a given request
+ *
+ * This is a helping function which purpose is to determine what
+ * ROW queue the given request should be added to (and
+ * dispatched from leter on)
+ *
+ * TODO: Right now only 3 queues are used REG_READ, REG_WRITE
+ * and REG_SWRITE
+ */
+static enum row_queue_prio get_queue_type(struct request *rq)
+{
+	const int data_dir = rq_data_dir(rq);
+	const bool is_sync = rq_is_sync(rq);
+
+	if (data_dir == READ)
+		return ROWQ_PRIO_REG_READ;
+	else if (is_sync)
+		return ROWQ_PRIO_REG_SWRITE;
+	else
+		return ROWQ_PRIO_REG_WRITE;
+}
+
+/*
+ * row_set_request() - Set ROW data structures associated with this request.
+ * @q:		requests queue
+ * @rq:		pointer to the request
+ * @gfp_mask:	ignored
+ *
+ */
+static int
+row_set_request(struct request_queue *q, struct request *rq, gfp_t gfp_mask)
+{
+	struct row_data *rd = (struct row_data *)q->elevator->elevator_data;
+	unsigned long flags;
+
+	spin_lock_irqsave(q->queue_lock, flags);
+	rq->elv.priv[0] =
+		(void *)(&rd->row_queues[get_queue_type(rq)]);
+	spin_unlock_irqrestore(q->queue_lock, flags);
+
+	return 0;
+}
+
+/********** Helping sysfs functions/defenitions for ROW attributes ******/
+static ssize_t row_var_show(int var, char *page)
+{
+	return snprintf(page, 100, "%d\n", var);
+}
+
+static ssize_t row_var_store(int *var, const char *page, size_t count)
+{
+	int err;
+	err = kstrtoul(page, 10, (unsigned long *)var);
+
+	return count;
+}
+
+#define SHOW_FUNCTION(__FUNC, __VAR, __CONV)				\
+static ssize_t __FUNC(struct elevator_queue *e, char *page)		\
+{									\
+	struct row_data *rowd = e->elevator_data;			\
+	int __data = __VAR;						\
+	if (__CONV)							\
+		__data = jiffies_to_msecs(__data);			\
+	return row_var_show(__data, (page));			\
+}
+SHOW_FUNCTION(row_hp_read_quantum_show,
+	rowd->row_queues[ROWQ_PRIO_HIGH_READ].disp_quantum, 0);
+SHOW_FUNCTION(row_rp_read_quantum_show,
+	rowd->row_queues[ROWQ_PRIO_REG_READ].disp_quantum, 0);
+SHOW_FUNCTION(row_hp_swrite_quantum_show,
+	rowd->row_queues[ROWQ_PRIO_HIGH_SWRITE].disp_quantum, 0);
+SHOW_FUNCTION(row_rp_swrite_quantum_show,
+	rowd->row_queues[ROWQ_PRIO_REG_SWRITE].disp_quantum, 0);
+SHOW_FUNCTION(row_rp_write_quantum_show,
+	rowd->row_queues[ROWQ_PRIO_REG_WRITE].disp_quantum, 0);
+SHOW_FUNCTION(row_lp_read_quantum_show,
+	rowd->row_queues[ROWQ_PRIO_LOW_READ].disp_quantum, 0);
+SHOW_FUNCTION(row_lp_swrite_quantum_show,
+	rowd->row_queues[ROWQ_PRIO_LOW_SWRITE].disp_quantum, 0);
+SHOW_FUNCTION(row_read_idle_show, rowd->read_idle.idle_time, 1);
+SHOW_FUNCTION(row_read_idle_freq_show, rowd->read_idle.freq, 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 row_data *rowd = e->elevator_data;			\
+	int __data;						\
+	int ret = row_var_store(&__data, (page), count);		\
+	if (__CONV)							\
+		__data = (int)msecs_to_jiffies(__data);			\
+	if (__data < (MIN))						\
+		__data = (MIN);						\
+	else if (__data > (MAX))					\
+		__data = (MAX);						\
+	*(__PTR) = __data;						\
+	return ret;							\
+}
+STORE_FUNCTION(row_hp_read_quantum_store,
+&rowd->row_queues[ROWQ_PRIO_HIGH_READ].disp_quantum, 1, INT_MAX, 0);
+STORE_FUNCTION(row_rp_read_quantum_store,
+			&rowd->row_queues[ROWQ_PRIO_REG_READ].disp_quantum,
+			1, INT_MAX, 0);
+STORE_FUNCTION(row_hp_swrite_quantum_store,
+			&rowd->row_queues[ROWQ_PRIO_HIGH_SWRITE].disp_quantum,
+			1, INT_MAX, 0);
+STORE_FUNCTION(row_rp_swrite_quantum_store,
+			&rowd->row_queues[ROWQ_PRIO_REG_SWRITE].disp_quantum,
+			1, INT_MAX, 0);
+STORE_FUNCTION(row_rp_write_quantum_store,
+			&rowd->row_queues[ROWQ_PRIO_REG_WRITE].disp_quantum,
+			1, INT_MAX, 0);
+STORE_FUNCTION(row_lp_read_quantum_store,
+			&rowd->row_queues[ROWQ_PRIO_LOW_READ].disp_quantum,
+			1, INT_MAX, 0);
+STORE_FUNCTION(row_lp_swrite_quantum_store,
+			&rowd->row_queues[ROWQ_PRIO_LOW_SWRITE].disp_quantum,
+			1, INT_MAX, 1);
+STORE_FUNCTION(row_read_idle_store, &rowd->read_idle.idle_time, 1, INT_MAX, 1);
+STORE_FUNCTION(row_read_idle_freq_store, &rowd->read_idle.freq, 1, INT_MAX, 0);
+
+#undef STORE_FUNCTION
+
+#define ROW_ATTR(name) \
+	__ATTR(name, S_IRUGO|S_IWUSR, row_##name##_show, \
+				      row_##name##_store)
+
+static struct elv_fs_entry row_attrs[] = {
+	ROW_ATTR(hp_read_quantum),
+	ROW_ATTR(rp_read_quantum),
+	ROW_ATTR(hp_swrite_quantum),
+	ROW_ATTR(rp_swrite_quantum),
+	ROW_ATTR(rp_write_quantum),
+	ROW_ATTR(lp_read_quantum),
+	ROW_ATTR(lp_swrite_quantum),
+	ROW_ATTR(read_idle),
+	ROW_ATTR(read_idle_freq),
+	__ATTR_NULL
+};
+
+static struct elevator_type iosched_row = {
+	.ops = {
+		.elevator_merge_req_fn		= row_merged_requests,
+		.elevator_dispatch_fn		= row_dispatch_requests,
+		.elevator_add_req_fn		= row_add_request,
+		.elevator_former_req_fn		= elv_rb_former_request,
+		.elevator_latter_req_fn		= elv_rb_latter_request,
+		.elevator_set_req_fn		= row_set_request,
+		.elevator_init_fn		= row_init_queue,
+		.elevator_exit_fn		= row_exit_queue,
+	},
+
+	.elevator_attrs = row_attrs,
+	.elevator_name = "row",
+	.elevator_owner = THIS_MODULE,
+};
+
+static int __init row_init(void)
+{
+	elv_register(&iosched_row);
+	return 0;
+}
+
+static void __exit row_exit(void)
+{
+	elv_unregister(&iosched_row);
+}
+
+module_init(row_init);
+module_exit(row_exit);
+
+MODULE_LICENSE("GPLv2");
+MODULE_DESCRIPTION("Read Over Write IO scheduler");
-- 
1.7.6
--
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