as-iosched.c

/*
 *  Anticipatory & deadline i/o scheduler.
 *
 *  Copyright (C) 2002 Jens Axboe <axboe@suse.de>
 *                     Nick Piggin <nickpiggin@yahoo.com.au>
 *
 */
#include <linux/kernel.h>
#include <linux/fs.h>
#include <linux/blkdev.h>
#include <linux/elevator.h>
#include <linux/bio.h>
#include <linux/config.h>
#include <linux/module.h>
#include <linux/slab.h>
#include <linux/init.h>
#include <linux/compiler.h>
#include <linux/hash.h>
#include <linux/rbtree.h>
#include <linux/interrupt.h>

#define REQ_SYNC	1
#define REQ_ASYNC	0

/*
 * See Documentation/block/as-iosched.txt
 */

/*
 * max time before a read is submitted.
 */
#define default_read_expire (HZ / 8)

/*
 * ditto for writes, these limits are not hard, even
 * if the disk is capable of satisfying them.
 */
#define default_write_expire (HZ / 4)

/*
 * read_batch_expire describes how long we will allow a stream of reads to
 * persist before looking to see whether it is time to switch over to writes.
 */
#define default_read_batch_expire (HZ / 2)

/*
 * write_batch_expire describes how long we want a stream of writes to run for.
 * This is not a hard limit, but a target we set for the auto-tuning thingy.
 * See, the problem is: we can send a lot of writes to disk cache / TCQ in
 * a short amount of time...
 */
#define default_write_batch_expire (HZ / 8)

/*
 * max time we may wait to anticipate a read (default around 6ms)
 */
#define default_antic_expire ((HZ / 150) ? HZ / 150 : 1)

/*
 * Keep track of up to 20ms thinktimes. We can go as big as we like here,
 * however huge values tend to interfere and not decay fast enough. A program
 * might be in a non-io phase of operation. Waiting on user input for example,
 * or doing a lengthy computation. A small penalty can be justified there, and
 * will still catch out those processes that constantly have large thinktimes.
 */
#define MAX_THINKTIME (HZ/50UL)

/* Bits in as_io_context.state */
enum as_io_states {
	AS_TASK_RUNNING=0,	/* Process has not exited */
	AS_TASK_IOSTARTED,	/* Process has started some IO */
	AS_TASK_IORUNNING,	/* Process has completed some IO */
};

enum anticipation_status {
	ANTIC_OFF=0,		/* Not anticipating (normal operation)	*/
	ANTIC_WAIT_REQ,		/* The last read has not yet completed  */
	ANTIC_WAIT_NEXT,	/* Currently anticipating a request vs
				   last read (which has completed) */
	ANTIC_FINISHED,		/* Anticipating but have found a candidate
				 * or timed out */
};

struct as_data {
	/*
	 * run time data
	 */

	struct request_queue *q;	/* the "owner" queue */

	/*
	 * requests (as_rq s) are present on both sort_list and fifo_list
	 */
	struct rb_root sort_list[2];
	struct list_head fifo_list[2];

	struct as_rq *next_arq[2];	/* next in sort order */
	sector_t last_sector[2];	/* last REQ_SYNC & REQ_ASYNC sectors */
	struct list_head *hash;		/* request hash */

	unsigned long exit_prob;	/* probability a task will exit while
					   being waited on */
	unsigned long exit_no_coop;	/* probablility an exited task will
					   not be part of a later cooperating
					   request */
	unsigned long new_ttime_total; 	/* mean thinktime on new proc */
	unsigned long new_ttime_mean;
	u64 new_seek_total;		/* mean seek on new proc */
	sector_t new_seek_mean;

	unsigned long current_batch_expires;
	unsigned long last_check_fifo[2];
	int changed_batch;		/* 1: waiting for old batch to end */
	int new_batch;			/* 1: waiting on first read complete */
	int batch_data_dir;		/* current batch REQ_SYNC / REQ_ASYNC */
	int write_batch_count;		/* max # of reqs in a write batch */
	int current_write_count;	/* how many requests left this batch */
	int write_batch_idled;		/* has the write batch gone idle? */
	mempool_t *arq_pool;

	enum anticipation_status antic_status;
	unsigned long antic_start;	/* jiffies: when it started */
	struct timer_list antic_timer;	/* anticipatory scheduling timer */
	struct work_struct antic_work;	/* Deferred unplugging */
	struct io_context *io_context;	/* Identify the expected process */
	int ioc_finished; /* IO associated with io_context is finished */
	int nr_dispatched;

	/*
	 * settings that change how the i/o scheduler behaves
	 */
	unsigned long fifo_expire[2];
	unsigned long batch_expire[2];
	unsigned long antic_expire;
};

#define list_entry_fifo(ptr)	list_entry((ptr), struct as_rq, fifo)

/*
 * per-request data.
 */
enum arq_state {
	AS_RQ_NEW=0,		/* New - not referenced and not on any lists */
	AS_RQ_QUEUED,		/* In the request queue. It belongs to the
				   scheduler */
	AS_RQ_DISPATCHED,	/* On the dispatch list. It belongs to the
				   driver now */
	AS_RQ_PRESCHED,		/* Debug poisoning for requests being used */
	AS_RQ_REMOVED,
	AS_RQ_MERGED,
	AS_RQ_POSTSCHED,	/* when they shouldn't be */
};

struct as_rq {
	/*
	 * rbtree index, key is the starting offset
	 */
	struct rb_node rb_node;
	sector_t rb_key;

	struct request *request;

	struct io_context *io_context;	/* The submitting task */

	/*
	 * request hash, key is the ending offset (for back merge lookup)
	 */
	struct list_head hash;
	unsigned int on_hash;

	/*
	 * expire fifo
	 */
	struct list_head fifo;
	unsigned long expires;

	unsigned int is_sync;
	enum arq_state state;
};

#define RQ_DATA(rq)	((struct as_rq *) (rq)->elevator_private)

static kmem_cache_t *arq_pool;

static void as_move_to_dispatch(struct as_data *ad, struct as_rq *arq);
static void as_antic_stop(struct as_data *ad);

/*
 * IO Context helper functions
 */

/* Called to deallocate the as_io_context */
static void free_as_io_context(struct as_io_context *aic)
{
	kfree(aic);
}

static void as_trim(struct io_context *ioc)
{
	kfree(ioc->aic);
	ioc->aic = NULL;
}

/* Called when the task exits */
static void exit_as_io_context(struct as_io_context *aic)
{
	WARN_ON(!test_bit(AS_TASK_RUNNING, &aic->state));
	clear_bit(AS_TASK_RUNNING, &aic->state);
}

static struct as_io_context *alloc_as_io_context(void)
{
	struct as_io_context *ret;

	ret = kmalloc(sizeof(*ret), GFP_ATOMIC);
	if (ret) {
		ret->dtor = free_as_io_context;
		ret->exit = exit_as_io_context;
		ret->state = 1 << AS_TASK_RUNNING;
		atomic_set(&ret->nr_queued, 0);
		atomic_set(&ret->nr_dispatched, 0);
		spin_lock_init(&ret->lock);
		ret->ttime_total = 0;
		ret->ttime_samples = 0;
		ret->ttime_mean = 0;
		ret->seek_total = 0;
		ret->seek_samples = 0;
		ret->seek_mean = 0;
	}

	return ret;
}

/*
 * If the current task has no AS IO context then create one and initialise it.
 * Then take a ref on the task's io context and return it.
 */
static struct io_context *as_get_io_context(void)
{
	struct io_context *ioc = get_io_context(GFP_ATOMIC);
	if (ioc && !ioc->aic) {
		ioc->aic = alloc_as_io_context();
		if (!ioc->aic) {
			put_io_context(ioc);
			ioc = NULL;
		}
	}
	return ioc;
}

static void as_put_io_context(struct as_rq *arq)
{
	struct as_io_context *aic;

	if (unlikely(!arq->io_context))
		return;

	aic = arq->io_context->aic;

	if (arq->is_sync == REQ_SYNC && aic) {
		spin_lock(&aic->lock);
		set_bit(AS_TASK_IORUNNING, &aic->state);
		aic->last_end_request = jiffies;
		spin_unlock(&aic->lock);
	}

	put_io_context(arq->io_context);
}

/*
 * the back merge hash support functions
 */
static const int as_hash_shift = 6;
#define AS_HASH_BLOCK(sec)	((sec) >> 3)
#define AS_HASH_FN(sec)		(hash_long(AS_HASH_BLOCK((sec)), as_hash_shift))
#define AS_HASH_ENTRIES		(1 << as_hash_shift)
#define rq_hash_key(rq)		((rq)->sector + (rq)->nr_sectors)
#define list_entry_hash(ptr)	list_entry((ptr), struct as_rq, hash)

static inline void __as_del_arq_hash(struct as_rq *arq)
{
	arq->on_hash = 0;
	list_del_init(&arq->hash);
}

static inline void as_del_arq_hash(struct as_rq *arq)
{
	if (arq->on_hash)
		__as_del_arq_hash(arq);
}

static void as_add_arq_hash(struct as_data *ad, struct as_rq *arq)
{
	struct request *rq = arq->request;

	BUG_ON(arq->on_hash);

	arq->on_hash = 1;
	list_add(&arq->hash, &ad->hash[AS_HASH_FN(rq_hash_key(rq))]);
}

/*
 * move hot entry to front of chain
 */
static inline void as_hot_arq_hash(struct as_data *ad, struct as_rq *arq)
{
	struct request *rq = arq->request;
	struct list_head *head = &ad->hash[AS_HASH_FN(rq_hash_key(rq))];

	if (!arq->on_hash) {
		WARN_ON(1);
		return;
	}

	if (arq->hash.prev != head) {
		list_del(&arq->hash);
		list_add(&arq->hash, head);
	}
}

static struct request *as_find_arq_hash(struct as_data *ad, sector_t offset)
{
	struct list_head *hash_list = &ad->hash[AS_HASH_FN(offset)];
	struct list_head *entry, *next = hash_list->next;

	while ((entry = next) != hash_list) {
		struct as_rq *arq = list_entry_hash(entry);
		struct request *__rq = arq->request;

		next = entry->next;

		BUG_ON(!arq->on_hash);

		if (!rq_mergeable(__rq)) {
			as_del_arq_hash(arq);
			continue;
		}

		if (rq_hash_key(__rq) == offset)
			return __rq;
	}

	return NULL;
}

/*
 * rb tree support functions
 */
#define RB_NONE		(2)
#define RB_EMPTY(root)	((root)->rb_node == NULL)
#define ON_RB(node)	((node)->rb_color != RB_NONE)
#define RB_CLEAR(node)	((node)->rb_color = RB_NONE)
#define rb_entry_arq(node)	rb_entry((node), struct as_rq, rb_node)
#define ARQ_RB_ROOT(ad, arq)	(&(ad)->sort_list[(arq)->is_sync])
#define rq_rb_key(rq)		(rq)->sector

/*
 * as_find_first_arq finds the first (lowest sector numbered) request
 * for the specified data_dir. Used to sweep back to the start of the disk
 * (1-way elevator) after we process the last (highest sector) request.
 */
static struct as_rq *as_find_first_arq(struct as_data *ad, int data_dir)
{
	struct rb_node *n = ad->sort_list[data_dir].rb_node;

	if (n == NULL)
		return NULL;

	for (;;) {
		if (n->rb_left == NULL)
			return rb_entry_arq(n);

		n = n->rb_left;
	}
}

/*
 * Add the request to the rb tree if it is unique.  If there is an alias (an
 * existing request against the same sector), which can happen when using
 * direct IO, then return the alias.
 */
static struct as_rq *__as_add_arq_rb(struct as_data *ad, struct as_rq *arq)
{
	struct rb_node **p = &ARQ_RB_ROOT(ad, arq)->rb_node;
	struct rb_node *parent = NULL;
	struct as_rq *__arq;
	struct request *rq = arq->request;

	arq->rb_key = rq_rb_key(rq);

	while (*p) {
		parent = *p;
		__arq = rb_entry_arq(parent);

		if (arq->rb_key < __arq->rb_key)
			p = &(*p)->rb_left;
		else if (arq->rb_key > __arq->rb_key)
			p = &(*p)->rb_right;
		else
			return __arq;
	}

	rb_link_node(&arq->rb_node, parent, p);
	rb_insert_color(&arq->rb_node, ARQ_RB_ROOT(ad, arq));

	return NULL;
}

static void as_add_arq_rb(struct as_data *ad, struct as_rq *arq)
{
	struct as_rq *alias;

	while ((unlikely(alias = __as_add_arq_rb(ad, arq)))) {
		as_move_to_dispatch(ad, alias);
		as_antic_stop(ad);
	}
}

static inline void as_del_arq_rb(struct as_data *ad, struct as_rq *arq)
{
	if (!ON_RB(&arq->rb_node)) {
		WARN_ON(1);
		return;
	}

	rb_erase(&arq->rb_node, ARQ_RB_ROOT(ad, arq));
	RB_CLEAR(&arq->rb_node);
}

static struct request *
as_find_arq_rb(struct as_data *ad, sector_t sector, int data_dir)
{
	struct rb_node *n = ad->sort_list[data_dir].rb_node;
	struct as_rq *arq;

	while (n) {
		arq = rb_entry_arq(n);

		if (sector < arq->rb_key)
			n = n->rb_left;
		else if (sector > arq->rb_key)
			n = n->rb_right;
		else
			return arq->request;
	}

	return NULL;
}

/*
 * IO Scheduler proper
 */

#define MAXBACK (1024 * 1024)	/*
				 * Maximum distance the disk will go backward
				 * for a request.
				 */

#define BACK_PENALTY	2

/*
 * as_choose_req selects the preferred one of two requests of the same data_dir
 * ignoring time - eg. timeouts, which is the job of as_dispatch_request
 */
static struct as_rq *
as_choose_req(struct as_data *ad, struct as_rq *arq1, struct as_rq *arq2)
{
	int data_dir;
	sector_t last, s1, s2, d1, d2;
	int r1_wrap=0, r2_wrap=0;	/* requests are behind the disk head */
	const sector_t maxback = MAXBACK;

	if (arq1 == NULL || arq1 == arq2)
		return arq2;
	if (arq2 == NULL)
		return arq1;

	data_dir = arq1->is_sync;

	last = ad->last_sector[data_dir];
	s1 = arq1->request->sector;
	s2 = arq2->request->sector;

	BUG_ON(data_dir != arq2->is_sync);

	/*
	 * Strict one way elevator _except_ in the case where we allow
	 * short backward seeks which are biased as twice the cost of a
	 * similar forward seek.
	 */
	if (s1 >= last)
		d1 = s1 - last;
	else if (s1+maxback >= last)
		d1 = (last - s1)*BACK_PENALTY;
	else {
		r1_wrap = 1;
		d1 = 0; /* shut up, gcc */
	}

	if (s2 >= last)
		d2 = s2 - last;
	else if (s2+maxback >= last)
		d2 = (last - s2)*BACK_PENALTY;
	else {
		r2_wrap = 1;
		d2 = 0;
	}

	/* Found required data */
	if (!r1_wrap && r2_wrap)
		return arq1;
	else if (!r2_wrap && r1_wrap)
		return arq2;
	else if (r1_wrap && r2_wrap) {
		/* both behind the head */
		if (s1 <= s2)
			return arq1;
		else
			return arq2;
	}

	/* Both requests in front of the head */
	if (d1 < d2)
		return arq1;
	else if (d2 < d1)
		return arq2;
	else {
		if (s1 >= s2)
			return arq1;
		else
			return arq2;
	}
}

/*
 * as_find_next_arq finds the next request after @prev in elevator order.
 * this with as_choose_req form the basis for how the scheduler chooses
 * what request to process next. Anticipation works on top of this.
 */
static struct as_rq *as_find_next_arq(struct as_data *ad, struct as_rq *last)
{
	const int data_dir = last->is_sync;
	struct as_rq *ret;
	struct rb_node *rbnext = rb_next(&last->rb_node);
	struct rb_node *rbprev = rb_prev(&last->rb_node);
	struct as_rq *arq_next, *arq_prev;

	BUG_ON(!ON_RB(&last->rb_node));

	if (rbprev)
		arq_prev = rb_entry_arq(rbprev);
	else
		arq_prev = NULL;

	if (rbnext)
		arq_next = rb_entry_arq(rbnext);
	else {
		arq_next = as_find_first_arq(ad, data_dir);
		if (arq_next == last)
			arq_next = NULL;
	}

	ret = as_choose_req(ad,	arq_next, arq_prev);

	return ret;
}

/*
 * anticipatory scheduling functions follow
 */

/*
 * as_antic_expired tells us when we have anticipated too long.
 * The funny "absolute difference" math on the elapsed time is to handle
 * jiffy wraps, and disks which have been idle for 0x80000000 jiffies.
 */
static int as_antic_expired(struct as_data *ad)
{
	long delta_jif;

	delta_jif = jiffies - ad->antic_start;
	if (unlikely(delta_jif < 0))
		delta_jif = -delta_jif;
	if (delta_jif < ad->antic_expire)
		return 0;

	return 1;
}

/*
 * as_antic_waitnext starts anticipating that a nice request will soon be
 * submitted. See also as_antic_waitreq
 */
static void as_antic_waitnext(struct as_data *ad)
{
	unsigned long timeout;

	BUG_ON(ad->antic_status != ANTIC_OFF
			&& ad->antic_status != ANTIC_WAIT_REQ);

	timeout = ad->antic_start + ad->antic_expire;

	mod_timer(&ad->antic_timer, timeout);

	ad->antic_status = ANTIC_WAIT_NEXT;
}

/*
 * as_antic_waitreq starts anticipating. We don't start timing the anticipation
 * until the request that we're anticipating on has finished. This means we
 * are timing from when the candidate process wakes up hopefully.
 */
static void as_antic_waitreq(struct as_data *ad)
{
	BUG_ON(ad->antic_status == ANTIC_FINISHED);
	if (ad->antic_status == ANTIC_OFF) {
		if (!ad->io_context || ad->ioc_finished)
			as_antic_waitnext(ad);
		else
			ad->antic_status = ANTIC_WAIT_REQ;
	}
}

/*
 * This is called directly by the functions in this file to stop anticipation.
 * We kill the timer and schedule a call to the request_fn asap.
 */
static void as_antic_stop(struct as_data *ad)
{
	int status = ad->antic_status;

	if (status == ANTIC_WAIT_REQ || status == ANTIC_WAIT_NEXT) {
		if (status == ANTIC_WAIT_NEXT)
			del_timer(&ad->antic_timer);
		ad->antic_status = ANTIC_FINISHED;
		/* see as_work_handler */
		kblockd_schedule_work(&ad->antic_work);
	}
}

/*
 * as_antic_timeout is the timer function set by as_antic_waitnext.
 */
static void as_antic_timeout(unsigned long data)
{
	struct request_queue *q = (struct request_queue *)data;
	struct as_data *ad = q->elevator->elevator_data;
	unsigned long flags;

	spin_lock_irqsave(q->queue_lock, flags);
	if (ad->antic_status == ANTIC_WAIT_REQ
			|| ad->antic_status == ANTIC_WAIT_NEXT) {
		struct as_io_context *aic = ad->io_context->aic;

		ad->antic_status = ANTIC_FINISHED;
		kblockd_schedule_work(&ad->antic_work);

		if (aic->ttime_samples == 0) {
			/* process anticipated on has exited or timed out*/
			ad->exit_prob = (7*ad->exit_prob + 256)/8;
		}
		if (!test_bit(AS_TASK_RUNNING, &aic->state)) {
			/* process not "saved" by a cooperating request */
			ad->exit_no_coop = (7*ad->exit_no_coop + 256)/8;
		}
	}
	spin_unlock_irqrestore(q->queue_lock, flags);
}

static void as_update_thinktime(struct as_data *ad, struct as_io_context *aic,
				unsigned long ttime)
{
	/* fixed point: 1.0 == 1<<8 */
	if (aic->ttime_samples == 0) {
		ad->new_ttime_total = (7*ad->new_ttime_total + 256*ttime) / 8;
		ad->new_ttime_mean = ad->new_ttime_total / 256;

		ad->exit_prob = (7*ad->exit_prob)/8;
	}
	aic->ttime_samples = (7*aic->ttime_samples + 256) / 8;
	aic->ttime_total = (7*aic->ttime_total + 256*ttime) / 8;
	aic->ttime_mean = (aic->ttime_total + 128) / aic->ttime_samples;
}

static void as_update_seekdist(struct as_data *ad, struct as_io_context *aic,
				sector_t sdist)
{
	u64 total;

	if (aic->seek_samples == 0) {
		ad->new_seek_total = (7*ad->new_seek_total + 256*(u64)sdist)/8;
		ad->new_seek_mean = ad->new_seek_total / 256;
	}

	/*
	 * Don't allow the seek distance to get too large from the
	 * odd fragment, pagein, etc
	 */
	if (aic->seek_samples <= 60) /* second&third seek */
		sdist = min(sdist, (aic->seek_mean * 4) + 2*1024*1024);
	else
		sdist = min(sdist, (aic->seek_mean * 4)	+ 2*1024*64);

	aic->seek_samples = (7*aic->seek_samples + 256) / 8;
	aic->seek_total = (7*aic->seek_total + (u64)256*sdist) / 8;
	total = aic->seek_total + (aic->seek_samples/2);
	do_div(total, aic->seek_samples);
	aic->seek_mean = (sector_t)total;
}

/*
 * as_update_iohist keeps a decaying histogram of IO thinktimes, and
 * updates @aic->ttime_mean based on that. It is called when a new
 * request is queued.
 */
static void as_update_iohist(struct as_data *ad, struct as_io_context *aic,
				struct request *rq)
{
	struct as_rq *arq = RQ_DATA(rq);
	int data_dir = arq->is_sync;
	unsigned long thinktime = 0;
	sector_t seek_dist;

	if (aic == NULL)
		return;

	if (data_dir == REQ_SYNC) {
		unsigned long in_flight = atomic_read(&aic->nr_queued)
					+ atomic_read(&aic->nr_dispatched);
		spin_lock(&aic->lock);
		if (test_bit(AS_TASK_IORUNNING, &aic->state) ||
			test_bit(AS_TASK_IOSTARTED, &aic->state)) {
			/* Calculate read -> read thinktime */
			if (test_bit(AS_TASK_IORUNNING, &aic->state)
							&& in_flight == 0) {
				thinktime = jiffies - aic->last_end_request;
				thinktime = min(thinktime, MAX_THINKTIME-1);
			}
			as_update_thinktime(ad, aic, thinktime);

			/* Calculate read -> read seek distance */
			if (aic->last_request_pos < rq->sector)
				seek_dist = rq->sector - aic->last_request_pos;
			else
				seek_dist = aic->last_request_pos - rq->sector;
			as_update_seekdist(ad, aic, seek_dist);
		}
		aic->last_request_pos = rq->sector + rq->nr_sectors;
		set_bit(AS_TASK_IOSTARTED, &aic->state);
		spin_unlock(&aic->lock);
	}
}

/*
 * as_close_req decides if one request is considered "close" to the
 * previous one issued.
 */
static int as_close_req(struct as_data *ad, struct as_io_context *aic,
				struct as_rq *arq)
{
	unsigned long delay;	/* milliseconds */
	sector_t last = ad->last_sector[ad->batch_data_dir];
	sector_t next = arq->request->sector;
	sector_t delta; /* acceptable close offset (in sectors) */
	sector_t s;

	if (ad->antic_status == ANTIC_OFF || !ad->ioc_finished)
		delay = 0;
	else
		delay = ((jiffies - ad->antic_start) * 1000) / HZ;

	if (delay == 0)
		delta = 8192;
	else if (delay <= 20 && delay <= ad->antic_expire)
		delta = 8192 << delay;
	else
		return 1;

	if ((last <= next + (delta>>1)) && (next <= last + delta))
		return 1;

	if (last < next)
		s = next - last;
	else
		s = last - next;

	if (aic->seek_samples == 0) {
		/*
		 * Process has just started IO. Use past statistics to
		 * gauge success possibility
		 */
		if (ad->new_seek_mean > s) {
			/* this request is better than what we're expecting */
			return 1;
		}

	} else {
		if (aic->seek_mean > s) {
			/* this request is better than what we're expecting */
			return 1;
		}
	}

	return 0;
}

/*
 * as_can_break_anticipation returns true if we have been anticipating this
 * request.
 *
 * It also returns true if the process against which we are anticipating
 * submits a write - that's presumably an fsync, O_SYNC write, etc. We want to
 * dispatch it ASAP, because we know that application will not be submitting
 * any new reads.
 *
 * If the task which has submitted the request has exited, break anticipation.
 *
 * If this task has queued some other IO, do not enter enticipation.
 */
static int as_can_break_anticipation(struct as_data *ad, struct as_rq *arq)
{
	struct io_context *ioc;
	struct as_io_context *aic;

	ioc = ad->io_context;
	BUG_ON(!ioc);

	if (arq && ioc == arq->io_context) {
		/* request from same process */
		return 1;
	}

	if (ad->ioc_finished && as_antic_expired(ad)) {
		/*
		 * In this situation status should really be FINISHED,
		 * however the timer hasn't had the chance to run yet.
		 */
		return 1;
	}

	aic = ioc->aic;
	if (!aic)
		return 0;

	if (atomic_read(&aic->nr_queued) > 0) {
		/* process has more requests queued */
		return 1;
	}

	if (atomic_read(&aic->nr_dispatched) > 0) {
		/* process has more requests dispatched */
		return 1;
	}

	if (arq && arq->is_sync == REQ_SYNC && as_close_req(ad, aic, arq)) {
		/*
		 * Found a close request that is not one of ours.
		 *
		 * This makes close requests from another process update
		 * our IO history. Is generally useful when there are
		 * two or more cooperating processes working in the same
		 * area.
		 */
		if (!test_bit(AS_TASK_RUNNING, &aic->state)) {
			if (aic->ttime_samples == 0)
				ad->exit_prob = (7*ad->exit_prob + 256)/8;

			ad->exit_no_coop = (7*ad->exit_no_coop)/8;
		}

		as_update_iohist(ad, aic, arq->request);
		return 1;
	}

	if (!test_bit(AS_TASK_RUNNING, &aic->state)) {
		/* process anticipated on has exited */
		if (aic->ttime_samples == 0)
			ad->exit_prob = (7*ad->exit_prob + 256)/8;

		if (ad->exit_no_coop > 128)
			return 1;
	}

	if (aic->ttime_samples == 0) {
		if (ad->new_ttime_mean > ad->antic_expire)
			return 1;
		if (ad->exit_prob * ad->exit_no_coop > 128*256)
			return 1;
	} else if (aic->ttime_mean > ad->antic_expire) {
		/* the process thinks too much between requests */
		return 1;
	}

	return 0;
}

/*
 * as_can_anticipate indicates weather we should either run arq
 * or keep anticipating a better request.
 */
static int as_can_anticipate(struct as_data *ad, struct as_rq *arq)
{
	if (!ad->io_context)
		/*
		 * Last request submitted was a write
		 */
		return 0;

	if (ad->antic_status == ANTIC_FINISHED)
		/*
		 * Don't restart if we have just finished. Run the next request
		 */
		return 0;

	if (as_can_break_anticipation(ad, arq))
		/*
		 * This request is a good candidate. Don't keep anticipating,
		 * run it.
		 */
		return 0;

	/*
	 * OK from here, we haven't finished, and don't have a decent request!
	 * Status is either ANTIC_OFF so start waiting,
	 * ANTIC_WAIT_REQ so continue waiting for request to finish
	 * or ANTIC_WAIT_NEXT so continue waiting for an acceptable request.
	 */

	return 1;
}

/*
 * as_update_arq must be called whenever a request (arq) is added to
 * the sort_list. This function keeps caches up to date, and checks if the
 * request might be one we are "anticipating"
 */
static void as_update_arq(struct as_data *ad, struct as_rq *arq)
{
	const int data_dir = arq->is_sync;

	/* keep the next_arq cache up to date */
	ad->next_arq[data_dir] = as_choose_req(ad, arq, ad->next_arq[data_dir]);

	/*
	 * have we been anticipating this request?
	 * or does it come from the same process as the one we are anticipating
	 * for?
	 */
	if (ad->antic_status == ANTIC_WAIT_REQ
			|| ad->antic_status == ANTIC_WAIT_NEXT) {
		if (as_can_break_anticipation(ad, arq))
			as_antic_stop(ad);
	}
}

/*
 * Gathers timings and resizes the write batch automatically
 */
static void update_write_batch(struct as_data *ad)
{
	unsigned long batch = ad->batch_expire[REQ_ASYNC];
	long write_time;

	write_time = (jiffies - ad->current_batch_expires) + batch;
	if (write_time < 0)
		write_time = 0;

	if (write_time > batch && !ad->write_batch_idled) {
		if (write_time > batch * 3)
			ad->write_batch_count /= 2;
		else
			ad->write_batch_count--;
	} else if (write_time < batch && ad->current_write_count == 0) {
		if (batch > write_time * 3)
			ad->write_batch_count *= 2;
		else
			ad->write_batch_count++;
	}

	if (ad->write_batch_count < 1)
		ad->write_batch_count = 1;
}

/*
 * as_completed_request is to be called when a request has completed and
 * returned something to the requesting process, be it an error or data.
 */
static void as_completed_request(request_queue_t *q, struct request *rq)
{
	struct as_data *ad = q->elevator->elevator_data;
	struct as_rq *arq = RQ_DATA(rq);

	WARN_ON(!list_empty(&rq->queuelist));

	if (arq->state != AS_RQ_REMOVED) {
		printk("arq->state %d\n", arq->state);
		WARN_ON(1);
		goto out;
	}