block: Fix the starving writes bug in the anticipatory IO scheduler

[safe/jmp/linux-2.6] / block / as-iosched.c

/*
 *  Anticipatory & deadline i/o scheduler.
 *
 *  Copyright (C) 2002 Jens Axboe <axboe@kernel.dk>
 *                     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/module.h>
#include <linux/slab.h>
#include <linux/init.h>
#include <linux/compiler.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 request *next_rq[2];     /* next in sort order */
        sector_t last_sector[2];        /* last REQ_SYNC & REQ_ASYNC sectors */

        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? */

        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;
};

/*
 * 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 */
};

#define RQ_IOC(rq)      ((struct io_context *) (rq)->elevator_private)
#define RQ_STATE(rq)    ((enum arq_state)(rq)->elevator_private2)
#define RQ_SET_STATE(rq, state) ((rq)->elevator_private2 = (void *) state)

static DEFINE_PER_CPU(unsigned long, ioc_count);
static struct completion *ioc_gone;

static void as_move_to_dispatch(struct as_data *ad, struct request *rq);
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);
        elv_ioc_count_dec(ioc_count);
        if (ioc_gone && !elv_ioc_count_read(ioc_count))
                complete(ioc_gone);
}

static void as_trim(struct io_context *ioc)
{
        spin_lock_irq(&ioc->lock);
        if (ioc->aic)
                free_as_io_context(ioc->aic);
        ioc->aic = NULL;
        spin_unlock_irq(&ioc->lock);
}

/* 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;
                elv_ioc_count_inc(ioc_count);
        }

        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(int node)
{
        struct io_context *ioc = get_io_context(GFP_ATOMIC, node);
        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 request *rq)
{
        struct as_io_context *aic;

        if (unlikely(!RQ_IOC(rq)))
                return;

        aic = RQ_IOC(rq)->aic;

        if (rq_is_sync(rq) && aic) {
                unsigned long flags;

                spin_lock_irqsave(&aic->lock, flags);
                set_bit(AS_TASK_IORUNNING, &aic->state);
                aic->last_end_request = jiffies;
                spin_unlock_irqrestore(&aic->lock, flags);
        }

        put_io_context(RQ_IOC(rq));
}

/*
 * rb tree support functions
 */
#define RQ_RB_ROOT(ad, rq)      (&(ad)->sort_list[rq_is_sync((rq))])

static void as_add_rq_rb(struct as_data *ad, struct request *rq)
{
        struct request *alias;

        while ((unlikely(alias = elv_rb_add(RQ_RB_ROOT(ad, rq), rq)))) {
                as_move_to_dispatch(ad, alias);
                as_antic_stop(ad);
        }
}

static inline void as_del_rq_rb(struct as_data *ad, struct request *rq)
{
        elv_rb_del(RQ_RB_ROOT(ad, rq), rq);
}

/*
 * 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 request *
as_choose_req(struct as_data *ad, struct request *rq1, struct request *rq2)
{
        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 (rq1 == NULL || rq1 == rq2)
                return rq2;
        if (rq2 == NULL)
                return rq1;

        data_dir = rq_is_sync(rq1);

        last = ad->last_sector[data_dir];
        s1 = rq1->sector;
        s2 = rq2->sector;

        BUG_ON(data_dir != rq_is_sync(rq2));

        /*
         * 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 rq1;
        else if (!r2_wrap && r1_wrap)
                return rq2;
        else if (r1_wrap && r2_wrap) {
                /* both behind the head */
                if (s1 <= s2)
                        return rq1;
                else
                        return rq2;
        }

        /* Both requests in front of the head */
        if (d1 < d2)
                return rq1;
        else if (d2 < d1)
                return rq2;
        else {
                if (s1 >= s2)
                        return rq1;
                else
                        return rq2;
        }
}

/*
 * as_find_next_rq 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 request *
as_find_next_rq(struct as_data *ad, struct request *last)
{
        struct rb_node *rbnext = rb_next(&last->rb_node);
        struct rb_node *rbprev = rb_prev(&last->rb_node);
        struct request *next = NULL, *prev = NULL;

        BUG_ON(RB_EMPTY_NODE(&last->rb_node));

        if (rbprev)
                prev = rb_entry_rq(rbprev);

        if (rbnext)
                next = rb_entry_rq(rbnext);
        else {
                const int data_dir = rq_is_sync(last);

                rbnext = rb_first(&ad->sort_list[data_dir]);
                if (rbnext && rbnext != &last->rb_node)
                        next = rb_entry_rq(rbnext);
        }

        return as_choose_req(ad, next, prev);
}

/*
 * 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;
                spin_lock(&ad->io_context->lock);
                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(&ad->io_context->lock);
        }
        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)
{
        int data_dir = rq_is_sync(rq);
        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 request *rq)
{
        unsigned long delay;    /* jiffies */
        sector_t last = ad->last_sector[ad->batch_data_dir];
        sector_t next = rq->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;

        if (delay == 0)
                delta = 8192;
        else if (delay <= (20 * HZ / 1000) && 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 request *rq)
{
        struct io_context *ioc;
        struct as_io_context *aic;

        ioc = ad->io_context;
        BUG_ON(!ioc);
        spin_lock(&ioc->lock);

        if (rq && ioc == RQ_IOC(rq)) {
                /* request from same process */
                spin_unlock(&ioc->lock);
                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.
                 */
                spin_unlock(&ioc->lock);
                return 1;
        }

        aic = ioc->aic;
        if (!aic) {
                spin_unlock(&ioc->lock);
                return 0;
        }

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

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

        if (rq && rq_is_sync(rq) && as_close_req(ad, aic, rq)) {
                /*
                 * 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, rq);
                spin_unlock(&ioc->lock);
                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) {
                        spin_unlock(&ioc->lock);
                        return 1;
                }
        }

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

/*
 * as_can_anticipate indicates whether we should either run rq
 * or keep anticipating a better request.
 */
static int as_can_anticipate(struct as_data *ad, struct request *rq)
{
        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, rq))
                /*
                 * 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_rq must be called whenever a request (rq) 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_rq(struct as_data *ad, struct request *rq)
{
        const int data_dir = rq_is_sync(rq);

        /* keep the next_rq cache up to date */
        ad->next_rq[data_dir] = as_choose_req(ad, rq, ad->next_rq[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, rq))
                        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(struct request_queue *q, struct request *rq)
{
        struct as_data *ad = q->elevator->elevator_data;

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

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

        if (ad->changed_batch && ad->nr_dispatched == 1) {
                ad->current_batch_expires = jiffies +
                                        ad->batch_expire[ad->batch_data_dir];
                kblockd_schedule_work(&ad->antic_work);
                ad->changed_batch = 0;

                if (ad->batch_data_dir == REQ_SYNC)
                        ad->new_batch = 1;
        }
        WARN_ON(ad->nr_dispatched == 0);
        ad->nr_dispatched--;

        /*
         * Start counting the batch from when a request of that direction is
         * actually serviced. This should help devices with big TCQ windows
         * and writeback caches
         */
        if (ad->new_batch && ad->batch_data_dir == rq_is_sync(rq)) {
                update_write_batch(ad);
                ad->current_batch_expires = jiffies +
                                ad->batch_expire[REQ_SYNC];
                ad->new_batch = 0;
        }

        if (ad->io_context == RQ_IOC(rq) && ad->io_context) {
                ad->antic_start = jiffies;
                ad->ioc_finished = 1;
                if (ad->antic_status == ANTIC_WAIT_REQ) {
                        /*
                         * We were waiting on this request, now anticipate
                         * the next one
                         */
                        as_antic_waitnext(ad);
                }
        }

        as_put_io_context(rq);
out:
        RQ_SET_STATE(rq, AS_RQ_POSTSCHED);
}

/*
 * as_remove_queued_request removes a request from the pre dispatch queue
 * without updating refcounts. It is expected the caller will drop the
 * reference unless it replaces the request at somepart of the elevator
 * (ie. the dispatch queue)
 */
static void as_remove_queued_request(struct request_queue *q,
                                     struct request *rq)
{
        const int data_dir = rq_is_sync(rq);
        struct as_data *ad = q->elevator->elevator_data;
        struct io_context *ioc;

        WARN_ON(RQ_STATE(rq) != AS_RQ_QUEUED);

        ioc = RQ_IOC(rq);
        if (ioc && ioc->aic) {
                BUG_ON(!atomic_read(&ioc->aic->nr_queued));
                atomic_dec(&ioc->aic->nr_queued);
        }

        /*
         * Update the "next_rq" cache if we are about to remove its
         * entry
         */
        if (ad->next_rq[data_dir] == rq)
                ad->next_rq[data_dir] = as_find_next_rq(ad, rq);

        rq_fifo_clear(rq);
        as_del_rq_rb(ad, rq);
}

/*
 * as_fifo_expired returns 0 if there are no expired requests on the fifo,
 * 1 otherwise.  It is ratelimited so that we only perform the check once per
 * `fifo_expire' interval.  Otherwise a large number of expired requests
 * would create a hopeless seekstorm.
 *
 * See as_antic_expired comment.
 */
static int as_fifo_expired(struct as_data *ad, int adir)
{
        struct request *rq;
        long delta_jif;

        delta_jif = jiffies - ad->last_check_fifo[adir];
        if (unlikely(delta_jif < 0))
                delta_jif = -delta_jif;
        if (delta_jif < ad->fifo_expire[adir])
                return 0;

        ad->last_check_fifo[adir] = jiffies;

        if (list_empty(&ad->fifo_list[adir]))
                return 0;

        rq = rq_entry_fifo(ad->fifo_list[adir].next);

        return time_after(jiffies, rq_fifo_time(rq));
}

/*
 * as_batch_expired returns true if the current batch has expired. A batch
 * is a set of reads or a set of writes.
 */
static inline int as_batch_expired(struct as_data *ad)
{
        if (ad->changed_batch || ad->new_batch)
                return 0;

        if (ad->batch_data_dir == REQ_SYNC)
                /* TODO! add a check so a complete fifo gets written? */
                return time_after(jiffies, ad->current_batch_expires);

        return time_after(jiffies, ad->current_batch_expires)
                || ad->current_write_count == 0;
}

/*
 * move an entry to dispatch queue
 */
static void as_move_to_dispatch(struct as_data *ad, struct request *rq)
{
        const int data_dir = rq_is_sync(rq);

        BUG_ON(RB_EMPTY_NODE(&rq->rb_node));

        as_antic_stop(ad);
        ad->antic_status = ANTIC_OFF;

        /*
         * This has to be set in order to be correctly updated by
         * as_find_next_rq
         */
        ad->last_sector[data_dir] = rq->sector + rq->nr_sectors;

        if (data_dir == REQ_SYNC) {
                struct io_context *ioc = RQ_IOC(rq);
                /* In case we have to anticipate after this */
                copy_io_context(&ad->io_context, &ioc);
        } else {
                if (ad->io_context) {
                        put_io_context(ad->io_context);
                        ad->io_context = NULL;
                }

                if (ad->current_write_count != 0)
                        ad->current_write_count--;
        }
        ad->ioc_finished = 0;

        ad->next_rq[data_dir] = as_find_next_rq(ad, rq);

        /*
         * take it off the sort and fifo list, add to dispatch queue
         */
        as_remove_queued_request(ad->q, rq);
        WARN_ON(RQ_STATE(rq) != AS_RQ_QUEUED);

        elv_dispatch_sort(ad->q, rq);

        RQ_SET_STATE(rq, AS_RQ_DISPATCHED);
        if (RQ_IOC(rq) && RQ_IOC(rq)->aic)
                atomic_inc(&RQ_IOC(rq)->aic->nr_dispatched);
        ad->nr_dispatched++;
}

/*
 * as_dispatch_request selects the best request according to
 * read/write expire, batch expire, etc, and moves it to the dispatch
 * queue. Returns 1 if a request was found, 0 otherwise.
 */
static int as_dispatch_request(struct request_queue *q, int force)
{
        struct as_data *ad = q->elevator->elevator_data;
        const int reads = !list_empty(&ad->fifo_list[REQ_SYNC]);
        const int writes = !list_empty(&ad->fifo_list[REQ_ASYNC]);
        struct request *rq;

        if (unlikely(force)) {
                /*
                 * Forced dispatch, accounting is useless.  Reset
                 * accounting states and dump fifo_lists.  Note that
                 * batch_data_dir is reset to REQ_SYNC to avoid
                 * screwing write batch accounting as write batch
                 * accounting occurs on W->R transition.
                 */
                int dispatched = 0;

                ad->batch_data_dir = REQ_SYNC;
                ad->changed_batch = 0;
                ad->new_batch = 0;

                while (ad->next_rq[REQ_SYNC]) {
                        as_move_to_dispatch(ad, ad->next_rq[REQ_SYNC]);
                        dispatched++;
                }
                ad->last_check_fifo[REQ_SYNC] = jiffies;

                while (ad->next_rq[REQ_ASYNC]) {
                        as_move_to_dispatch(ad, ad->next_rq[REQ_ASYNC]);
                        dispatched++;
                }
                ad->last_check_fifo[REQ_ASYNC] = jiffies;

                return dispatched;
        }

        /* Signal that the write batch was uncontended, so we can't time it */
        if (ad->batch_data_dir == REQ_ASYNC && !reads) {
                if (ad->current_write_count == 0 || !writes)
                        ad->write_batch_idled = 1;
        }

        if (!(reads || writes)
                || ad->antic_status == ANTIC_WAIT_REQ
                || ad->antic_status == ANTIC_WAIT_NEXT
                || ad->changed_batch)
                return 0;

        if (!(reads && writes && as_batch_expired(ad))) {
                /*
                 * batch is still running or no reads or no writes
                 */
                rq = ad->next_rq[ad->batch_data_dir];

                if (ad->batch_data_dir == REQ_SYNC && ad->antic_expire) {
                        if (as_fifo_expired(ad, REQ_SYNC))
                                goto fifo_expired;

                        if (as_can_anticipate(ad, rq)) {
                                as_antic_waitreq(ad);
                                return 0;
                        }
                }

                if (rq) {
                        /* we have a "next request" */
                        if (reads && !writes)
                                ad->current_batch_expires =
                                        jiffies + ad->batch_expire[REQ_SYNC];
                        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(&ad->sort_list[REQ_SYNC]));

                if (writes && ad->batch_data_dir == REQ_SYNC)
                        /*
                         * Last batch was a read, switch to writes
                         */
                        goto dispatch_writes;

                if (ad->batch_data_dir == REQ_ASYNC) {
                        WARN_ON(ad->new_batch);
                        ad->changed_batch = 1;
                }
                ad->batch_data_dir = REQ_SYNC;
                rq = rq_entry_fifo(ad->fifo_list[REQ_SYNC].next);
                ad->last_check_fifo[ad->batch_data_dir] = jiffies;
                goto dispatch_request;
        }

        /*
         * the last batch was a read
         */

        if (writes) {
dispatch_writes:
                BUG_ON(RB_EMPTY_ROOT(&ad->sort_list[REQ_ASYNC]));

                if (ad->batch_data_dir == REQ_SYNC) {
                        ad->changed_batch = 1;

                        /*
                         * new_batch might be 1 when the queue runs out of
                         * reads. A subsequent submission of a write might
                         * cause a change of batch before the read is finished.
                         */
                        ad->new_batch = 0;
                }
                ad->batch_data_dir = REQ_ASYNC;
                ad->current_write_count = ad->write_batch_count;
                ad->write_batch_idled = 0;
                rq = rq_entry_fifo(ad->fifo_list[REQ_ASYNC].next);
                ad->last_check_fifo[REQ_ASYNC] = jiffies;
                goto dispatch_request;
        }

        BUG();
        return 0;

dispatch_request:
        /*
         * If a request has expired, service it.
         */

        if (as_fifo_expired(ad, ad->batch_data_dir)) {
fifo_expired:
                rq = rq_entry_fifo(ad->fifo_list[ad->batch_data_dir].next);
        }

        if (ad->changed_batch) {
                WARN_ON(ad->new_batch);

                if (ad->nr_dispatched)
                        return 0;

                if (ad->batch_data_dir == REQ_ASYNC)
                        ad->current_batch_expires = jiffies +
                                        ad->batch_expire[REQ_ASYNC];
                else
                        ad->new_batch = 1;

                ad->changed_batch = 0;
        }

        /*
         * rq is the selected appropriate request.
         */
        as_move_to_dispatch(ad, rq);

        return 1;
}

/*
 * add rq to rbtree and fifo
 */
static void as_add_request(struct request_queue *q, struct request *rq)
{
        struct as_data *ad = q->elevator->elevator_data;
        int data_dir;

        RQ_SET_STATE(rq, AS_RQ_NEW);

        data_dir = rq_is_sync(rq);

        rq->elevator_private = as_get_io_context(q->node);

        if (RQ_IOC(rq)) {
                as_update_iohist(ad, RQ_IOC(rq)->aic, rq);
                atomic_inc(&RQ_IOC(rq)->aic->nr_queued);
        }

        as_add_rq_rb(ad, rq);

        /*
         * set expire time and add to fifo list
         */
        rq_set_fifo_time(rq, jiffies + ad->fifo_expire[data_dir]);
        list_add_tail(&rq->queuelist, &ad->fifo_list[data_dir]);

        as_update_rq(ad, rq); /* keep state machine up to date */
        RQ_SET_STATE(rq, AS_RQ_QUEUED);
}

static void as_activate_request(struct request_queue *q, struct request *rq)
{
        WARN_ON(RQ_STATE(rq) != AS_RQ_DISPATCHED);
        RQ_SET_STATE(rq, AS_RQ_REMOVED);
        if (RQ_IOC(rq) && RQ_IOC(rq)->aic)
                atomic_dec(&RQ_IOC(rq)->aic->nr_dispatched);
}

static void as_deactivate_request(struct request_queue *q, struct request *rq)
{
        WARN_ON(RQ_STATE(rq) != AS_RQ_REMOVED);
        RQ_SET_STATE(rq, AS_RQ_DISPATCHED);
        if (RQ_IOC(rq) && RQ_IOC(rq)->aic)
                atomic_inc(&RQ_IOC(rq)->aic->nr_dispatched);
}

/*
 * as_queue_empty tells us if there are requests left in the device. It may
 * not be the case that a driver can get the next request even if the queue
 * is not empty - it is used in the block layer to check for plugging and
 * merging opportunities
 */
static int as_queue_empty(struct request_queue *q)
{
        struct as_data *ad = q->elevator->elevator_data;

        return list_empty(&ad->fifo_list[REQ_ASYNC])
                && list_empty(&ad->fifo_list[REQ_SYNC]);
}

static int
as_merge(struct request_queue *q, struct request **req, struct bio *bio)
{
        struct as_data *ad = q->elevator->elevator_data;
        sector_t rb_key = bio->bi_sector + bio_sectors(bio);
        struct request *__rq;

        /*
         * check for front merge
         */
        __rq = elv_rb_find(&ad->sort_list[bio_data_dir(bio)], rb_key);
        if (__rq && elv_rq_merge_ok(__rq, bio)) {
                *req = __rq;
                return ELEVATOR_FRONT_MERGE;
        }

        return ELEVATOR_NO_MERGE;
}

static void as_merged_request(struct request_queue *q, struct request *req,
                              int type)
{
        struct as_data *ad = q->elevator->elevator_data;

        /*
         * if the merge was a front merge, we need to reposition request
         */
        if (type == ELEVATOR_FRONT_MERGE) {
                as_del_rq_rb(ad, req);
                as_add_rq_rb(ad, req);
                /*
                 * Note! At this stage of this and the next function, our next
                 * request may not be optimal - eg the request may have "grown"
                 * behind the disk head. We currently don't bother adjusting.
                 */
        }
}

static void as_merged_requests(struct request_queue *q, struct request *req,
                                struct request *next)
{
        /*
         * if next expires before rq, assign its expire time to arq
         * and move into next position (next will be deleted) in fifo
         */
        if (!list_empty(&req->queuelist) && !list_empty(&next->queuelist)) {
                if (time_before(rq_fifo_time(next), rq_fifo_time(req))) {
                        list_move(&req->queuelist, &next->queuelist);
                        rq_set_fifo_time(req, rq_fifo_time(next));
                }
        }

        /*
         * kill knowledge of next, this one is a goner
         */
        as_remove_queued_request(q, next);
        as_put_io_context(next);

        RQ_SET_STATE(next, AS_RQ_MERGED);
}

/*
 * This is executed in a "deferred" process context, by kblockd. It calls the
 * driver's request_fn so the driver can submit that request.
 *
 * IMPORTANT! This guy will reenter the elevator, so set up all queue global
 * state before calling, and don't rely on any state over calls.
 *
 * FIXME! dispatch queue is not a queue at all!
 */
static void as_work_handler(struct work_struct *work)
{
        struct as_data *ad = container_of(work, struct as_data, antic_work);
        struct request_queue *q = ad->q;
        unsigned long flags;

        spin_lock_irqsave(q->queue_lock, flags);
        blk_start_queueing(q);
        spin_unlock_irqrestore(q->queue_lock, flags);
}

static int as_may_queue(struct request_queue *q, int rw)
{
        int ret = ELV_MQUEUE_MAY;
        struct as_data *ad = q->elevator->elevator_data;
        struct io_context *ioc;
        if (ad->antic_status == ANTIC_WAIT_REQ ||
                        ad->antic_status == ANTIC_WAIT_NEXT) {
                ioc = as_get_io_context(q->node);
                if (ad->io_context == ioc)
                        ret = ELV_MQUEUE_MUST;
                put_io_context(ioc);
        }

        return ret;
}

static void as_exit_queue(elevator_t *e)
{
        struct as_data *ad = e->elevator_data;

        del_timer_sync(&ad->antic_timer);
        kblockd_flush_work(&ad->antic_work);

        BUG_ON(!list_empty(&ad->fifo_list[REQ_SYNC]));
        BUG_ON(!list_empty(&ad->fifo_list[REQ_ASYNC]));

        put_io_context(ad->io_context);
        kfree(ad);
}

/*
 * initialize elevator private data (as_data).
 */
static void *as_init_queue(struct request_queue *q)
{
        struct as_data *ad;

        ad = kmalloc_node(sizeof(*ad), GFP_KERNEL | __GFP_ZERO, q->node);
        if (!ad)
                return NULL;

        ad->q = q; /* Identify what queue the data belongs to */

        /* anticipatory scheduling helpers */
        ad->antic_timer.function = as_antic_timeout;
        ad->antic_timer.data = (unsigned long)q;
        init_timer(&ad->antic_timer);
        INIT_WORK(&ad->antic_work, as_work_handler);

        INIT_LIST_HEAD(&ad->fifo_list[REQ_SYNC]);
        INIT_LIST_HEAD(&ad->fifo_list[REQ_ASYNC]);
        ad->sort_list[REQ_SYNC] = RB_ROOT;
        ad->sort_list[REQ_ASYNC] = RB_ROOT;
        ad->fifo_expire[REQ_SYNC] = default_read_expire;
        ad->fifo_expire[REQ_ASYNC] = default_write_expire;
        ad->antic_expire = default_antic_expire;
        ad->batch_expire[REQ_SYNC] = default_read_batch_expire;
        ad->batch_expire[REQ_ASYNC] = default_write_batch_expire;

        ad->current_batch_expires = jiffies + ad->batch_expire[REQ_SYNC];
        ad->write_batch_count = ad->batch_expire[REQ_ASYNC] / 10;
        if (ad->write_batch_count < 2)
                ad->write_batch_count = 2;

        return ad;
}

/*
 * sysfs parts below
 */

static ssize_t
as_var_show(unsigned int var, char *page)
{
        return sprintf(page, "%d\n", var);
}

static ssize_t
as_var_store(unsigned long *var, const char *page, size_t count)
{
        char *p = (char *) page;

        *var = simple_strtoul(p, &p, 10);
        return count;
}

static ssize_t est_time_show(elevator_t *e, char *page)
{
        struct as_data *ad = e->elevator_data;
        int pos = 0;

        pos += sprintf(page+pos, "%lu %% exit probability\n",
                                100*ad->exit_prob/256);
        pos += sprintf(page+pos, "%lu %% probability of exiting without a "
                                "cooperating process submitting IO\n",
                                100*ad->exit_no_coop/256);
        pos += sprintf(page+pos, "%lu ms new thinktime\n", ad->new_ttime_mean);
        pos += sprintf(page+pos, "%llu sectors new seek distance\n",
                                (unsigned long long)ad->new_seek_mean);

        return pos;
}

#define SHOW_FUNCTION(__FUNC, __VAR)                            \
static ssize_t __FUNC(elevator_t *e, char *page)                \
{                                                               \
        struct as_data *ad = e->elevator_data;                  \
        return as_var_show(jiffies_to_msecs((__VAR)), (page));  \
}
SHOW_FUNCTION(as_read_expire_show, ad->fifo_expire[REQ_SYNC]);
SHOW_FUNCTION(as_write_expire_show, ad->fifo_expire[REQ_ASYNC]);
SHOW_FUNCTION(as_antic_expire_show, ad->antic_expire);
SHOW_FUNCTION(as_read_batch_expire_show, ad->batch_expire[REQ_SYNC]);
SHOW_FUNCTION(as_write_batch_expire_show, ad->batch_expire[REQ_ASYNC]);
#undef SHOW_FUNCTION

#define STORE_FUNCTION(__FUNC, __PTR, MIN, MAX)                         \
static ssize_t __FUNC(elevator_t *e, const char *page, size_t count)    \
{                                                                       \
        struct as_data *ad = e->elevator_data;                          \
        int ret = as_var_store(__PTR, (page), count);                   \
        if (*(__PTR) < (MIN))                                           \
                *(__PTR) = (MIN);                                       \
        else if (*(__PTR) > (MAX))                                      \
                *(__PTR) = (MAX);                                       \
        *(__PTR) = msecs_to_jiffies(*(__PTR));                          \
        return ret;                                                     \
}
STORE_FUNCTION(as_read_expire_store, &ad->fifo_expire[REQ_SYNC], 0, INT_MAX);
STORE_FUNCTION(as_write_expire_store, &ad->fifo_expire[REQ_ASYNC], 0, INT_MAX);
STORE_FUNCTION(as_antic_expire_store, &ad->antic_expire, 0, INT_MAX);
STORE_FUNCTION(as_read_batch_expire_store,
                        &ad->batch_expire[REQ_SYNC], 0, INT_MAX);
STORE_FUNCTION(as_write_batch_expire_store,
                        &ad->batch_expire[REQ_ASYNC], 0, INT_MAX);
#undef STORE_FUNCTION

#define AS_ATTR(name) \
        __ATTR(name, S_IRUGO|S_IWUSR, as_##name##_show, as_##name##_store)

static struct elv_fs_entry as_attrs[] = {
        __ATTR_RO(est_time),
        AS_ATTR(read_expire),
        AS_ATTR(write_expire),
        AS_ATTR(antic_expire),
        AS_ATTR(read_batch_expire),
        AS_ATTR(write_batch_expire),
        __ATTR_NULL
};

static struct elevator_type iosched_as = {
        .ops = {
                .elevator_merge_fn =            as_merge,
                .elevator_merged_fn =           as_merged_request,
                .elevator_merge_req_fn =        as_merged_requests,
                .elevator_dispatch_fn =         as_dispatch_request,
                .elevator_add_req_fn =          as_add_request,
                .elevator_activate_req_fn =     as_activate_request,
                .elevator_deactivate_req_fn =   as_deactivate_request,
                .elevator_queue_empty_fn =      as_queue_empty,
                .elevator_completed_req_fn =    as_completed_request,
                .elevator_former_req_fn =       elv_rb_former_request,
                .elevator_latter_req_fn =       elv_rb_latter_request,
                .elevator_may_queue_fn =        as_may_queue,
                .elevator_init_fn =             as_init_queue,
                .elevator_exit_fn =             as_exit_queue,
                .trim =                         as_trim,
        },

        .elevator_attrs = as_attrs,
        .elevator_name = "anticipatory",
        .elevator_owner = THIS_MODULE,
};

static int __init as_init(void)
{
        elv_register(&iosched_as);

        return 0;
}

static void __exit as_exit(void)
{
        DECLARE_COMPLETION_ONSTACK(all_gone);
        elv_unregister(&iosched_as);
        ioc_gone = &all_gone;
        /* ioc_gone's update must be visible before reading ioc_count */
        smp_wmb();
        if (elv_ioc_count_read(ioc_count))
                wait_for_completion(ioc_gone);
        synchronize_rcu();
}

module_init(as_init);
module_exit(as_exit);

MODULE_AUTHOR("Nick Piggin");
MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("anticipatory IO scheduler");