FreeBSD/Linux Kernel Cross Reference
sys/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>
    
    /*
     * 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 SYNC & 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 SYNC / 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, as_ioc_count);
   static struct completion *ioc_gone;
   static DEFINE_SPINLOCK(ioc_gone_lock);
   
   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(as_ioc_count);
           if (ioc_gone) {
                   /*
                    * AS scheduler is exiting, grab exit lock and check
                    * the pending io context count. If it hits zero,
                    * complete ioc_gone and set it back to NULL.
                    */
                   spin_lock(&ioc_gone_lock);
                   if (ioc_gone && !elv_ioc_count_read(as_ioc_count)) {
                           complete(ioc_gone);
                           ioc_gone = NULL;
                   }
                   spin_unlock(&ioc_gone_lock);
           }
   }
   
   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(as_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 = blk_rq_pos(rq1);
           s2 = blk_rq_pos(rq2);
   
           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->q, &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(q, &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 == BLK_RW_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 < blk_rq_pos(rq))
                                   seek_dist = blk_rq_pos(rq) -
                                               aic->last_request_pos;
                           else
                                   seek_dist = aic->last_request_pos -
                                               blk_rq_pos(rq);
                           as_update_seekdist(ad, aic, seek_dist);
                   }
                   aic->last_request_pos = blk_rq_pos(rq) + blk_rq_sectors(rq);
                   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 = blk_rq_pos(rq);
           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 0 /* disable for now, we need to check tag level as well */
           /*
            * SSD device without seek penalty, disable idling
            */
           if (blk_queue_nonrot(ad->q)) axman
                   return 0;
   #endif
   
           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[BLK_RW_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) {
                   WARN(1, "rq->state %d\n", RQ_STATE(rq));
                   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(q, &ad->antic_work);
                   ad->changed_batch = 0;
   
                   if (ad->batch_data_dir == BLK_RW_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[BLK_RW_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 == BLK_RW_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] = blk_rq_pos(rq) + blk_rq_sectors(rq);
   
           if (data_dir == BLK_RW_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[BLK_RW_SYNC]);
          const int writes = !list_empty(&ad->fifo_list[BLK_RW_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 BLK_RW_SYNC to avoid
                   * screwing write batch accounting as write batch
                   * accounting occurs on W->R transition.
                   */
                  int dispatched = 0;
  
                  ad->batch_data_dir = BLK_RW_SYNC;
                  ad->changed_batch = 0;
                  ad->new_batch = 0;
  
                  while (ad->next_rq[BLK_RW_SYNC]) {
                          as_move_to_dispatch(ad, ad->next_rq[BLK_RW_SYNC]);
                          dispatched++;
                  }
                  ad->last_check_fifo[BLK_RW_SYNC] = jiffies;
  
                  while (ad->next_rq[BLK_RW_ASYNC]) {
                          as_move_to_dispatch(ad, ad->next_rq[BLK_RW_ASYNC]);
                          dispatched++;
                  }
                  ad->last_check_fifo[BLK_RW_ASYNC] = jiffies;
  
                  return dispatched;
          }
  
          /* Signal that the write batch was uncontended, so we can't time it */
          if (ad->batch_data_dir == BLK_RW_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 == BLK_RW_SYNC && ad->antic_expire) {
                          if (as_fifo_expired(ad, BLK_RW_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[BLK_RW_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[BLK_RW_SYNC]));
  
                  if (writes && ad->batch_data_dir == BLK_RW_SYNC)
                          /*
                           * Last batch was a read, switch to writes
                           */
                          goto dispatch_writes;
  
                  if (ad->batch_data_dir == BLK_RW_ASYNC) {
                          WARN_ON(ad->new_batch);
                          ad->changed_batch = 1;
                  }
                  ad->batch_data_dir = BLK_RW_SYNC;
                  rq = rq_entry_fifo(ad->fifo_list[BLK_RW_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[BLK_RW_ASYNC]));
  
                  if (ad->batch_data_dir == BLK_RW_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 = BLK_RW_ASYNC;
                  ad->current_write_count = ad->write_batch_count;
                  ad->write_batch_idled = 0;
                  rq = rq_entry_fifo(ad->fifo_list[BLK_RW_ASYNC].next);
                  ad->last_check_fifo[BLK_RW_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 == BLK_RW_ASYNC)
                          ad->current_batch_expires = jiffies +
                                          ad->batch_expire[BLK_RW_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[BLK_RW_ASYNC])
                  && list_empty(&ad->fifo_list[BLK_RW_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);
  
          blk_run_queue(ad->q);
  }
  
  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(struct elevator_queue *e)
  {
          struct as_data *ad = e->elevator_data;
  
          del_timer_sync(&ad->antic_timer);
          cancel_work_sync(&ad->antic_work);
  
          BUG_ON(!list_empty(&ad->fifo_list[BLK_RW_SYNC]));
          BUG_ON(!list_empty(&ad->fifo_list[BLK_RW_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[BLK_RW_SYNC]);
          INIT_LIST_HEAD(&ad->fifo_list[BLK_RW_ASYNC]);
          ad->sort_list[BLK_RW_SYNC] = RB_ROOT;
          ad->sort_list[BLK_RW_ASYNC] = RB_ROOT;
          ad->fifo_expire[BLK_RW_SYNC] = default_read_expire;
          ad->fifo_expire[BLK_RW_ASYNC] = default_write_expire;
          ad->antic_expire = default_antic_expire;
          ad->batch_expire[BLK_RW_SYNC] = default_read_batch_expire;
          ad->batch_expire[BLK_RW_ASYNC] = default_write_batch_expire;
  
          ad->current_batch_expires = jiffies + ad->batch_expire[BLK_RW_SYNC];
          ad->write_batch_count = ad->batch_expire[BLK_RW_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(struct elevator_queue *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(struct elevator_queue *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[BLK_RW_SYNC]);
  SHOW_FUNCTION(as_write_expire_show, ad->fifo_expire[BLK_RW_ASYNC]);
  SHOW_FUNCTION(as_antic_expire_show, ad->antic_expire);
  SHOW_FUNCTION(as_read_batch_expire_show, ad->batch_expire[BLK_RW_SYNC]);
  SHOW_FUNCTION(as_write_batch_expire_show, ad->batch_expire[BLK_RW_ASYNC]);
  #undef SHOW_FUNCTION
  
  #define STORE_FUNCTION(__FUNC, __PTR, MIN, MAX)                         \
  static ssize_t __FUNC(struct elevator_queue *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[BLK_RW_SYNC], 0, INT_MAX);
  STORE_FUNCTION(as_write_expire_store,
                          &ad->fifo_expire[BLK_RW_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[BLK_RW_SYNC], 0, INT_MAX);
  STORE_FUNCTION(as_write_batch_expire_store,
                          &ad->batch_expire[BLK_RW_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(as_ioc_count))
                  wait_for_completion(&all_gone);
          synchronize_rcu();
  }
  
  module_init(as_init);
  module_exit(as_exit);
  
  MODULE_AUTHOR("Nick Piggin");
  MODULE_LICENSE("GPL");
  MODULE_DESCRIPTION("anticipatory IO scheduler");

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