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deadline-iosched.c

/*
 *  Deadline i/o scheduler.
 *
 *  Copyright (C) 2002 Jens Axboe <axboe@kernel.dk>
 */
#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>

/*
 * See Documentation/block/deadline-iosched.txt
 */
static const int read_expire = HZ / 2;  /* max time before a read is submitted. */
static const int write_expire = 5 * HZ; /* ditto for writes, these limits are SOFT! */
static const int writes_starved = 2;    /* max times reads can starve a write */
static const int fifo_batch = 16;       /* # of sequential requests treated as one
                             by the above parameters. For throughput. */

struct deadline_data {
      /*
       * run time data
       */

      /*
       * requests (deadline_rq s) are present on both sort_list and fifo_list
       */
      struct rb_root sort_list[2];  
      struct list_head fifo_list[2];
      
      /*
       * next in sort order. read, write or both are NULL
       */
      struct request *next_rq[2];
      unsigned int batching;        /* number of sequential requests made */
      sector_t last_sector;         /* head position */
      unsigned int starved;         /* times reads have starved writes */

      /*
       * settings that change how the i/o scheduler behaves
       */
      int fifo_expire[2];
      int fifo_batch;
      int writes_starved;
      int front_merges;
};

static void deadline_move_request(struct deadline_data *, struct request *);

#define RQ_RB_ROOT(dd, rq)    (&(dd)->sort_list[rq_data_dir((rq))])

/*
 * get the request after `rq' in sector-sorted order
 */
static inline struct request *
deadline_latter_request(struct request *rq)
{
      struct rb_node *node = rb_next(&rq->rb_node);

      if (node)
            return rb_entry_rq(node);

      return NULL;
}

static void
deadline_add_rq_rb(struct deadline_data *dd, struct request *rq)
{
      struct rb_root *root = RQ_RB_ROOT(dd, rq);
      struct request *__alias;

retry:
      __alias = elv_rb_add(root, rq);
      if (unlikely(__alias)) {
            deadline_move_request(dd, __alias);
            goto retry;
      }
}

static inline void
deadline_del_rq_rb(struct deadline_data *dd, struct request *rq)
{
      const int data_dir = rq_data_dir(rq);

      if (dd->next_rq[data_dir] == rq)
            dd->next_rq[data_dir] = deadline_latter_request(rq);

      elv_rb_del(RQ_RB_ROOT(dd, rq), rq);
}

/*
 * add rq to rbtree and fifo
 */
static void
deadline_add_request(struct request_queue *q, struct request *rq)
{
      struct deadline_data *dd = q->elevator->elevator_data;
      const int data_dir = rq_data_dir(rq);

      deadline_add_rq_rb(dd, rq);

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

/*
 * remove rq from rbtree and fifo.
 */
static void deadline_remove_request(struct request_queue *q, struct request *rq)
{
      struct deadline_data *dd = q->elevator->elevator_data;

      rq_fifo_clear(rq);
      deadline_del_rq_rb(dd, rq);
}

static int
deadline_merge(struct request_queue *q, struct request **req, struct bio *bio)
{
      struct deadline_data *dd = q->elevator->elevator_data;
      struct request *__rq;
      int ret;

      /*
       * check for front merge
       */
      if (dd->front_merges) {
            sector_t sector = bio->bi_sector + bio_sectors(bio);

            __rq = elv_rb_find(&dd->sort_list[bio_data_dir(bio)], sector);
            if (__rq) {
                  BUG_ON(sector != __rq->sector);

                  if (elv_rq_merge_ok(__rq, bio)) {
                        ret = ELEVATOR_FRONT_MERGE;
                        goto out;
                  }
            }
      }

      return ELEVATOR_NO_MERGE;
out:
      *req = __rq;
      return ret;
}

static void deadline_merged_request(struct request_queue *q,
                            struct request *req, int type)
{
      struct deadline_data *dd = q->elevator->elevator_data;

      /*
       * if the merge was a front merge, we need to reposition request
       */
      if (type == ELEVATOR_FRONT_MERGE) {
            elv_rb_del(RQ_RB_ROOT(dd, req), req);
            deadline_add_rq_rb(dd, req);
      }
}

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

/*
 * move request from sort list to dispatch queue.
 */
static inline void
deadline_move_to_dispatch(struct deadline_data *dd, struct request *rq)
{
      struct request_queue *q = rq->q;

      deadline_remove_request(q, rq);
      elv_dispatch_add_tail(q, rq);
}

/*
 * move an entry to dispatch queue
 */
static void
deadline_move_request(struct deadline_data *dd, struct request *rq)
{
      const int data_dir = rq_data_dir(rq);

      dd->next_rq[READ] = NULL;
      dd->next_rq[WRITE] = NULL;
      dd->next_rq[data_dir] = deadline_latter_request(rq);

      dd->last_sector = rq->sector + rq->nr_sectors;

      /*
       * take it off the sort and fifo list, move
       * to dispatch queue
       */
      deadline_move_to_dispatch(dd, rq);
}

/*
 * deadline_check_fifo returns 0 if there are no expired reads on the fifo,
 * 1 otherwise. Requires !list_empty(&dd->fifo_list[data_dir])
 */
static inline int deadline_check_fifo(struct deadline_data *dd, int ddir)
{
      struct request *rq = rq_entry_fifo(dd->fifo_list[ddir].next);

      /*
       * rq is expired!
       */
      if (time_after(jiffies, rq_fifo_time(rq)))
            return 1;

      return 0;
}

/*
 * deadline_dispatch_requests selects the best request according to
 * read/write expire, fifo_batch, etc
 */
static int deadline_dispatch_requests(struct request_queue *q, int force)
{
      struct deadline_data *dd = q->elevator->elevator_data;
      const int reads = !list_empty(&dd->fifo_list[READ]);
      const int writes = !list_empty(&dd->fifo_list[WRITE]);
      struct request *rq;
      int data_dir;

      /*
       * batches are currently reads XOR writes
       */
      if (dd->next_rq[WRITE])
            rq = dd->next_rq[WRITE];
      else
            rq = dd->next_rq[READ];

      if (rq) {
            /* we have a "next request" */
            
            if (dd->last_sector != rq->sector)
                  /* end the batch on a non sequential request */
                  dd->batching += dd->fifo_batch;
            
            if (dd->batching < dd->fifo_batch)
                  /* we are still entitled to batch */
                  goto dispatch_request;
      }

      /*
       * at this point we are not running a batch. select the appropriate
       * data direction (read / write)
       */

      if (reads) {
            BUG_ON(RB_EMPTY_ROOT(&dd->sort_list[READ]));

            if (writes && (dd->starved++ >= dd->writes_starved))
                  goto dispatch_writes;

            data_dir = READ;

            goto dispatch_find_request;
      }

      /*
       * there are either no reads or writes have been starved
       */

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

            dd->starved = 0;

            data_dir = WRITE;

            goto dispatch_find_request;
      }

      return 0;

dispatch_find_request:
      /*
       * we are not running a batch, find best request for selected data_dir
       */
      if (deadline_check_fifo(dd, data_dir) || !dd->next_rq[data_dir]) {
            /*
             * A deadline has expired, the last request was in the other
             * direction, or we have run out of higher-sectored requests.
             * Start again from the request with the earliest expiry time.
             */
            rq = rq_entry_fifo(dd->fifo_list[data_dir].next);
      } else {
            /*
             * The last req was the same dir and we have a next request in
             * sort order. No expired requests so continue on from here.
             */
            rq = dd->next_rq[data_dir];
      }

      dd->batching = 0;

dispatch_request:
      /*
       * rq is the selected appropriate request.
       */
      dd->batching++;
      deadline_move_request(dd, rq);

      return 1;
}

static int deadline_queue_empty(struct request_queue *q)
{
      struct deadline_data *dd = q->elevator->elevator_data;

      return list_empty(&dd->fifo_list[WRITE])
            && list_empty(&dd->fifo_list[READ]);
}

static void deadline_exit_queue(elevator_t *e)
{
      struct deadline_data *dd = e->elevator_data;

      BUG_ON(!list_empty(&dd->fifo_list[READ]));
      BUG_ON(!list_empty(&dd->fifo_list[WRITE]));

      kfree(dd);
}

/*
 * initialize elevator private data (deadline_data).
 */
static void *deadline_init_queue(struct request_queue *q)
{
      struct deadline_data *dd;

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

      INIT_LIST_HEAD(&dd->fifo_list[READ]);
      INIT_LIST_HEAD(&dd->fifo_list[WRITE]);
      dd->sort_list[READ] = RB_ROOT;
      dd->sort_list[WRITE] = RB_ROOT;
      dd->fifo_expire[READ] = read_expire;
      dd->fifo_expire[WRITE] = write_expire;
      dd->writes_starved = writes_starved;
      dd->front_merges = 1;
      dd->fifo_batch = fifo_batch;
      return dd;
}

/*
 * sysfs parts below
 */

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

static ssize_t
deadline_var_store(int *var, const char *page, size_t count)
{
      char *p = (char *) page;

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

#define SHOW_FUNCTION(__FUNC, __VAR, __CONV)                      \
static ssize_t __FUNC(elevator_t *e, char *page)                  \
{                                                     \
      struct deadline_data *dd = e->elevator_data;                \
      int __data = __VAR;                                   \
      if (__CONV)                                     \
            __data = jiffies_to_msecs(__data);              \
      return deadline_var_show(__data, (page));             \
}
SHOW_FUNCTION(deadline_read_expire_show, dd->fifo_expire[READ], 1);
SHOW_FUNCTION(deadline_write_expire_show, dd->fifo_expire[WRITE], 1);
SHOW_FUNCTION(deadline_writes_starved_show, dd->writes_starved, 0);
SHOW_FUNCTION(deadline_front_merges_show, dd->front_merges, 0);
SHOW_FUNCTION(deadline_fifo_batch_show, dd->fifo_batch, 0);
#undef SHOW_FUNCTION

#define STORE_FUNCTION(__FUNC, __PTR, MIN, MAX, __CONV)                 \
static ssize_t __FUNC(elevator_t *e, const char *page, size_t count)    \
{                                                     \
      struct deadline_data *dd = e->elevator_data;                \
      int __data;                                     \
      int ret = deadline_var_store(&__data, (page), count);       \
      if (__data < (MIN))                                   \
            __data = (MIN);                                 \
      else if (__data > (MAX))                              \
            __data = (MAX);                                 \
      if (__CONV)                                     \
            *(__PTR) = msecs_to_jiffies(__data);                  \
      else                                            \
            *(__PTR) = __data;                              \
      return ret;                                     \
}
STORE_FUNCTION(deadline_read_expire_store, &dd->fifo_expire[READ], 0, INT_MAX, 1);
STORE_FUNCTION(deadline_write_expire_store, &dd->fifo_expire[WRITE], 0, INT_MAX, 1);
STORE_FUNCTION(deadline_writes_starved_store, &dd->writes_starved, INT_MIN, INT_MAX, 0);
STORE_FUNCTION(deadline_front_merges_store, &dd->front_merges, 0, 1, 0);
STORE_FUNCTION(deadline_fifo_batch_store, &dd->fifo_batch, 0, INT_MAX, 0);
#undef STORE_FUNCTION

#define DD_ATTR(name) \
      __ATTR(name, S_IRUGO|S_IWUSR, deadline_##name##_show, \
                              deadline_##name##_store)

static struct elv_fs_entry deadline_attrs[] = {
      DD_ATTR(read_expire),
      DD_ATTR(write_expire),
      DD_ATTR(writes_starved),
      DD_ATTR(front_merges),
      DD_ATTR(fifo_batch),
      __ATTR_NULL
};

static struct elevator_type iosched_deadline = {
      .ops = {
            .elevator_merge_fn =          deadline_merge,
            .elevator_merged_fn =         deadline_merged_request,
            .elevator_merge_req_fn =      deadline_merged_requests,
            .elevator_dispatch_fn =       deadline_dispatch_requests,
            .elevator_add_req_fn =        deadline_add_request,
            .elevator_queue_empty_fn =    deadline_queue_empty,
            .elevator_former_req_fn =     elv_rb_former_request,
            .elevator_latter_req_fn =     elv_rb_latter_request,
            .elevator_init_fn =           deadline_init_queue,
            .elevator_exit_fn =           deadline_exit_queue,
      },

      .elevator_attrs = deadline_attrs,
      .elevator_name = "deadline",
      .elevator_owner = THIS_MODULE,
};

static int __init deadline_init(void)
{
      elv_register(&iosched_deadline);

      return 0;
}

static void __exit deadline_exit(void)
{
      elv_unregister(&iosched_deadline);
}

module_init(deadline_init);
module_exit(deadline_exit);

MODULE_AUTHOR("Jens Axboe");
MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("deadline IO scheduler");

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