deadline-iosched.c

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/*
 *  Deadline i/o scheduler.
 *
 *  Copyright (C) 2002 Jens Axboe <[email protected]>
 */
#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 *);

static inline struct rb_root *
deadline_rb_root(struct deadline_data *dd, struct request *rq)
{
    return &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 = deadline_rb_root(dd, rq);

    elv_rb_add(root, rq);
}

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(deadline_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 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 != blk_rq_pos(__rq));

            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(deadline_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_end_sector(rq);

    /*
     * 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 requests 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 && dd->batching < dd->fifo_batch)
        /* we have a next request 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 void deadline_exit_queue(struct elevator_queue *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 int 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 -ENOMEM;

    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;

    q->elevator->elevator_data = dd;
    return 0;
}

/*
 * 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(struct elevator_queue *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(struct elevator_queue *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_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)
{
    return elv_register(&iosched_deadline);
}

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");