[PATCH] cfq-iosched: merging problem

[linux-2.6/linux-acpi-2.6/ibm-acpi-2.6.git] / block / cfq-iosched.c

/*
 *  CFQ, or complete fairness queueing, disk scheduler.
 *
 *  Based on ideas from a previously unfinished io
 *  scheduler (round robin per-process disk scheduling) and Andrea Arcangeli.
 *
 *  Copyright (C) 2003 Jens Axboe <axboe@kernel.dk>
 */
#include <linux/module.h>
#include <linux/blkdev.h>
#include <linux/elevator.h>
#include <linux/hash.h>
#include <linux/rbtree.h>
#include <linux/ioprio.h>

/*
 * tunables
 */
static const int cfq_quantum = 4;               /* max queue in one round of service */
static const int cfq_fifo_expire[2] = { HZ / 4, HZ / 8 };
static const int cfq_back_max = 16 * 1024;      /* maximum backwards seek, in KiB */
static const int cfq_back_penalty = 2;          /* penalty of a backwards seek */

static const int cfq_slice_sync = HZ / 10;
static int cfq_slice_async = HZ / 25;
static const int cfq_slice_async_rq = 2;
static int cfq_slice_idle = HZ / 125;

#define CFQ_IDLE_GRACE          (HZ / 10)
#define CFQ_SLICE_SCALE         (5)

#define CFQ_KEY_ASYNC           (0)

/*
 * for the hash of cfqq inside the cfqd
 */
#define CFQ_QHASH_SHIFT         6
#define CFQ_QHASH_ENTRIES       (1 << CFQ_QHASH_SHIFT)
#define list_entry_qhash(entry) hlist_entry((entry), struct cfq_queue, cfq_hash)

#define list_entry_cfqq(ptr)    list_entry((ptr), struct cfq_queue, cfq_list)

#define RQ_CIC(rq)              ((struct cfq_io_context*)(rq)->elevator_private)
#define RQ_CFQQ(rq)             ((rq)->elevator_private2)

static struct kmem_cache *cfq_pool;
static struct kmem_cache *cfq_ioc_pool;

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

#define CFQ_PRIO_LISTS          IOPRIO_BE_NR
#define cfq_class_idle(cfqq)    ((cfqq)->ioprio_class == IOPRIO_CLASS_IDLE)
#define cfq_class_rt(cfqq)      ((cfqq)->ioprio_class == IOPRIO_CLASS_RT)

#define ASYNC                   (0)
#define SYNC                    (1)

#define cfq_cfqq_dispatched(cfqq)       \
        ((cfqq)->on_dispatch[ASYNC] + (cfqq)->on_dispatch[SYNC])

#define cfq_cfqq_class_sync(cfqq)       ((cfqq)->key != CFQ_KEY_ASYNC)

#define cfq_cfqq_sync(cfqq)             \
        (cfq_cfqq_class_sync(cfqq) || (cfqq)->on_dispatch[SYNC])

#define sample_valid(samples)   ((samples) > 80)

/*
 * Per block device queue structure
 */
struct cfq_data {
        request_queue_t *queue;

        /*
         * rr list of queues with requests and the count of them
         */
        struct list_head rr_list[CFQ_PRIO_LISTS];
        struct list_head busy_rr;
        struct list_head cur_rr;
        struct list_head idle_rr;
        unsigned int busy_queues;

        /*
         * cfqq lookup hash
         */
        struct hlist_head *cfq_hash;

        int rq_in_driver;
        int hw_tag;

        /*
         * idle window management
         */
        struct timer_list idle_slice_timer;
        struct work_struct unplug_work;

        struct cfq_queue *active_queue;
        struct cfq_io_context *active_cic;
        int cur_prio, cur_end_prio;
        unsigned int dispatch_slice;

        struct timer_list idle_class_timer;

        sector_t last_sector;
        unsigned long last_end_request;

        /*
         * tunables, see top of file
         */
        unsigned int cfq_quantum;
        unsigned int cfq_fifo_expire[2];
        unsigned int cfq_back_penalty;
        unsigned int cfq_back_max;
        unsigned int cfq_slice[2];
        unsigned int cfq_slice_async_rq;
        unsigned int cfq_slice_idle;

        struct list_head cic_list;
};

/*
 * Per process-grouping structure
 */
struct cfq_queue {
        /* reference count */
        atomic_t ref;
        /* parent cfq_data */
        struct cfq_data *cfqd;
        /* cfqq lookup hash */
        struct hlist_node cfq_hash;
        /* hash key */
        unsigned int key;
        /* member of the rr/busy/cur/idle cfqd list */
        struct list_head cfq_list;
        /* sorted list of pending requests */
        struct rb_root sort_list;
        /* if fifo isn't expired, next request to serve */
        struct request *next_rq;
        /* requests queued in sort_list */
        int queued[2];
        /* currently allocated requests */
        int allocated[2];
        /* pending metadata requests */
        int meta_pending;
        /* fifo list of requests in sort_list */
        struct list_head fifo;

        unsigned long slice_start;
        unsigned long slice_end;
        unsigned long slice_left;

        /* number of requests that are on the dispatch list */
        int on_dispatch[2];

        /* io prio of this group */
        unsigned short ioprio, org_ioprio;
        unsigned short ioprio_class, org_ioprio_class;

        /* various state flags, see below */
        unsigned int flags;
};

enum cfqq_state_flags {
        CFQ_CFQQ_FLAG_on_rr = 0,
        CFQ_CFQQ_FLAG_wait_request,
        CFQ_CFQQ_FLAG_must_alloc,
        CFQ_CFQQ_FLAG_must_alloc_slice,
        CFQ_CFQQ_FLAG_must_dispatch,
        CFQ_CFQQ_FLAG_fifo_expire,
        CFQ_CFQQ_FLAG_idle_window,
        CFQ_CFQQ_FLAG_prio_changed,
        CFQ_CFQQ_FLAG_queue_new,
};

#define CFQ_CFQQ_FNS(name)                                              \
static inline void cfq_mark_cfqq_##name(struct cfq_queue *cfqq)         \
{                                                                       \
        cfqq->flags |= (1 << CFQ_CFQQ_FLAG_##name);                     \
}                                                                       \
static inline void cfq_clear_cfqq_##name(struct cfq_queue *cfqq)        \
{                                                                       \
        cfqq->flags &= ~(1 << CFQ_CFQQ_FLAG_##name);                    \
}                                                                       \
static inline int cfq_cfqq_##name(const struct cfq_queue *cfqq)         \
{                                                                       \
        return (cfqq->flags & (1 << CFQ_CFQQ_FLAG_##name)) != 0;        \
}

CFQ_CFQQ_FNS(on_rr);
CFQ_CFQQ_FNS(wait_request);
CFQ_CFQQ_FNS(must_alloc);
CFQ_CFQQ_FNS(must_alloc_slice);
CFQ_CFQQ_FNS(must_dispatch);
CFQ_CFQQ_FNS(fifo_expire);
CFQ_CFQQ_FNS(idle_window);
CFQ_CFQQ_FNS(prio_changed);
CFQ_CFQQ_FNS(queue_new);
#undef CFQ_CFQQ_FNS

static struct cfq_queue *cfq_find_cfq_hash(struct cfq_data *, unsigned int, unsigned short);
static void cfq_dispatch_insert(request_queue_t *, struct request *);
static struct cfq_queue *cfq_get_queue(struct cfq_data *cfqd, unsigned int key, struct task_struct *tsk, gfp_t gfp_mask);

/*
 * scheduler run of queue, if there are requests pending and no one in the
 * driver that will restart queueing
 */
static inline void cfq_schedule_dispatch(struct cfq_data *cfqd)
{
        if (cfqd->busy_queues)
                kblockd_schedule_work(&cfqd->unplug_work);
}

static int cfq_queue_empty(request_queue_t *q)
{
        struct cfq_data *cfqd = q->elevator->elevator_data;

        return !cfqd->busy_queues;
}

static inline pid_t cfq_queue_pid(struct task_struct *task, int rw, int is_sync)
{
        /*
         * Use the per-process queue, for read requests and syncronous writes
         */
        if (!(rw & REQ_RW) || is_sync)
                return task->pid;

        return CFQ_KEY_ASYNC;
}

/*
 * Lifted from AS - choose which of rq1 and rq2 that is best served now.
 * We choose the request that is closest to the head right now. Distance
 * behind the head is penalized and only allowed to a certain extent.
 */
static struct request *
cfq_choose_req(struct cfq_data *cfqd, struct request *rq1, struct request *rq2)
{
        sector_t last, s1, s2, d1 = 0, d2 = 0;
        unsigned long back_max;
#define CFQ_RQ1_WRAP    0x01 /* request 1 wraps */
#define CFQ_RQ2_WRAP    0x02 /* request 2 wraps */
        unsigned wrap = 0; /* bit mask: requests behind the disk head? */

        if (rq1 == NULL || rq1 == rq2)
                return rq2;
        if (rq2 == NULL)
                return rq1;

        if (rq_is_sync(rq1) && !rq_is_sync(rq2))
                return rq1;
        else if (rq_is_sync(rq2) && !rq_is_sync(rq1))
                return rq2;
        if (rq_is_meta(rq1) && !rq_is_meta(rq2))
                return rq1;
        else if (rq_is_meta(rq2) && !rq_is_meta(rq1))
                return rq2;

        s1 = rq1->sector;
        s2 = rq2->sector;

        last = cfqd->last_sector;

        /*
         * by definition, 1KiB is 2 sectors
         */
        back_max = cfqd->cfq_back_max * 2;

        /*
         * 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 + back_max >= last)
                d1 = (last - s1) * cfqd->cfq_back_penalty;
        else
                wrap |= CFQ_RQ1_WRAP;

        if (s2 >= last)
                d2 = s2 - last;
        else if (s2 + back_max >= last)
                d2 = (last - s2) * cfqd->cfq_back_penalty;
        else
                wrap |= CFQ_RQ2_WRAP;

        /* Found required data */

        /*
         * By doing switch() on the bit mask "wrap" we avoid having to
         * check two variables for all permutations: --> faster!
         */
        switch (wrap) {
        case 0: /* common case for CFQ: rq1 and rq2 not wrapped */
                if (d1 < d2)
                        return rq1;
                else if (d2 < d1)
                        return rq2;
                else {
                        if (s1 >= s2)
                                return rq1;
                        else
                                return rq2;
                }

        case CFQ_RQ2_WRAP:
                return rq1;
        case CFQ_RQ1_WRAP:
                return rq2;
        case (CFQ_RQ1_WRAP|CFQ_RQ2_WRAP): /* both rqs wrapped */
        default:
                /*
                 * Since both rqs are wrapped,
                 * start with the one that's further behind head
                 * (--> only *one* back seek required),
                 * since back seek takes more time than forward.
                 */
                if (s1 <= s2)
                        return rq1;
                else
                        return rq2;
        }
}

/*
 * would be nice to take fifo expire time into account as well
 */
static struct request *
cfq_find_next_rq(struct cfq_data *cfqd, struct cfq_queue *cfqq,
                  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 {
                rbnext = rb_first(&cfqq->sort_list);
                if (rbnext && rbnext != &last->rb_node)
                        next = rb_entry_rq(rbnext);
        }

        return cfq_choose_req(cfqd, next, prev);
}

static void cfq_resort_rr_list(struct cfq_queue *cfqq, int preempted)
{
        struct cfq_data *cfqd = cfqq->cfqd;
        struct list_head *list;

        BUG_ON(!cfq_cfqq_on_rr(cfqq));

        list_del(&cfqq->cfq_list);

        if (cfq_class_rt(cfqq))
                list = &cfqd->cur_rr;
        else if (cfq_class_idle(cfqq))
                list = &cfqd->idle_rr;
        else {
                /*
                 * if cfqq has requests in flight, don't allow it to be
                 * found in cfq_set_active_queue before it has finished them.
                 * this is done to increase fairness between a process that
                 * has lots of io pending vs one that only generates one
                 * sporadically or synchronously
                 */
                if (cfq_cfqq_dispatched(cfqq))
                        list = &cfqd->busy_rr;
                else
                        list = &cfqd->rr_list[cfqq->ioprio];
        }

        /*
         * If this queue was preempted or is new (never been serviced), let
         * it be added first for fairness but beind other new queues.
         * Otherwise, just add to the back  of the list.
         */
        if (preempted || cfq_cfqq_queue_new(cfqq)) {
                struct list_head *n = list;
                struct cfq_queue *__cfqq;

                while (n->next != list) {
                        __cfqq = list_entry_cfqq(n->next);
                        if (!cfq_cfqq_queue_new(__cfqq))
                                break;

                        n = n->next;
                }

                list = n;
        }

        list_add_tail(&cfqq->cfq_list, list);
}

/*
 * add to busy list of queues for service, trying to be fair in ordering
 * the pending list according to last request service
 */
static inline void
cfq_add_cfqq_rr(struct cfq_data *cfqd, struct cfq_queue *cfqq)
{
        BUG_ON(cfq_cfqq_on_rr(cfqq));
        cfq_mark_cfqq_on_rr(cfqq);
        cfqd->busy_queues++;

        cfq_resort_rr_list(cfqq, 0);
}

static inline void
cfq_del_cfqq_rr(struct cfq_data *cfqd, struct cfq_queue *cfqq)
{
        BUG_ON(!cfq_cfqq_on_rr(cfqq));
        cfq_clear_cfqq_on_rr(cfqq);
        list_del_init(&cfqq->cfq_list);

        BUG_ON(!cfqd->busy_queues);
        cfqd->busy_queues--;
}

/*
 * rb tree support functions
 */
static inline void cfq_del_rq_rb(struct request *rq)
{
        struct cfq_queue *cfqq = RQ_CFQQ(rq);
        struct cfq_data *cfqd = cfqq->cfqd;
        const int sync = rq_is_sync(rq);

        BUG_ON(!cfqq->queued[sync]);
        cfqq->queued[sync]--;

        elv_rb_del(&cfqq->sort_list, rq);

        if (cfq_cfqq_on_rr(cfqq) && RB_EMPTY_ROOT(&cfqq->sort_list))
                cfq_del_cfqq_rr(cfqd, cfqq);
}

static void cfq_add_rq_rb(struct request *rq)
{
        struct cfq_queue *cfqq = RQ_CFQQ(rq);
        struct cfq_data *cfqd = cfqq->cfqd;
        struct request *__alias;

        cfqq->queued[rq_is_sync(rq)]++;

        /*
         * looks a little odd, but the first insert might return an alias.
         * if that happens, put the alias on the dispatch list
         */
        while ((__alias = elv_rb_add(&cfqq->sort_list, rq)) != NULL)
                cfq_dispatch_insert(cfqd->queue, __alias);

        if (!cfq_cfqq_on_rr(cfqq))
                cfq_add_cfqq_rr(cfqd, cfqq);
}

static inline void
cfq_reposition_rq_rb(struct cfq_queue *cfqq, struct request *rq)
{
        elv_rb_del(&cfqq->sort_list, rq);
        cfqq->queued[rq_is_sync(rq)]--;
        cfq_add_rq_rb(rq);
}

static struct request *
cfq_find_rq_fmerge(struct cfq_data *cfqd, struct bio *bio)
{
        struct task_struct *tsk = current;
        pid_t key = cfq_queue_pid(tsk, bio_data_dir(bio), bio_sync(bio));
        struct cfq_queue *cfqq;

        cfqq = cfq_find_cfq_hash(cfqd, key, tsk->ioprio);
        if (cfqq) {
                sector_t sector = bio->bi_sector + bio_sectors(bio);

                return elv_rb_find(&cfqq->sort_list, sector);
        }

        return NULL;
}

static void cfq_activate_request(request_queue_t *q, struct request *rq)
{
        struct cfq_data *cfqd = q->elevator->elevator_data;

        cfqd->rq_in_driver++;

        /*
         * If the depth is larger 1, it really could be queueing. But lets
         * make the mark a little higher - idling could still be good for
         * low queueing, and a low queueing number could also just indicate
         * a SCSI mid layer like behaviour where limit+1 is often seen.
         */
        if (!cfqd->hw_tag && cfqd->rq_in_driver > 4)
                cfqd->hw_tag = 1;
}

static void cfq_deactivate_request(request_queue_t *q, struct request *rq)
{
        struct cfq_data *cfqd = q->elevator->elevator_data;

        WARN_ON(!cfqd->rq_in_driver);
        cfqd->rq_in_driver--;
}

static void cfq_remove_request(struct request *rq)
{
        struct cfq_queue *cfqq = RQ_CFQQ(rq);

        if (cfqq->next_rq == rq)
                cfqq->next_rq = cfq_find_next_rq(cfqq->cfqd, cfqq, rq);

        list_del_init(&rq->queuelist);
        cfq_del_rq_rb(rq);

        if (rq_is_meta(rq)) {
                WARN_ON(!cfqq->meta_pending);
                cfqq->meta_pending--;
        }
}

static int
cfq_merge(request_queue_t *q, struct request **req, struct bio *bio)
{
        struct cfq_data *cfqd = q->elevator->elevator_data;
        struct request *__rq;

        __rq = cfq_find_rq_fmerge(cfqd, bio);
        if (__rq && elv_rq_merge_ok(__rq, bio)) {
                *req = __rq;
                return ELEVATOR_FRONT_MERGE;
        }

        return ELEVATOR_NO_MERGE;
}

static void cfq_merged_request(request_queue_t *q, struct request *req,
                               int type)
{
        if (type == ELEVATOR_FRONT_MERGE) {
                struct cfq_queue *cfqq = RQ_CFQQ(req);

                cfq_reposition_rq_rb(cfqq, req);
        }
}

static void
cfq_merged_requests(request_queue_t *q, struct request *rq,
                    struct request *next)
{
        /*
         * reposition in fifo if next is older than rq
         */
        if (!list_empty(&rq->queuelist) && !list_empty(&next->queuelist) &&
            time_before(next->start_time, rq->start_time))
                list_move(&rq->queuelist, &next->queuelist);

        cfq_remove_request(next);
}

static int cfq_allow_merge(request_queue_t *q, struct request *rq,
                           struct bio *bio)
{
        struct cfq_data *cfqd = q->elevator->elevator_data;
        const int rw = bio_data_dir(bio);
        struct cfq_queue *cfqq;
        pid_t key;

        /*
         * Disallow merge of a sync bio into an async request.
         */
        if ((bio_data_dir(bio) == READ || bio_sync(bio)) && !rq_is_sync(rq))
                return 0;

        /*
         * Lookup the cfqq that this bio will be queued with. Allow
         * merge only if rq is queued there.
         */
        key = cfq_queue_pid(current, rw, bio_sync(bio));
        cfqq = cfq_find_cfq_hash(cfqd, key, current->ioprio);

        if (cfqq == RQ_CFQQ(rq))
                return 1;

        return 0;
}

static inline void
__cfq_set_active_queue(struct cfq_data *cfqd, struct cfq_queue *cfqq)
{
        if (cfqq) {
                /*
                 * stop potential idle class queues waiting service
                 */
                del_timer(&cfqd->idle_class_timer);

                cfqq->slice_start = jiffies;
                cfqq->slice_end = 0;
                cfqq->slice_left = 0;
                cfq_clear_cfqq_must_alloc_slice(cfqq);
                cfq_clear_cfqq_fifo_expire(cfqq);
        }

        cfqd->active_queue = cfqq;
}

/*
 * current cfqq expired its slice (or was too idle), select new one
 */
static void
__cfq_slice_expired(struct cfq_data *cfqd, struct cfq_queue *cfqq,
                    int preempted)
{
        unsigned long now = jiffies;

        if (cfq_cfqq_wait_request(cfqq))
                del_timer(&cfqd->idle_slice_timer);

        if (!preempted && !cfq_cfqq_dispatched(cfqq))
                cfq_schedule_dispatch(cfqd);

        cfq_clear_cfqq_must_dispatch(cfqq);
        cfq_clear_cfqq_wait_request(cfqq);
        cfq_clear_cfqq_queue_new(cfqq);

        /*
         * store what was left of this slice, if the queue idled out
         * or was preempted
         */
        if (time_after(cfqq->slice_end, now))
                cfqq->slice_left = cfqq->slice_end - now;
        else
                cfqq->slice_left = 0;

        if (cfq_cfqq_on_rr(cfqq))
                cfq_resort_rr_list(cfqq, preempted);

        if (cfqq == cfqd->active_queue)
                cfqd->active_queue = NULL;

        if (cfqd->active_cic) {
                put_io_context(cfqd->active_cic->ioc);
                cfqd->active_cic = NULL;
        }

        cfqd->dispatch_slice = 0;
}

static inline void cfq_slice_expired(struct cfq_data *cfqd, int preempted)
{
        struct cfq_queue *cfqq = cfqd->active_queue;

        if (cfqq)
                __cfq_slice_expired(cfqd, cfqq, preempted);
}

/*
 * 0
 * 0,1
 * 0,1,2
 * 0,1,2,3
 * 0,1,2,3,4
 * 0,1,2,3,4,5
 * 0,1,2,3,4,5,6
 * 0,1,2,3,4,5,6,7
 */
static int cfq_get_next_prio_level(struct cfq_data *cfqd)
{
        int prio, wrap;

        prio = -1;
        wrap = 0;
        do {
                int p;

                for (p = cfqd->cur_prio; p <= cfqd->cur_end_prio; p++) {
                        if (!list_empty(&cfqd->rr_list[p])) {
                                prio = p;
                                break;
                        }
                }

                if (prio != -1)
                        break;
                cfqd->cur_prio = 0;
                if (++cfqd->cur_end_prio == CFQ_PRIO_LISTS) {
                        cfqd->cur_end_prio = 0;
                        if (wrap)
                                break;
                        wrap = 1;
                }
        } while (1);

        if (unlikely(prio == -1))
                return -1;

        BUG_ON(prio >= CFQ_PRIO_LISTS);

        list_splice_init(&cfqd->rr_list[prio], &cfqd->cur_rr);

        cfqd->cur_prio = prio + 1;
        if (cfqd->cur_prio > cfqd->cur_end_prio) {
                cfqd->cur_end_prio = cfqd->cur_prio;
                cfqd->cur_prio = 0;
        }
        if (cfqd->cur_end_prio == CFQ_PRIO_LISTS) {
                cfqd->cur_prio = 0;
                cfqd->cur_end_prio = 0;
        }

        return prio;
}

static struct cfq_queue *cfq_set_active_queue(struct cfq_data *cfqd)
{
        struct cfq_queue *cfqq = NULL;

        if (!list_empty(&cfqd->cur_rr) || cfq_get_next_prio_level(cfqd) != -1) {
                /*
                 * if current list is non-empty, grab first entry. if it is
                 * empty, get next prio level and grab first entry then if any
                 * are spliced
                 */
                cfqq = list_entry_cfqq(cfqd->cur_rr.next);
        } else if (!list_empty(&cfqd->busy_rr)) {
                /*
                 * If no new queues are available, check if the busy list has
                 * some before falling back to idle io.
                 */
                cfqq = list_entry_cfqq(cfqd->busy_rr.next);
        } else if (!list_empty(&cfqd->idle_rr)) {
                /*
                 * if we have idle queues and no rt or be queues had pending
                 * requests, either allow immediate service if the grace period
                 * has passed or arm the idle grace timer
                 */
                unsigned long end = cfqd->last_end_request + CFQ_IDLE_GRACE;

                if (time_after_eq(jiffies, end))
                        cfqq = list_entry_cfqq(cfqd->idle_rr.next);
                else
                        mod_timer(&cfqd->idle_class_timer, end);
        }

        __cfq_set_active_queue(cfqd, cfqq);
        return cfqq;
}

#define CIC_SEEKY(cic) ((cic)->seek_mean > (128 * 1024))

static int cfq_arm_slice_timer(struct cfq_data *cfqd, struct cfq_queue *cfqq)

{
        struct cfq_io_context *cic;
        unsigned long sl;

        WARN_ON(!RB_EMPTY_ROOT(&cfqq->sort_list));
        WARN_ON(cfqq != cfqd->active_queue);

        /*
         * idle is disabled, either manually or by past process history
         */
        if (!cfqd->cfq_slice_idle)
                return 0;
        if (!cfq_cfqq_idle_window(cfqq))
                return 0;
        /*
         * task has exited, don't wait
         */
        cic = cfqd->active_cic;
        if (!cic || !cic->ioc->task)
                return 0;

        cfq_mark_cfqq_must_dispatch(cfqq);
        cfq_mark_cfqq_wait_request(cfqq);

        sl = min(cfqq->slice_end - 1, (unsigned long) cfqd->cfq_slice_idle);

        /*
         * we don't want to idle for seeks, but we do want to allow
         * fair distribution of slice time for a process doing back-to-back
         * seeks. so allow a little bit of time for him to submit a new rq
         */
        if (sample_valid(cic->seek_samples) && CIC_SEEKY(cic))
                sl = min(sl, msecs_to_jiffies(2));

        mod_timer(&cfqd->idle_slice_timer, jiffies + sl);
        return 1;
}

static void cfq_dispatch_insert(request_queue_t *q, struct request *rq)
{
        struct cfq_data *cfqd = q->elevator->elevator_data;
        struct cfq_queue *cfqq = RQ_CFQQ(rq);

        cfq_remove_request(rq);
        cfqq->on_dispatch[rq_is_sync(rq)]++;
        elv_dispatch_sort(q, rq);

        rq = list_entry(q->queue_head.prev, struct request, queuelist);
        cfqd->last_sector = rq->sector + rq->nr_sectors;
}

/*
 * return expired entry, or NULL to just start from scratch in rbtree
 */
static inline struct request *cfq_check_fifo(struct cfq_queue *cfqq)
{
        struct cfq_data *cfqd = cfqq->cfqd;
        struct request *rq;
        int fifo;

        if (cfq_cfqq_fifo_expire(cfqq))
                return NULL;
        if (list_empty(&cfqq->fifo))
                return NULL;

        fifo = cfq_cfqq_class_sync(cfqq);
        rq = rq_entry_fifo(cfqq->fifo.next);

        if (time_after(jiffies, rq->start_time + cfqd->cfq_fifo_expire[fifo])) {
                cfq_mark_cfqq_fifo_expire(cfqq);
                return rq;
        }

        return NULL;
}

/*
 * Scale schedule slice based on io priority. Use the sync time slice only
 * if a queue is marked sync and has sync io queued. A sync queue with async
 * io only, should not get full sync slice length.
 */
static inline int
cfq_prio_to_slice(struct cfq_data *cfqd, struct cfq_queue *cfqq)
{
        const int base_slice = cfqd->cfq_slice[cfq_cfqq_sync(cfqq)];

        WARN_ON(cfqq->ioprio >= IOPRIO_BE_NR);

        return base_slice + (base_slice/CFQ_SLICE_SCALE * (4 - cfqq->ioprio));
}

static inline void
cfq_set_prio_slice(struct cfq_data *cfqd, struct cfq_queue *cfqq)
{
        cfqq->slice_end = cfq_prio_to_slice(cfqd, cfqq) + jiffies;
}

static inline int
cfq_prio_to_maxrq(struct cfq_data *cfqd, struct cfq_queue *cfqq)
{
        const int base_rq = cfqd->cfq_slice_async_rq;

        WARN_ON(cfqq->ioprio >= IOPRIO_BE_NR);

        return 2 * (base_rq + base_rq * (CFQ_PRIO_LISTS - 1 - cfqq->ioprio));
}

/*
 * get next queue for service
 */
static struct cfq_queue *cfq_select_queue(struct cfq_data *cfqd)
{
        unsigned long now = jiffies;
        struct cfq_queue *cfqq;

        cfqq = cfqd->active_queue;
        if (!cfqq)
                goto new_queue;

        /*
         * slice has expired
         */
        if (!cfq_cfqq_must_dispatch(cfqq) && time_after(now, cfqq->slice_end))
                goto expire;

        /*
         * if queue has requests, dispatch one. if not, check if
         * enough slice is left to wait for one
         */
        if (!RB_EMPTY_ROOT(&cfqq->sort_list))
                goto keep_queue;
        else if (cfq_cfqq_dispatched(cfqq)) {
                cfqq = NULL;
                goto keep_queue;
        } else if (cfq_cfqq_class_sync(cfqq)) {
                if (cfq_arm_slice_timer(cfqd, cfqq))
                        return NULL;
        }

expire:
        cfq_slice_expired(cfqd, 0);
new_queue:
        cfqq = cfq_set_active_queue(cfqd);
keep_queue:
        return cfqq;
}

static int
__cfq_dispatch_requests(struct cfq_data *cfqd, struct cfq_queue *cfqq,
                        int max_dispatch)
{
        int dispatched = 0;

        BUG_ON(RB_EMPTY_ROOT(&cfqq->sort_list));

        do {
                struct request *rq;

                /*
                 * follow expired path, else get first next available
                 */
                if ((rq = cfq_check_fifo(cfqq)) == NULL)
                        rq = cfqq->next_rq;

                /*
                 * finally, insert request into driver dispatch list
                 */
                cfq_dispatch_insert(cfqd->queue, rq);

                cfqd->dispatch_slice++;
                dispatched++;

                if (!cfqd->active_cic) {
                        atomic_inc(&RQ_CIC(rq)->ioc->refcount);
                        cfqd->active_cic = RQ_CIC(rq);
                }

                if (RB_EMPTY_ROOT(&cfqq->sort_list))
                        break;

        } while (dispatched < max_dispatch);

        /*
         * if slice end isn't set yet, set it.
         */
        if (!cfqq->slice_end)
                cfq_set_prio_slice(cfqd, cfqq);

        /*
         * expire an async queue immediately if it has used up its slice. idle
         * queue always expire after 1 dispatch round.
         */
        if ((!cfq_cfqq_sync(cfqq) &&
            cfqd->dispatch_slice >= cfq_prio_to_maxrq(cfqd, cfqq)) ||
            cfq_class_idle(cfqq) ||
            !cfq_cfqq_idle_window(cfqq))
                cfq_slice_expired(cfqd, 0);

        return dispatched;
}

static int
cfq_forced_dispatch_cfqqs(struct list_head *list)
{
        struct cfq_queue *cfqq, *next;
        int dispatched;

        dispatched = 0;
        list_for_each_entry_safe(cfqq, next, list, cfq_list) {
                while (cfqq->next_rq) {
                        cfq_dispatch_insert(cfqq->cfqd->queue, cfqq->next_rq);
                        dispatched++;
                }
                BUG_ON(!list_empty(&cfqq->fifo));
        }

        return dispatched;
}

static int
cfq_forced_dispatch(struct cfq_data *cfqd)
{
        int i, dispatched = 0;

        for (i = 0; i < CFQ_PRIO_LISTS; i++)
                dispatched += cfq_forced_dispatch_cfqqs(&cfqd->rr_list[i]);

        dispatched += cfq_forced_dispatch_cfqqs(&cfqd->busy_rr);
        dispatched += cfq_forced_dispatch_cfqqs(&cfqd->cur_rr);
        dispatched += cfq_forced_dispatch_cfqqs(&cfqd->idle_rr);

        cfq_slice_expired(cfqd, 0);

        BUG_ON(cfqd->busy_queues);

        return dispatched;
}

static int
cfq_dispatch_requests(request_queue_t *q, int force)
{
        struct cfq_data *cfqd = q->elevator->elevator_data;
        struct cfq_queue *cfqq, *prev_cfqq;
        int dispatched;

        if (!cfqd->busy_queues)
                return 0;

        if (unlikely(force))
                return cfq_forced_dispatch(cfqd);

        dispatched = 0;
        prev_cfqq = NULL;
        while ((cfqq = cfq_select_queue(cfqd)) != NULL) {
                int max_dispatch;

                /*
                 * Don't repeat dispatch from the previous queue.
                 */
                if (prev_cfqq == cfqq)
                        break;

                cfq_clear_cfqq_must_dispatch(cfqq);
                cfq_clear_cfqq_wait_request(cfqq);
                del_timer(&cfqd->idle_slice_timer);

                max_dispatch = cfqd->cfq_quantum;
                if (cfq_class_idle(cfqq))
                        max_dispatch = 1;

                dispatched += __cfq_dispatch_requests(cfqd, cfqq, max_dispatch);

                /*
                 * If the dispatch cfqq has idling enabled and is still
                 * the active queue, break out.
                 */
                if (cfq_cfqq_idle_window(cfqq) && cfqd->active_queue)
                        break;

                prev_cfqq = cfqq;
        }

        return dispatched;
}

/*
 * task holds one reference to the queue, dropped when task exits. each rq
 * in-flight on this queue also holds a reference, dropped when rq is freed.
 *
 * queue lock must be held here.
 */
static void cfq_put_queue(struct cfq_queue *cfqq)
{
        struct cfq_data *cfqd = cfqq->cfqd;

        BUG_ON(atomic_read(&cfqq->ref) <= 0);

        if (!atomic_dec_and_test(&cfqq->ref))
                return;

        BUG_ON(rb_first(&cfqq->sort_list));
        BUG_ON(cfqq->allocated[READ] + cfqq->allocated[WRITE]);
        BUG_ON(cfq_cfqq_on_rr(cfqq));

        if (unlikely(cfqd->active_queue == cfqq))
                __cfq_slice_expired(cfqd, cfqq, 0);

        /*
         * it's on the empty list and still hashed
         */
        list_del(&cfqq->cfq_list);
        hlist_del(&cfqq->cfq_hash);
        kmem_cache_free(cfq_pool, cfqq);
}

static struct cfq_queue *
__cfq_find_cfq_hash(struct cfq_data *cfqd, unsigned int key, unsigned int prio,
                    const int hashval)
{
        struct hlist_head *hash_list = &cfqd->cfq_hash[hashval];
        struct hlist_node *entry;
        struct cfq_queue *__cfqq;

        hlist_for_each_entry(__cfqq, entry, hash_list, cfq_hash) {
                const unsigned short __p = IOPRIO_PRIO_VALUE(__cfqq->org_ioprio_class, __cfqq->org_ioprio);

                if (__cfqq->key == key && (__p == prio || !prio))
                        return __cfqq;
        }

        return NULL;
}

static struct cfq_queue *
cfq_find_cfq_hash(struct cfq_data *cfqd, unsigned int key, unsigned short prio)
{
        return __cfq_find_cfq_hash(cfqd, key, prio, hash_long(key, CFQ_QHASH_SHIFT));
}

static void cfq_free_io_context(struct io_context *ioc)
{
        struct cfq_io_context *__cic;
        struct rb_node *n;
        int freed = 0;

        while ((n = rb_first(&ioc->cic_root)) != NULL) {
                __cic = rb_entry(n, struct cfq_io_context, rb_node);
                rb_erase(&__cic->rb_node, &ioc->cic_root);
                kmem_cache_free(cfq_ioc_pool, __cic);
                freed++;
        }

        elv_ioc_count_mod(ioc_count, -freed);

        if (ioc_gone && !elv_ioc_count_read(ioc_count))
                complete(ioc_gone);
}

static void cfq_exit_cfqq(struct cfq_data *cfqd, struct cfq_queue *cfqq)
{
        if (unlikely(cfqq == cfqd->active_queue))
                __cfq_slice_expired(cfqd, cfqq, 0);

        cfq_put_queue(cfqq);
}

static void __cfq_exit_single_io_context(struct cfq_data *cfqd,
                                         struct cfq_io_context *cic)
{
        list_del_init(&cic->queue_list);
        smp_wmb();
        cic->key = NULL;

        if (cic->cfqq[ASYNC]) {
                cfq_exit_cfqq(cfqd, cic->cfqq[ASYNC]);
                cic->cfqq[ASYNC] = NULL;
        }

        if (cic->cfqq[SYNC]) {
                cfq_exit_cfqq(cfqd, cic->cfqq[SYNC]);
                cic->cfqq[SYNC] = NULL;
        }
}


/*
 * Called with interrupts disabled
 */
static void cfq_exit_single_io_context(struct cfq_io_context *cic)
{
        struct cfq_data *cfqd = cic->key;

        if (cfqd) {
                request_queue_t *q = cfqd->queue;

                spin_lock_irq(q->queue_lock);
                __cfq_exit_single_io_context(cfqd, cic);
                spin_unlock_irq(q->queue_lock);
        }
}

static void cfq_exit_io_context(struct io_context *ioc)
{
        struct cfq_io_context *__cic;
        struct rb_node *n;

        /*
         * put the reference this task is holding to the various queues
         */

        n = rb_first(&ioc->cic_root);
        while (n != NULL) {
                __cic = rb_entry(n, struct cfq_io_context, rb_node);

                cfq_exit_single_io_context(__cic);
                n = rb_next(n);
        }
}

static struct cfq_io_context *
cfq_alloc_io_context(struct cfq_data *cfqd, gfp_t gfp_mask)
{
        struct cfq_io_context *cic;

        cic = kmem_cache_alloc_node(cfq_ioc_pool, gfp_mask, cfqd->queue->node);
        if (cic) {
                memset(cic, 0, sizeof(*cic));
                cic->last_end_request = jiffies;
                INIT_LIST_HEAD(&cic->queue_list);
                cic->dtor = cfq_free_io_context;
                cic->exit = cfq_exit_io_context;
                elv_ioc_count_inc(ioc_count);
        }

        return cic;
}

static void cfq_init_prio_data(struct cfq_queue *cfqq)
{
        struct task_struct *tsk = current;
        int ioprio_class;

        if (!cfq_cfqq_prio_changed(cfqq))
                return;

        ioprio_class = IOPRIO_PRIO_CLASS(tsk->ioprio);
        switch (ioprio_class) {
                default:
                        printk(KERN_ERR "cfq: bad prio %x\n", ioprio_class);
                case IOPRIO_CLASS_NONE:
                        /*
                         * no prio set, place us in the middle of the BE classes
                         */
                        cfqq->ioprio = task_nice_ioprio(tsk);
                        cfqq->ioprio_class = IOPRIO_CLASS_BE;
                        break;
                case IOPRIO_CLASS_RT:
                        cfqq->ioprio = task_ioprio(tsk);
                        cfqq->ioprio_class = IOPRIO_CLASS_RT;
                        break;
                case IOPRIO_CLASS_BE:
                        cfqq->ioprio = task_ioprio(tsk);
                        cfqq->ioprio_class = IOPRIO_CLASS_BE;
                        break;
                case IOPRIO_CLASS_IDLE:
                        cfqq->ioprio_class = IOPRIO_CLASS_IDLE;
                        cfqq->ioprio = 7;
                        cfq_clear_cfqq_idle_window(cfqq);
                        break;
        }

        /*
         * keep track of original prio settings in case we have to temporarily
         * elevate the priority of this queue
         */
        cfqq->org_ioprio = cfqq->ioprio;
        cfqq->org_ioprio_class = cfqq->ioprio_class;

        if (cfq_cfqq_on_rr(cfqq))
                cfq_resort_rr_list(cfqq, 0);

        cfq_clear_cfqq_prio_changed(cfqq);
}

static inline void changed_ioprio(struct cfq_io_context *cic)
{
        struct cfq_data *cfqd = cic->key;
        struct cfq_queue *cfqq;
        unsigned long flags;

        if (unlikely(!cfqd))
                return;

        spin_lock_irqsave(cfqd->queue->queue_lock, flags);

        cfqq = cic->cfqq[ASYNC];
        if (cfqq) {
                struct cfq_queue *new_cfqq;
                new_cfqq = cfq_get_queue(cfqd, CFQ_KEY_ASYNC, cic->ioc->task,
                                         GFP_ATOMIC);
                if (new_cfqq) {
                        cic->cfqq[ASYNC] = new_cfqq;
                        cfq_put_queue(cfqq);
                }
        }

        cfqq = cic->cfqq[SYNC];
        if (cfqq)
                cfq_mark_cfqq_prio_changed(cfqq);

        spin_unlock_irqrestore(cfqd->queue->queue_lock, flags);
}

static void cfq_ioc_set_ioprio(struct io_context *ioc)
{
        struct cfq_io_context *cic;
        struct rb_node *n;

        ioc->ioprio_changed = 0;

        n = rb_first(&ioc->cic_root);
        while (n != NULL) {
                cic = rb_entry(n, struct cfq_io_context, rb_node);

                changed_ioprio(cic);
                n = rb_next(n);
        }
}

static struct cfq_queue *
cfq_get_queue(struct cfq_data *cfqd, unsigned int key, struct task_struct *tsk,
              gfp_t gfp_mask)
{
        const int hashval = hash_long(key, CFQ_QHASH_SHIFT);
        struct cfq_queue *cfqq, *new_cfqq = NULL;
        unsigned short ioprio;

retry:
        ioprio = tsk->ioprio;
        cfqq = __cfq_find_cfq_hash(cfqd, key, ioprio, hashval);

        if (!cfqq) {
                if (new_cfqq) {
                        cfqq = new_cfqq;
                        new_cfqq = NULL;
                } else if (gfp_mask & __GFP_WAIT) {
                        /*
                         * Inform the allocator of the fact that we will
                         * just repeat this allocation if it fails, to allow
                         * the allocator to do whatever it needs to attempt to
                         * free memory.
                         */
                        spin_unlock_irq(cfqd->queue->queue_lock);
                        new_cfqq = kmem_cache_alloc_node(cfq_pool, gfp_mask|__GFP_NOFAIL, cfqd->queue->node);
                        spin_lock_irq(cfqd->queue->queue_lock);
                        goto retry;
                } else {
                        cfqq = kmem_cache_alloc_node(cfq_pool, gfp_mask, cfqd->queue->node);
                        if (!cfqq)
                                goto out;
                }

                memset(cfqq, 0, sizeof(*cfqq));

                INIT_HLIST_NODE(&cfqq->cfq_hash);
                INIT_LIST_HEAD(&cfqq->cfq_list);
                INIT_LIST_HEAD(&cfqq->fifo);

                cfqq->key = key;
                hlist_add_head(&cfqq->cfq_hash, &cfqd->cfq_hash[hashval]);
                atomic_set(&cfqq->ref, 0);
                cfqq->cfqd = cfqd;
                /*
                 * set ->slice_left to allow preemption for a new process
                 */
                cfqq->slice_left = 2 * cfqd->cfq_slice_idle;
                cfq_mark_cfqq_idle_window(cfqq);
                cfq_mark_cfqq_prio_changed(cfqq);
                cfq_mark_cfqq_queue_new(cfqq);
                cfq_init_prio_data(cfqq);
        }

        if (new_cfqq)
                kmem_cache_free(cfq_pool, new_cfqq);

        atomic_inc(&cfqq->ref);
out:
        WARN_ON((gfp_mask & __GFP_WAIT) && !cfqq);
        return cfqq;
}

static void
cfq_drop_dead_cic(struct io_context *ioc, struct cfq_io_context *cic)
{
        WARN_ON(!list_empty(&cic->queue_list));
        rb_erase(&cic->rb_node, &ioc->cic_root);
        kmem_cache_free(cfq_ioc_pool, cic);
        elv_ioc_count_dec(ioc_count);
}

static struct cfq_io_context *
cfq_cic_rb_lookup(struct cfq_data *cfqd, struct io_context *ioc)
{
        struct rb_node *n;
        struct cfq_io_context *cic;
        void *k, *key = cfqd;

restart:
        n = ioc->cic_root.rb_node;
        while (n) {
                cic = rb_entry(n, struct cfq_io_context, rb_node);
                /* ->key must be copied to avoid race with cfq_exit_queue() */
                k = cic->key;
                if (unlikely(!k)) {
                        cfq_drop_dead_cic(ioc, cic);
                        goto restart;
                }

                if (key < k)
                        n = n->rb_left;
                else if (key > k)
                        n = n->rb_right;
                else
                        return cic;
        }

        return NULL;
}

static inline void
cfq_cic_link(struct cfq_data *cfqd, struct io_context *ioc,
             struct cfq_io_context *cic)
{
        struct rb_node **p;
        struct rb_node *parent;
        struct cfq_io_context *__cic;
        unsigned long flags;
        void *k;

        cic->ioc = ioc;
        cic->key = cfqd;

restart:
        parent = NULL;
        p = &ioc->cic_root.rb_node;
        while (*p) {
                parent = *p;
                __cic = rb_entry(parent, struct cfq_io_context, rb_node);
                /* ->key must be copied to avoid race with cfq_exit_queue() */
                k = __cic->key;
                if (unlikely(!k)) {
                        cfq_drop_dead_cic(ioc, __cic);
                        goto restart;
                }

                if (cic->key < k)
                        p = &(*p)->rb_left;
                else if (cic->key > k)
                        p = &(*p)->rb_right;
                else
                        BUG();
        }

        rb_link_node(&cic->rb_node, parent, p);
        rb_insert_color(&cic->rb_node, &ioc->cic_root);

        spin_lock_irqsave(cfqd->queue->queue_lock, flags);
        list_add(&cic->queue_list, &cfqd->cic_list);
        spin_unlock_irqrestore(cfqd->queue->queue_lock, flags);
}

/*
 * Setup general io context and cfq io context. There can be several cfq
 * io contexts per general io context, if this process is doing io to more
 * than one device managed by cfq.
 */
static struct cfq_io_context *
cfq_get_io_context(struct cfq_data *cfqd, gfp_t gfp_mask)
{
        struct io_context *ioc = NULL;
        struct cfq_io_context *cic;

        might_sleep_if(gfp_mask & __GFP_WAIT);

        ioc = get_io_context(gfp_mask, cfqd->queue->node);
        if (!ioc)
                return NULL;

        cic = cfq_cic_rb_lookup(cfqd, ioc);
        if (cic)
                goto out;

        cic = cfq_alloc_io_context(cfqd, gfp_mask);
        if (cic == NULL)
                goto err;

        cfq_cic_link(cfqd, ioc, cic);
out:
        smp_read_barrier_depends();
        if (unlikely(ioc->ioprio_changed))
                cfq_ioc_set_ioprio(ioc);

        return cic;
err:
        put_io_context(ioc);
        return NULL;
}

static void
cfq_update_io_thinktime(struct cfq_data *cfqd, struct cfq_io_context *cic)
{
        unsigned long elapsed, ttime;

        /*
         * if this context already has stuff queued, thinktime is from
         * last queue not last end
         */
#if 0
        if (time_after(cic->last_end_request, cic->last_queue))
                elapsed = jiffies - cic->last_end_request;
        else
                elapsed = jiffies - cic->last_queue;
#else
                elapsed = jiffies - cic->last_end_request;
#endif

        ttime = min(elapsed, 2UL * cfqd->cfq_slice_idle);

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

static void
cfq_update_io_seektime(struct cfq_io_context *cic, struct request *rq)
{
        sector_t sdist;
        u64 total;

        if (cic->last_request_pos < rq->sector)
                sdist = rq->sector - cic->last_request_pos;
        else
                sdist = cic->last_request_pos - rq->sector;

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

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

/*
 * Disable idle window if the process thinks too long or seeks so much that
 * it doesn't matter
 */
static void
cfq_update_idle_window(struct cfq_data *cfqd, struct cfq_queue *cfqq,
                       struct cfq_io_context *cic)
{
        int enable_idle = cfq_cfqq_idle_window(cfqq);

        if (!cic->ioc->task || !cfqd->cfq_slice_idle ||
            (cfqd->hw_tag && CIC_SEEKY(cic)))
                enable_idle = 0;
        else if (sample_valid(cic->ttime_samples)) {
                if (cic->ttime_mean > cfqd->cfq_slice_idle)
                        enable_idle = 0;
                else
                        enable_idle = 1;
        }

        if (enable_idle)
                cfq_mark_cfqq_idle_window(cfqq);
        else
                cfq_clear_cfqq_idle_window(cfqq);
}


/*
 * Check if new_cfqq should preempt the currently active queue. Return 0 for
 * no or if we aren't sure, a 1 will cause a preempt.
 */
static int
cfq_should_preempt(struct cfq_data *cfqd, struct cfq_queue *new_cfqq,
                   struct request *rq)
{
        struct cfq_queue *cfqq = cfqd->active_queue;

        if (cfq_class_idle(new_cfqq))
                return 0;

        if (!cfqq)
                return 0;

        if (cfq_class_idle(cfqq))
                return 1;
        if (!cfq_cfqq_wait_request(new_cfqq))
                return 0;
        /*
         * if it doesn't have slice left, forget it
         */
        if (new_cfqq->slice_left < cfqd->cfq_slice_idle)
                return 0;
        /*
         * if the new request is sync, but the currently running queue is
         * not, let the sync request have priority.
         */
        if (rq_is_sync(rq) && !cfq_cfqq_sync(cfqq))
                return 1;
        /*
         * So both queues are sync. Let the new request get disk time if
         * it's a metadata request and the current queue is doing regular IO.
         */
        if (rq_is_meta(rq) && !cfqq->meta_pending)
                return 1;

        return 0;
}

/*
 * cfqq preempts the active queue. if we allowed preempt with no slice left,
 * let it have half of its nominal slice.
 */
static void cfq_preempt_queue(struct cfq_data *cfqd, struct cfq_queue *cfqq)
{
        cfq_slice_expired(cfqd, 1);

        if (!cfqq->slice_left)
                cfqq->slice_left = cfq_prio_to_slice(cfqd, cfqq) / 2;

        /*
         * Put the new queue at the front of the of the current list,
         * so we know that it will be selected next.
         */
        BUG_ON(!cfq_cfqq_on_rr(cfqq));
        list_move(&cfqq->cfq_list, &cfqd->cur_rr);

        cfqq->slice_end = cfqq->slice_left + jiffies;
}

/*
 * Called when a new fs request (rq) is added (to cfqq). Check if there's
 * something we should do about it
 */
static void
cfq_rq_enqueued(struct cfq_data *cfqd, struct cfq_queue *cfqq,
                struct request *rq)
{
        struct cfq_io_context *cic = RQ_CIC(rq);

        if (rq_is_meta(rq))
                cfqq->meta_pending++;

        /*
         * check if this request is a better next-serve candidate)) {
         */
        cfqq->next_rq = cfq_choose_req(cfqd, cfqq->next_rq, rq);
        BUG_ON(!cfqq->next_rq);

        /*
         * we never wait for an async request and we don't allow preemption
         * of an async request. so just return early
         */
        if (!rq_is_sync(rq)) {
                /*
                 * sync process issued an async request, if it's waiting
                 * then expire it and kick rq handling.
                 */
                if (cic == cfqd->active_cic &&
                    del_timer(&cfqd->idle_slice_timer)) {
                        cfq_slice_expired(cfqd, 0);
                        blk_start_queueing(cfqd->queue);
                }
                return;
        }

        cfq_update_io_thinktime(cfqd, cic);
        cfq_update_io_seektime(cic, rq);
        cfq_update_idle_window(cfqd, cfqq, cic);

        cic->last_queue = jiffies;
        cic->last_request_pos = rq->sector + rq->nr_sectors;

        if (cfqq == cfqd->active_queue) {
                /*
                 * if we are waiting for a request for this queue, let it rip
                 * immediately and flag that we must not expire this queue
                 * just now
                 */
                if (cfq_cfqq_wait_request(cfqq)) {
                        cfq_mark_cfqq_must_dispatch(cfqq);
                        del_timer(&cfqd->idle_slice_timer);
                        blk_start_queueing(cfqd->queue);
                }
        } else if (cfq_should_preempt(cfqd, cfqq, rq)) {
                /*
                 * not the active queue - expire current slice if it is
                 * idle and has expired it's mean thinktime or this new queue
                 * has some old slice time left and is of higher priority
                 */
                cfq_preempt_queue(cfqd, cfqq);
                cfq_mark_cfqq_must_dispatch(cfqq);
                blk_start_queueing(cfqd->queue);
        }
}

static void cfq_insert_request(request_queue_t *q, struct request *rq)
{
        struct cfq_data *cfqd = q->elevator->elevator_data;
        struct cfq_queue *cfqq = RQ_CFQQ(rq);

        cfq_init_prio_data(cfqq);

        cfq_add_rq_rb(rq);

        list_add_tail(&rq->queuelist, &cfqq->fifo);

        cfq_rq_enqueued(cfqd, cfqq, rq);
}

static void cfq_completed_request(request_queue_t *q, struct request *rq)
{
        struct cfq_queue *cfqq = RQ_CFQQ(rq);
        struct cfq_data *cfqd = cfqq->cfqd;
        const int sync = rq_is_sync(rq);
        unsigned long now;

        now = jiffies;

        WARN_ON(!cfqd->rq_in_driver);
        WARN_ON(!cfqq->on_dispatch[sync]);
        cfqd->rq_in_driver--;
        cfqq->on_dispatch[sync]--;

        if (!cfq_class_idle(cfqq))
                cfqd->last_end_request = now;

        if (!cfq_cfqq_dispatched(cfqq) && cfq_cfqq_on_rr(cfqq))
                cfq_resort_rr_list(cfqq, 0);

        if (sync)
                RQ_CIC(rq)->last_end_request = now;

        /*
         * If this is the active queue, check if it needs to be expired,
         * or if we want to idle in case it has no pending requests.
         */
        if (cfqd->active_queue == cfqq) {
                if (time_after(now, cfqq->slice_end))
                        cfq_slice_expired(cfqd, 0);
                else if (sync && RB_EMPTY_ROOT(&cfqq->sort_list)) {
                        if (!cfq_arm_slice_timer(cfqd, cfqq))
                                cfq_schedule_dispatch(cfqd);
                }
        }
}

/*
 * we temporarily boost lower priority queues if they are holding fs exclusive
 * resources. they are boosted to normal prio (CLASS_BE/4)
 */
static void cfq_prio_boost(struct cfq_queue *cfqq)
{
        const int ioprio_class = cfqq->ioprio_class;
        const int ioprio = cfqq->ioprio;

        if (has_fs_excl()) {
                /*
                 * boost idle prio on transactions that would lock out other
                 * users of the filesystem
                 */
                if (cfq_class_idle(cfqq))
                        cfqq->ioprio_class = IOPRIO_CLASS_BE;
                if (cfqq->ioprio > IOPRIO_NORM)
                        cfqq->ioprio = IOPRIO_NORM;
        } else {
                /*
                 * check if we need to unboost the queue
                 */
                if (cfqq->ioprio_class != cfqq->org_ioprio_class)
                        cfqq->ioprio_class = cfqq->org_ioprio_class;
                if (cfqq->ioprio != cfqq->org_ioprio)
                        cfqq->ioprio = cfqq->org_ioprio;
        }

        /*
         * refile between round-robin lists if we moved the priority class
         */
        if ((ioprio_class != cfqq->ioprio_class || ioprio != cfqq->ioprio) &&
            cfq_cfqq_on_rr(cfqq))
                cfq_resort_rr_list(cfqq, 0);
}

static inline int __cfq_may_queue(struct cfq_queue *cfqq)
{
        if ((cfq_cfqq_wait_request(cfqq) || cfq_cfqq_must_alloc(cfqq)) &&
            !cfq_cfqq_must_alloc_slice(cfqq)) {
                cfq_mark_cfqq_must_alloc_slice(cfqq);
                return ELV_MQUEUE_MUST;
        }

        return ELV_MQUEUE_MAY;
}

static int cfq_may_queue(request_queue_t *q, int rw)
{
        struct cfq_data *cfqd = q->elevator->elevator_data;
        struct task_struct *tsk = current;
        struct cfq_queue *cfqq;
        unsigned int key;

        key = cfq_queue_pid(tsk, rw, rw & REQ_RW_SYNC);

        /*
         * don't force setup of a queue from here, as a call to may_queue
         * does not necessarily imply that a request actually will be queued.
         * so just lookup a possibly existing queue, or return 'may queue'
         * if that fails
         */
        cfqq = cfq_find_cfq_hash(cfqd, key, tsk->ioprio);
        if (cfqq) {
                cfq_init_prio_data(cfqq);
                cfq_prio_boost(cfqq);

                return __cfq_may_queue(cfqq);
        }

        return ELV_MQUEUE_MAY;
}

/*
 * queue lock held here
 */
static void cfq_put_request(struct request *rq)
{
        struct cfq_queue *cfqq = RQ_CFQQ(rq);

        if (cfqq) {
                const int rw = rq_data_dir(rq);

                BUG_ON(!cfqq->allocated[rw]);
                cfqq->allocated[rw]--;

                put_io_context(RQ_CIC(rq)->ioc);

                rq->elevator_private = NULL;
                rq->elevator_private2 = NULL;

                cfq_put_queue(cfqq);
        }
}

/*
 * Allocate cfq data structures associated with this request.
 */
static int
cfq_set_request(request_queue_t *q, struct request *rq, gfp_t gfp_mask)
{
        struct cfq_data *cfqd = q->elevator->elevator_data;
        struct task_struct *tsk = current;
        struct cfq_io_context *cic;
        const int rw = rq_data_dir(rq);
        const int is_sync = rq_is_sync(rq);
        pid_t key = cfq_queue_pid(tsk, rw, is_sync);
        struct cfq_queue *cfqq;
        unsigned long flags;

        might_sleep_if(gfp_mask & __GFP_WAIT);

        cic = cfq_get_io_context(cfqd, gfp_mask);

        spin_lock_irqsave(q->queue_lock, flags);

        if (!cic)
                goto queue_fail;

        if (!cic->cfqq[is_sync]) {
                cfqq = cfq_get_queue(cfqd, key, tsk, gfp_mask);
                if (!cfqq)
                        goto queue_fail;

                cic->cfqq[is_sync] = cfqq;
        } else
                cfqq = cic->cfqq[is_sync];

        cfqq->allocated[rw]++;
        cfq_clear_cfqq_must_alloc(cfqq);
        atomic_inc(&cfqq->ref);

        spin_unlock_irqrestore(q->queue_lock, flags);

        rq->elevator_private = cic;
        rq->elevator_private2 = cfqq;
        return 0;

queue_fail:
        if (cic)
                put_io_context(cic->ioc);

        cfq_schedule_dispatch(cfqd);
        spin_unlock_irqrestore(q->queue_lock, flags);
        return 1;
}

static void cfq_kick_queue(struct work_struct *work)
{
        struct cfq_data *cfqd =
                container_of(work, struct cfq_data, unplug_work);
        request_queue_t *q = cfqd->queue;
        unsigned long flags;

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

/*
 * Timer running if the active_queue is currently idling inside its time slice
 */
static void cfq_idle_slice_timer(unsigned long data)
{
        struct cfq_data *cfqd = (struct cfq_data *) data;
        struct cfq_queue *cfqq;
        unsigned long flags;

        spin_lock_irqsave(cfqd->queue->queue_lock, flags);

        if ((cfqq = cfqd->active_queue) != NULL) {
                unsigned long now = jiffies;

                /*
                 * expired
                 */
                if (time_after(now, cfqq->slice_end))
                        goto expire;

                /*
                 * only expire and reinvoke request handler, if there are
                 * other queues with pending requests
                 */
                if (!cfqd->busy_queues)
                        goto out_cont;

                /*
                 * not expired and it has a request pending, let it dispatch
                 */
                if (!RB_EMPTY_ROOT(&cfqq->sort_list)) {
                        cfq_mark_cfqq_must_dispatch(cfqq);
                        goto out_kick;
                }
        }
expire:
        cfq_slice_expired(cfqd, 0);
out_kick:
        cfq_schedule_dispatch(cfqd);
out_cont:
        spin_unlock_irqrestore(cfqd->queue->queue_lock, flags);
}

/*
 * Timer running if an idle class queue is waiting for service
 */
static void cfq_idle_class_timer(unsigned long data)
{
        struct cfq_data *cfqd = (struct cfq_data *) data;
        unsigned long flags, end;

        spin_lock_irqsave(cfqd->queue->queue_lock, flags);

        /*
         * race with a non-idle queue, reset timer
         */
        end = cfqd->last_end_request + CFQ_IDLE_GRACE;
        if (!time_after_eq(jiffies, end))
                mod_timer(&cfqd->idle_class_timer, end);
        else
                cfq_schedule_dispatch(cfqd);

        spin_unlock_irqrestore(cfqd->queue->queue_lock, flags);
}

static void cfq_shutdown_timer_wq(struct cfq_data *cfqd)
{
        del_timer_sync(&cfqd->idle_slice_timer);
        del_timer_sync(&cfqd->idle_class_timer);
        blk_sync_queue(cfqd->queue);
}

static void cfq_exit_queue(elevator_t *e)
{
        struct cfq_data *cfqd = e->elevator_data;
        request_queue_t *q = cfqd->queue;

        cfq_shutdown_timer_wq(cfqd);

        spin_lock_irq(q->queue_lock);

        if (cfqd->active_queue)
                __cfq_slice_expired(cfqd, cfqd->active_queue, 0);

        while (!list_empty(&cfqd->cic_list)) {
                struct cfq_io_context *cic = list_entry(cfqd->cic_list.next,
                                                        struct cfq_io_context,
                                                        queue_list);

                __cfq_exit_single_io_context(cfqd, cic);
        }

        spin_unlock_irq(q->queue_lock);

        cfq_shutdown_timer_wq(cfqd);

        kfree(cfqd->cfq_hash);
        kfree(cfqd);
}

static void *cfq_init_queue(request_queue_t *q)
{
        struct cfq_data *cfqd;
        int i;

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

        memset(cfqd, 0, sizeof(*cfqd));

        for (i = 0; i < CFQ_PRIO_LISTS; i++)
                INIT_LIST_HEAD(&cfqd->rr_list[i]);

        INIT_LIST_HEAD(&cfqd->busy_rr);
        INIT_LIST_HEAD(&cfqd->cur_rr);
        INIT_LIST_HEAD(&cfqd->idle_rr);
        INIT_LIST_HEAD(&cfqd->cic_list);

        cfqd->cfq_hash = kmalloc_node(sizeof(struct hlist_head) * CFQ_QHASH_ENTRIES, GFP_KERNEL, q->node);
        if (!cfqd->cfq_hash)
                goto out_free;

        for (i = 0; i < CFQ_QHASH_ENTRIES; i++)
                INIT_HLIST_HEAD(&cfqd->cfq_hash[i]);

        cfqd->queue = q;

        init_timer(&cfqd->idle_slice_timer);
        cfqd->idle_slice_timer.function = cfq_idle_slice_timer;
        cfqd->idle_slice_timer.data = (unsigned long) cfqd;

        init_timer(&cfqd->idle_class_timer);
        cfqd->idle_class_timer.function = cfq_idle_class_timer;
        cfqd->idle_class_timer.data = (unsigned long) cfqd;

        INIT_WORK(&cfqd->unplug_work, cfq_kick_queue);

        cfqd->cfq_quantum = cfq_quantum;
        cfqd->cfq_fifo_expire[0] = cfq_fifo_expire[0];
        cfqd->cfq_fifo_expire[1] = cfq_fifo_expire[1];
        cfqd->cfq_back_max = cfq_back_max;
        cfqd->cfq_back_penalty = cfq_back_penalty;
        cfqd->cfq_slice[0] = cfq_slice_async;
        cfqd->cfq_slice[1] = cfq_slice_sync;
        cfqd->cfq_slice_async_rq = cfq_slice_async_rq;
        cfqd->cfq_slice_idle = cfq_slice_idle;

        return cfqd;
out_free:
        kfree(cfqd);
        return NULL;
}

static void cfq_slab_kill(void)
{
        if (cfq_pool)
                kmem_cache_destroy(cfq_pool);
        if (cfq_ioc_pool)
                kmem_cache_destroy(cfq_ioc_pool);
}

static int __init cfq_slab_setup(void)
{
        cfq_pool = kmem_cache_create("cfq_pool", sizeof(struct cfq_queue), 0, 0,
                                        NULL, NULL);
        if (!cfq_pool)
                goto fail;

        cfq_ioc_pool = kmem_cache_create("cfq_ioc_pool",
                        sizeof(struct cfq_io_context), 0, 0, NULL, NULL);
        if (!cfq_ioc_pool)
                goto fail;

        return 0;
fail:
        cfq_slab_kill();
        return -ENOMEM;
}

/*
 * sysfs parts below -->
 */

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

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

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

#define SHOW_FUNCTION(__FUNC, __VAR, __CONV)                            \
static ssize_t __FUNC(elevator_t *e, char *page)                        \
{                                                                       \
        struct cfq_data *cfqd = e->elevator_data;                       \
        unsigned int __data = __VAR;                                    \
        if (__CONV)                                                     \
                __data = jiffies_to_msecs(__data);                      \
        return cfq_var_show(__data, (page));                            \
}
SHOW_FUNCTION(cfq_quantum_show, cfqd->cfq_quantum, 0);
SHOW_FUNCTION(cfq_fifo_expire_sync_show, cfqd->cfq_fifo_expire[1], 1);
SHOW_FUNCTION(cfq_fifo_expire_async_show, cfqd->cfq_fifo_expire[0], 1);
SHOW_FUNCTION(cfq_back_seek_max_show, cfqd->cfq_back_max, 0);
SHOW_FUNCTION(cfq_back_seek_penalty_show, cfqd->cfq_back_penalty, 0);
SHOW_FUNCTION(cfq_slice_idle_show, cfqd->cfq_slice_idle, 1);
SHOW_FUNCTION(cfq_slice_sync_show, cfqd->cfq_slice[1], 1);
SHOW_FUNCTION(cfq_slice_async_show, cfqd->cfq_slice[0], 1);
SHOW_FUNCTION(cfq_slice_async_rq_show, cfqd->cfq_slice_async_rq, 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 cfq_data *cfqd = e->elevator_data;                       \
        unsigned int __data;                                            \
        int ret = cfq_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(cfq_quantum_store, &cfqd->cfq_quantum, 1, UINT_MAX, 0);
STORE_FUNCTION(cfq_fifo_expire_sync_store, &cfqd->cfq_fifo_expire[1], 1, UINT_MAX, 1);
STORE_FUNCTION(cfq_fifo_expire_async_store, &cfqd->cfq_fifo_expire[0], 1, UINT_MAX, 1);
STORE_FUNCTION(cfq_back_seek_max_store, &cfqd->cfq_back_max, 0, UINT_MAX, 0);
STORE_FUNCTION(cfq_back_seek_penalty_store, &cfqd->cfq_back_penalty, 1, UINT_MAX, 0);
STORE_FUNCTION(cfq_slice_idle_store, &cfqd->cfq_slice_idle, 0, UINT_MAX, 1);
STORE_FUNCTION(cfq_slice_sync_store, &cfqd->cfq_slice[1], 1, UINT_MAX, 1);
STORE_FUNCTION(cfq_slice_async_store, &cfqd->cfq_slice[0], 1, UINT_MAX, 1);
STORE_FUNCTION(cfq_slice_async_rq_store, &cfqd->cfq_slice_async_rq, 1, UINT_MAX, 0);
#undef STORE_FUNCTION

#define CFQ_ATTR(name) \
        __ATTR(name, S_IRUGO|S_IWUSR, cfq_##name##_show, cfq_##name##_store)

static struct elv_fs_entry cfq_attrs[] = {
        CFQ_ATTR(quantum),
        CFQ_ATTR(fifo_expire_sync),
        CFQ_ATTR(fifo_expire_async),
        CFQ_ATTR(back_seek_max),
        CFQ_ATTR(back_seek_penalty),
        CFQ_ATTR(slice_sync),
        CFQ_ATTR(slice_async),
        CFQ_ATTR(slice_async_rq),
        CFQ_ATTR(slice_idle),
        __ATTR_NULL
};

static struct elevator_type iosched_cfq = {
        .ops = {
                .elevator_merge_fn =            cfq_merge,
                .elevator_merged_fn =           cfq_merged_request,
                .elevator_merge_req_fn =        cfq_merged_requests,
                .elevator_allow_merge_fn =      cfq_allow_merge,
                .elevator_dispatch_fn =         cfq_dispatch_requests,
                .elevator_add_req_fn =          cfq_insert_request,
                .elevator_activate_req_fn =     cfq_activate_request,
                .elevator_deactivate_req_fn =   cfq_deactivate_request,
                .elevator_queue_empty_fn =      cfq_queue_empty,
                .elevator_completed_req_fn =    cfq_completed_request,
                .elevator_former_req_fn =       elv_rb_former_request,
                .elevator_latter_req_fn =       elv_rb_latter_request,
                .elevator_set_req_fn =          cfq_set_request,
                .elevator_put_req_fn =          cfq_put_request,
                .elevator_may_queue_fn =        cfq_may_queue,
                .elevator_init_fn =             cfq_init_queue,
                .elevator_exit_fn =             cfq_exit_queue,
                .trim =                         cfq_free_io_context,
        },
        .elevator_attrs =       cfq_attrs,
        .elevator_name =        "cfq",
        .elevator_owner =       THIS_MODULE,
};

static int __init cfq_init(void)
{
        int ret;

        /*
         * could be 0 on HZ < 1000 setups
         */
        if (!cfq_slice_async)
                cfq_slice_async = 1;
        if (!cfq_slice_idle)
                cfq_slice_idle = 1;

        if (cfq_slab_setup())
                return -ENOMEM;

        ret = elv_register(&iosched_cfq);
        if (ret)
                cfq_slab_kill();

        return ret;
}

static void __exit cfq_exit(void)
{
        DECLARE_COMPLETION_ONSTACK(all_gone);
        elv_unregister(&iosched_cfq);
        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();
        cfq_slab_kill();
}

module_init(cfq_init);
module_exit(cfq_exit);

MODULE_AUTHOR("Jens Axboe");
MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("Completely Fair Queueing IO scheduler");