sh: __copy_user() optimizations for small copies.

[firewire-audio.git] / kernel / sched_fair.c

/*
 * Completely Fair Scheduling (CFS) Class (SCHED_NORMAL/SCHED_BATCH)
 *
 *  Copyright (C) 2007 Red Hat, Inc., Ingo Molnar <mingo@redhat.com>
 *
 *  Interactivity improvements by Mike Galbraith
 *  (C) 2007 Mike Galbraith <efault@gmx.de>
 *
 *  Various enhancements by Dmitry Adamushko.
 *  (C) 2007 Dmitry Adamushko <dmitry.adamushko@gmail.com>
 *
 *  Group scheduling enhancements by Srivatsa Vaddagiri
 *  Copyright IBM Corporation, 2007
 *  Author: Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
 *
 *  Scaled math optimizations by Thomas Gleixner
 *  Copyright (C) 2007, Thomas Gleixner <tglx@linutronix.de>
 *
 *  Adaptive scheduling granularity, math enhancements by Peter Zijlstra
 *  Copyright (C) 2007 Red Hat, Inc., Peter Zijlstra <pzijlstr@redhat.com>
 */

/*
 * Targeted preemption latency for CPU-bound tasks:
 * (default: 20ms, units: nanoseconds)
 *
 * NOTE: this latency value is not the same as the concept of
 * 'timeslice length' - timeslices in CFS are of variable length.
 * (to see the precise effective timeslice length of your workload,
 *  run vmstat and monitor the context-switches field)
 *
 * On SMP systems the value of this is multiplied by the log2 of the
 * number of CPUs. (i.e. factor 2x on 2-way systems, 3x on 4-way
 * systems, 4x on 8-way systems, 5x on 16-way systems, etc.)
 * Targeted preemption latency for CPU-bound tasks:
 */
unsigned int sysctl_sched_latency __read_mostly = 20000000ULL;

/*
 * Minimal preemption granularity for CPU-bound tasks:
 * (default: 2 msec, units: nanoseconds)
 */
unsigned int sysctl_sched_min_granularity __read_mostly = 2000000ULL;

/*
 * sys_sched_yield() compat mode
 *
 * This option switches the agressive yield implementation of the
 * old scheduler back on.
 */
unsigned int __read_mostly sysctl_sched_compat_yield;

/*
 * SCHED_BATCH wake-up granularity.
 * (default: 25 msec, units: nanoseconds)
 *
 * This option delays the preemption effects of decoupled workloads
 * and reduces their over-scheduling. Synchronous workloads will still
 * have immediate wakeup/sleep latencies.
 */
unsigned int sysctl_sched_batch_wakeup_granularity __read_mostly = 25000000UL;

/*
 * SCHED_OTHER wake-up granularity.
 * (default: 1 msec, units: nanoseconds)
 *
 * This option delays the preemption effects of decoupled workloads
 * and reduces their over-scheduling. Synchronous workloads will still
 * have immediate wakeup/sleep latencies.
 */
unsigned int sysctl_sched_wakeup_granularity __read_mostly = 1000000UL;

unsigned int sysctl_sched_stat_granularity __read_mostly;

/*
 * Initialized in sched_init_granularity() [to 5 times the base granularity]:
 */
unsigned int sysctl_sched_runtime_limit __read_mostly;

/*
 * Debugging: various feature bits
 */
enum {
        SCHED_FEAT_FAIR_SLEEPERS        = 1,
        SCHED_FEAT_SLEEPER_AVG          = 2,
        SCHED_FEAT_SLEEPER_LOAD_AVG     = 4,
        SCHED_FEAT_PRECISE_CPU_LOAD     = 8,
        SCHED_FEAT_START_DEBIT          = 16,
        SCHED_FEAT_SKIP_INITIAL         = 32,
};

unsigned int sysctl_sched_features __read_mostly =
                SCHED_FEAT_FAIR_SLEEPERS        *1 |
                SCHED_FEAT_SLEEPER_AVG          *0 |
                SCHED_FEAT_SLEEPER_LOAD_AVG     *1 |
                SCHED_FEAT_PRECISE_CPU_LOAD     *1 |
                SCHED_FEAT_START_DEBIT          *1 |
                SCHED_FEAT_SKIP_INITIAL         *0;

extern struct sched_class fair_sched_class;

/**************************************************************
 * CFS operations on generic schedulable entities:
 */

#ifdef CONFIG_FAIR_GROUP_SCHED

/* cpu runqueue to which this cfs_rq is attached */
static inline struct rq *rq_of(struct cfs_rq *cfs_rq)
{
        return cfs_rq->rq;
}

/* currently running entity (if any) on this cfs_rq */
static inline struct sched_entity *cfs_rq_curr(struct cfs_rq *cfs_rq)
{
        return cfs_rq->curr;
}

/* An entity is a task if it doesn't "own" a runqueue */
#define entity_is_task(se)      (!se->my_q)

static inline void
set_cfs_rq_curr(struct cfs_rq *cfs_rq, struct sched_entity *se)
{
        cfs_rq->curr = se;
}

#else   /* CONFIG_FAIR_GROUP_SCHED */

static inline struct rq *rq_of(struct cfs_rq *cfs_rq)
{
        return container_of(cfs_rq, struct rq, cfs);
}

static inline struct sched_entity *cfs_rq_curr(struct cfs_rq *cfs_rq)
{
        struct rq *rq = rq_of(cfs_rq);

        if (unlikely(rq->curr->sched_class != &fair_sched_class))
                return NULL;

        return &rq->curr->se;
}

#define entity_is_task(se)      1

static inline void
set_cfs_rq_curr(struct cfs_rq *cfs_rq, struct sched_entity *se) { }

#endif  /* CONFIG_FAIR_GROUP_SCHED */

static inline struct task_struct *task_of(struct sched_entity *se)
{
        return container_of(se, struct task_struct, se);
}


/**************************************************************
 * Scheduling class tree data structure manipulation methods:
 */

/*
 * Enqueue an entity into the rb-tree:
 */
static inline void
__enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se)
{
        struct rb_node **link = &cfs_rq->tasks_timeline.rb_node;
        struct rb_node *parent = NULL;
        struct sched_entity *entry;
        s64 key = se->fair_key;
        int leftmost = 1;

        /*
         * Find the right place in the rbtree:
         */
        while (*link) {
                parent = *link;
                entry = rb_entry(parent, struct sched_entity, run_node);
                /*
                 * We dont care about collisions. Nodes with
                 * the same key stay together.
                 */
                if (key - entry->fair_key < 0) {
                        link = &parent->rb_left;
                } else {
                        link = &parent->rb_right;
                        leftmost = 0;
                }
        }

        /*
         * Maintain a cache of leftmost tree entries (it is frequently
         * used):
         */
        if (leftmost)
                cfs_rq->rb_leftmost = &se->run_node;

        rb_link_node(&se->run_node, parent, link);
        rb_insert_color(&se->run_node, &cfs_rq->tasks_timeline);
        update_load_add(&cfs_rq->load, se->load.weight);
        cfs_rq->nr_running++;
        se->on_rq = 1;

        schedstat_add(cfs_rq, wait_runtime, se->wait_runtime);
}

static inline void
__dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se)
{
        if (cfs_rq->rb_leftmost == &se->run_node)
                cfs_rq->rb_leftmost = rb_next(&se->run_node);
        rb_erase(&se->run_node, &cfs_rq->tasks_timeline);
        update_load_sub(&cfs_rq->load, se->load.weight);
        cfs_rq->nr_running--;
        se->on_rq = 0;

        schedstat_add(cfs_rq, wait_runtime, -se->wait_runtime);
}

static inline struct rb_node *first_fair(struct cfs_rq *cfs_rq)
{
        return cfs_rq->rb_leftmost;
}

static struct sched_entity *__pick_next_entity(struct cfs_rq *cfs_rq)
{
        return rb_entry(first_fair(cfs_rq), struct sched_entity, run_node);
}

/**************************************************************
 * Scheduling class statistics methods:
 */

/*
 * Calculate the preemption granularity needed to schedule every
 * runnable task once per sysctl_sched_latency amount of time.
 * (down to a sensible low limit on granularity)
 *
 * For example, if there are 2 tasks running and latency is 10 msecs,
 * we switch tasks every 5 msecs. If we have 3 tasks running, we have
 * to switch tasks every 3.33 msecs to get a 10 msecs observed latency
 * for each task. We do finer and finer scheduling up to until we
 * reach the minimum granularity value.
 *
 * To achieve this we use the following dynamic-granularity rule:
 *
 *    gran = lat/nr - lat/nr/nr
 *
 * This comes out of the following equations:
 *
 *    kA1 + gran = kB1
 *    kB2 + gran = kA2
 *    kA2 = kA1
 *    kB2 = kB1 - d + d/nr
 *    lat = d * nr
 *
 * Where 'k' is key, 'A' is task A (waiting), 'B' is task B (running),
 * '1' is start of time, '2' is end of time, 'd' is delay between
 * 1 and 2 (during which task B was running), 'nr' is number of tasks
 * running, 'lat' is the the period of each task. ('lat' is the
 * sched_latency that we aim for.)
 */
static long
sched_granularity(struct cfs_rq *cfs_rq)
{
        unsigned int gran = sysctl_sched_latency;
        unsigned int nr = cfs_rq->nr_running;

        if (nr > 1) {
                gran = gran/nr - gran/nr/nr;
                gran = max(gran, sysctl_sched_min_granularity);
        }

        return gran;
}

/*
 * We rescale the rescheduling granularity of tasks according to their
 * nice level, but only linearly, not exponentially:
 */
static long
niced_granularity(struct sched_entity *curr, unsigned long granularity)
{
        u64 tmp;

        if (likely(curr->load.weight == NICE_0_LOAD))
                return granularity;
        /*
         * Positive nice levels get the same granularity as nice-0:
         */
        if (likely(curr->load.weight < NICE_0_LOAD)) {
                tmp = curr->load.weight * (u64)granularity;
                return (long) (tmp >> NICE_0_SHIFT);
        }
        /*
         * Negative nice level tasks get linearly finer
         * granularity:
         */
        tmp = curr->load.inv_weight * (u64)granularity;

        /*
         * It will always fit into 'long':
         */
        return (long) (tmp >> (WMULT_SHIFT-NICE_0_SHIFT));
}

static inline void
limit_wait_runtime(struct cfs_rq *cfs_rq, struct sched_entity *se)
{
        long limit = sysctl_sched_runtime_limit;

        /*
         * Niced tasks have the same history dynamic range as
         * non-niced tasks:
         */
        if (unlikely(se->wait_runtime > limit)) {
                se->wait_runtime = limit;
                schedstat_inc(se, wait_runtime_overruns);
                schedstat_inc(cfs_rq, wait_runtime_overruns);
        }
        if (unlikely(se->wait_runtime < -limit)) {
                se->wait_runtime = -limit;
                schedstat_inc(se, wait_runtime_underruns);
                schedstat_inc(cfs_rq, wait_runtime_underruns);
        }
}

static inline void
__add_wait_runtime(struct cfs_rq *cfs_rq, struct sched_entity *se, long delta)
{
        se->wait_runtime += delta;
        schedstat_add(se, sum_wait_runtime, delta);
        limit_wait_runtime(cfs_rq, se);
}

static void
add_wait_runtime(struct cfs_rq *cfs_rq, struct sched_entity *se, long delta)
{
        schedstat_add(cfs_rq, wait_runtime, -se->wait_runtime);
        __add_wait_runtime(cfs_rq, se, delta);
        schedstat_add(cfs_rq, wait_runtime, se->wait_runtime);
}

/*
 * Update the current task's runtime statistics. Skip current tasks that
 * are not in our scheduling class.
 */
static inline void
__update_curr(struct cfs_rq *cfs_rq, struct sched_entity *curr)
{
        unsigned long delta, delta_exec, delta_fair, delta_mine;
        struct load_weight *lw = &cfs_rq->load;
        unsigned long load = lw->weight;

        delta_exec = curr->delta_exec;
        schedstat_set(curr->exec_max, max((u64)delta_exec, curr->exec_max));

        curr->sum_exec_runtime += delta_exec;
        cfs_rq->exec_clock += delta_exec;

        if (unlikely(!load))
                return;

        delta_fair = calc_delta_fair(delta_exec, lw);
        delta_mine = calc_delta_mine(delta_exec, curr->load.weight, lw);

        if (cfs_rq->sleeper_bonus > sysctl_sched_min_granularity) {
                delta = min((u64)delta_mine, cfs_rq->sleeper_bonus);
                delta = min(delta, (unsigned long)(
                        (long)sysctl_sched_runtime_limit - curr->wait_runtime));
                cfs_rq->sleeper_bonus -= delta;
                delta_mine -= delta;
        }

        cfs_rq->fair_clock += delta_fair;
        /*
         * We executed delta_exec amount of time on the CPU,
         * but we were only entitled to delta_mine amount of
         * time during that period (if nr_running == 1 then
         * the two values are equal)
         * [Note: delta_mine - delta_exec is negative]:
         */
        add_wait_runtime(cfs_rq, curr, delta_mine - delta_exec);
}

static void update_curr(struct cfs_rq *cfs_rq)
{
        struct sched_entity *curr = cfs_rq_curr(cfs_rq);
        unsigned long delta_exec;

        if (unlikely(!curr))
                return;

        /*
         * Get the amount of time the current task was running
         * since the last time we changed load (this cannot
         * overflow on 32 bits):
         */
        delta_exec = (unsigned long)(rq_of(cfs_rq)->clock - curr->exec_start);

        curr->delta_exec += delta_exec;

        if (unlikely(curr->delta_exec > sysctl_sched_stat_granularity)) {
                __update_curr(cfs_rq, curr);
                curr->delta_exec = 0;
        }
        curr->exec_start = rq_of(cfs_rq)->clock;
}

static inline void
update_stats_wait_start(struct cfs_rq *cfs_rq, struct sched_entity *se)
{
        se->wait_start_fair = cfs_rq->fair_clock;
        schedstat_set(se->wait_start, rq_of(cfs_rq)->clock);
}

/*
 * We calculate fair deltas here, so protect against the random effects
 * of a multiplication overflow by capping it to the runtime limit:
 */
#if BITS_PER_LONG == 32
static inline unsigned long
calc_weighted(unsigned long delta, unsigned long weight, int shift)
{
        u64 tmp = (u64)delta * weight >> shift;

        if (unlikely(tmp > sysctl_sched_runtime_limit*2))
                return sysctl_sched_runtime_limit*2;
        return tmp;
}
#else
static inline unsigned long
calc_weighted(unsigned long delta, unsigned long weight, int shift)
{
        return delta * weight >> shift;
}
#endif

/*
 * Task is being enqueued - update stats:
 */
static void update_stats_enqueue(struct cfs_rq *cfs_rq, struct sched_entity *se)
{
        s64 key;

        /*
         * Are we enqueueing a waiting task? (for current tasks
         * a dequeue/enqueue event is a NOP)
         */
        if (se != cfs_rq_curr(cfs_rq))
                update_stats_wait_start(cfs_rq, se);
        /*
         * Update the key:
         */
        key = cfs_rq->fair_clock;

        /*
         * Optimize the common nice 0 case:
         */
        if (likely(se->load.weight == NICE_0_LOAD)) {
                key -= se->wait_runtime;
        } else {
                u64 tmp;

                if (se->wait_runtime < 0) {
                        tmp = -se->wait_runtime;
                        key += (tmp * se->load.inv_weight) >>
                                        (WMULT_SHIFT - NICE_0_SHIFT);
                } else {
                        tmp = se->wait_runtime;
                        key -= (tmp * se->load.inv_weight) >>
                                        (WMULT_SHIFT - NICE_0_SHIFT);
                }
        }

        se->fair_key = key;
}

/*
 * Note: must be called with a freshly updated rq->fair_clock.
 */
static inline void
__update_stats_wait_end(struct cfs_rq *cfs_rq, struct sched_entity *se)
{
        unsigned long delta_fair = se->delta_fair_run;

        schedstat_set(se->wait_max, max(se->wait_max,
                        rq_of(cfs_rq)->clock - se->wait_start));

        if (unlikely(se->load.weight != NICE_0_LOAD))
                delta_fair = calc_weighted(delta_fair, se->load.weight,
                                                        NICE_0_SHIFT);

        add_wait_runtime(cfs_rq, se, delta_fair);
}

static void
update_stats_wait_end(struct cfs_rq *cfs_rq, struct sched_entity *se)
{
        unsigned long delta_fair;

        if (unlikely(!se->wait_start_fair))
                return;

        delta_fair = (unsigned long)min((u64)(2*sysctl_sched_runtime_limit),
                        (u64)(cfs_rq->fair_clock - se->wait_start_fair));

        se->delta_fair_run += delta_fair;
        if (unlikely(abs(se->delta_fair_run) >=
                                sysctl_sched_stat_granularity)) {
                __update_stats_wait_end(cfs_rq, se);
                se->delta_fair_run = 0;
        }

        se->wait_start_fair = 0;
        schedstat_set(se->wait_start, 0);
}

static inline void
update_stats_dequeue(struct cfs_rq *cfs_rq, struct sched_entity *se)
{
        update_curr(cfs_rq);
        /*
         * Mark the end of the wait period if dequeueing a
         * waiting task:
         */
        if (se != cfs_rq_curr(cfs_rq))
                update_stats_wait_end(cfs_rq, se);
}

/*
 * We are picking a new current task - update its stats:
 */
static inline void
update_stats_curr_start(struct cfs_rq *cfs_rq, struct sched_entity *se)
{
        /*
         * We are starting a new run period:
         */
        se->exec_start = rq_of(cfs_rq)->clock;
}

/*
 * We are descheduling a task - update its stats:
 */
static inline void
update_stats_curr_end(struct cfs_rq *cfs_rq, struct sched_entity *se)
{
        se->exec_start = 0;
}

/**************************************************
 * Scheduling class queueing methods:
 */

static void __enqueue_sleeper(struct cfs_rq *cfs_rq, struct sched_entity *se)
{
        unsigned long load = cfs_rq->load.weight, delta_fair;
        long prev_runtime;

        /*
         * Do not boost sleepers if there's too much bonus 'in flight'
         * already:
         */
        if (unlikely(cfs_rq->sleeper_bonus > sysctl_sched_runtime_limit))
                return;

        if (sysctl_sched_features & SCHED_FEAT_SLEEPER_LOAD_AVG)
                load = rq_of(cfs_rq)->cpu_load[2];

        delta_fair = se->delta_fair_sleep;

        /*
         * Fix up delta_fair with the effect of us running
         * during the whole sleep period:
         */
        if (sysctl_sched_features & SCHED_FEAT_SLEEPER_AVG)
                delta_fair = div64_likely32((u64)delta_fair * load,
                                                load + se->load.weight);

        if (unlikely(se->load.weight != NICE_0_LOAD))
                delta_fair = calc_weighted(delta_fair, se->load.weight,
                                                        NICE_0_SHIFT);

        prev_runtime = se->wait_runtime;
        __add_wait_runtime(cfs_rq, se, delta_fair);
        delta_fair = se->wait_runtime - prev_runtime;

        /*
         * Track the amount of bonus we've given to sleepers:
         */
        cfs_rq->sleeper_bonus += delta_fair;
}

static void enqueue_sleeper(struct cfs_rq *cfs_rq, struct sched_entity *se)
{
        struct task_struct *tsk = task_of(se);
        unsigned long delta_fair;

        if ((entity_is_task(se) && tsk->policy == SCHED_BATCH) ||
                         !(sysctl_sched_features & SCHED_FEAT_FAIR_SLEEPERS))
                return;

        delta_fair = (unsigned long)min((u64)(2*sysctl_sched_runtime_limit),
                (u64)(cfs_rq->fair_clock - se->sleep_start_fair));

        se->delta_fair_sleep += delta_fair;
        if (unlikely(abs(se->delta_fair_sleep) >=
                                sysctl_sched_stat_granularity)) {
                __enqueue_sleeper(cfs_rq, se);
                se->delta_fair_sleep = 0;
        }

        se->sleep_start_fair = 0;

#ifdef CONFIG_SCHEDSTATS
        if (se->sleep_start) {
                u64 delta = rq_of(cfs_rq)->clock - se->sleep_start;

                if ((s64)delta < 0)
                        delta = 0;

                if (unlikely(delta > se->sleep_max))
                        se->sleep_max = delta;

                se->sleep_start = 0;
                se->sum_sleep_runtime += delta;
        }
        if (se->block_start) {
                u64 delta = rq_of(cfs_rq)->clock - se->block_start;

                if ((s64)delta < 0)
                        delta = 0;

                if (unlikely(delta > se->block_max))
                        se->block_max = delta;

                se->block_start = 0;
                se->sum_sleep_runtime += delta;
        }
#endif
}

static void
enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int wakeup)
{
        /*
         * Update the fair clock.
         */
        update_curr(cfs_rq);

        if (wakeup)
                enqueue_sleeper(cfs_rq, se);

        update_stats_enqueue(cfs_rq, se);
        __enqueue_entity(cfs_rq, se);
}

static void
dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int sleep)
{
        update_stats_dequeue(cfs_rq, se);
        if (sleep) {
                se->sleep_start_fair = cfs_rq->fair_clock;
#ifdef CONFIG_SCHEDSTATS
                if (entity_is_task(se)) {
                        struct task_struct *tsk = task_of(se);

                        if (tsk->state & TASK_INTERRUPTIBLE)
                                se->sleep_start = rq_of(cfs_rq)->clock;
                        if (tsk->state & TASK_UNINTERRUPTIBLE)
                                se->block_start = rq_of(cfs_rq)->clock;
                }
#endif
        }
        __dequeue_entity(cfs_rq, se);
}

/*
 * Preempt the current task with a newly woken task if needed:
 */
static void
__check_preempt_curr_fair(struct cfs_rq *cfs_rq, struct sched_entity *se,
                          struct sched_entity *curr, unsigned long granularity)
{
        s64 __delta = curr->fair_key - se->fair_key;
        unsigned long ideal_runtime, delta_exec;

        /*
         * ideal_runtime is compared against sum_exec_runtime, which is
         * walltime, hence do not scale.
         */
        ideal_runtime = max(sysctl_sched_latency / cfs_rq->nr_running,
                        (unsigned long)sysctl_sched_min_granularity);

        /*
         * If we executed more than what the latency constraint suggests,
         * reduce the rescheduling granularity. This way the total latency
         * of how much a task is not scheduled converges to
         * sysctl_sched_latency:
         */
        delta_exec = curr->sum_exec_runtime - curr->prev_sum_exec_runtime;
        if (delta_exec > ideal_runtime)
                granularity = 0;

        /*
         * Take scheduling granularity into account - do not
         * preempt the current task unless the best task has
         * a larger than sched_granularity fairness advantage:
         *
         * scale granularity as key space is in fair_clock.
         */
        if (__delta > niced_granularity(curr, granularity))
                resched_task(rq_of(cfs_rq)->curr);
}

static inline void
set_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *se)
{
        /*
         * Any task has to be enqueued before it get to execute on
         * a CPU. So account for the time it spent waiting on the
         * runqueue. (note, here we rely on pick_next_task() having
         * done a put_prev_task_fair() shortly before this, which
         * updated rq->fair_clock - used by update_stats_wait_end())
         */
        update_stats_wait_end(cfs_rq, se);
        update_stats_curr_start(cfs_rq, se);
        set_cfs_rq_curr(cfs_rq, se);
        se->prev_sum_exec_runtime = se->sum_exec_runtime;
}

static struct sched_entity *pick_next_entity(struct cfs_rq *cfs_rq)
{
        struct sched_entity *se = __pick_next_entity(cfs_rq);

        set_next_entity(cfs_rq, se);

        return se;
}

static void put_prev_entity(struct cfs_rq *cfs_rq, struct sched_entity *prev)
{
        /*
         * If still on the runqueue then deactivate_task()
         * was not called and update_curr() has to be done:
         */
        if (prev->on_rq)
                update_curr(cfs_rq);

        update_stats_curr_end(cfs_rq, prev);

        if (prev->on_rq)
                update_stats_wait_start(cfs_rq, prev);
        set_cfs_rq_curr(cfs_rq, NULL);
}

static void entity_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr)
{
        struct sched_entity *next;

        /*
         * Dequeue and enqueue the task to update its
         * position within the tree:
         */
        dequeue_entity(cfs_rq, curr, 0);
        enqueue_entity(cfs_rq, curr, 0);

        /*
         * Reschedule if another task tops the current one.
         */
        next = __pick_next_entity(cfs_rq);
        if (next == curr)
                return;

        __check_preempt_curr_fair(cfs_rq, next, curr,
                        sched_granularity(cfs_rq));
}

/**************************************************
 * CFS operations on tasks:
 */

#ifdef CONFIG_FAIR_GROUP_SCHED

/* Walk up scheduling entities hierarchy */
#define for_each_sched_entity(se) \
                for (; se; se = se->parent)

static inline struct cfs_rq *task_cfs_rq(struct task_struct *p)
{
        return p->se.cfs_rq;
}

/* runqueue on which this entity is (to be) queued */
static inline struct cfs_rq *cfs_rq_of(struct sched_entity *se)
{
        return se->cfs_rq;
}

/* runqueue "owned" by this group */
static inline struct cfs_rq *group_cfs_rq(struct sched_entity *grp)
{
        return grp->my_q;
}

/* Given a group's cfs_rq on one cpu, return its corresponding cfs_rq on
 * another cpu ('this_cpu')
 */
static inline struct cfs_rq *cpu_cfs_rq(struct cfs_rq *cfs_rq, int this_cpu)
{
        /* A later patch will take group into account */
        return &cpu_rq(this_cpu)->cfs;
}

/* Iterate thr' all leaf cfs_rq's on a runqueue */
#define for_each_leaf_cfs_rq(rq, cfs_rq) \
        list_for_each_entry(cfs_rq, &rq->leaf_cfs_rq_list, leaf_cfs_rq_list)

/* Do the two (enqueued) tasks belong to the same group ? */
static inline int is_same_group(struct task_struct *curr, struct task_struct *p)
{
        if (curr->se.cfs_rq == p->se.cfs_rq)
                return 1;

        return 0;
}

#else   /* CONFIG_FAIR_GROUP_SCHED */

#define for_each_sched_entity(se) \
                for (; se; se = NULL)

static inline struct cfs_rq *task_cfs_rq(struct task_struct *p)
{
        return &task_rq(p)->cfs;
}

static inline struct cfs_rq *cfs_rq_of(struct sched_entity *se)
{
        struct task_struct *p = task_of(se);
        struct rq *rq = task_rq(p);

        return &rq->cfs;
}

/* runqueue "owned" by this group */
static inline struct cfs_rq *group_cfs_rq(struct sched_entity *grp)
{
        return NULL;
}

static inline struct cfs_rq *cpu_cfs_rq(struct cfs_rq *cfs_rq, int this_cpu)
{
        return &cpu_rq(this_cpu)->cfs;
}

#define for_each_leaf_cfs_rq(rq, cfs_rq) \
                for (cfs_rq = &rq->cfs; cfs_rq; cfs_rq = NULL)

static inline int is_same_group(struct task_struct *curr, struct task_struct *p)
{
        return 1;
}

#endif  /* CONFIG_FAIR_GROUP_SCHED */

/*
 * The enqueue_task method is called before nr_running is
 * increased. Here we update the fair scheduling stats and
 * then put the task into the rbtree:
 */
static void enqueue_task_fair(struct rq *rq, struct task_struct *p, int wakeup)
{
        struct cfs_rq *cfs_rq;
        struct sched_entity *se = &p->se;

        for_each_sched_entity(se) {
                if (se->on_rq)
                        break;
                cfs_rq = cfs_rq_of(se);
                enqueue_entity(cfs_rq, se, wakeup);
        }
}

/*
 * The dequeue_task method is called before nr_running is
 * decreased. We remove the task from the rbtree and
 * update the fair scheduling stats:
 */
static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int sleep)
{
        struct cfs_rq *cfs_rq;
        struct sched_entity *se = &p->se;

        for_each_sched_entity(se) {
                cfs_rq = cfs_rq_of(se);
                dequeue_entity(cfs_rq, se, sleep);
                /* Don't dequeue parent if it has other entities besides us */
                if (cfs_rq->load.weight)
                        break;
        }
}

/*
 * sched_yield() support is very simple - we dequeue and enqueue.
 *
 * If compat_yield is turned on then we requeue to the end of the tree.
 */
static void yield_task_fair(struct rq *rq, struct task_struct *p)
{
        struct cfs_rq *cfs_rq = task_cfs_rq(p);
        struct rb_node **link = &cfs_rq->tasks_timeline.rb_node;
        struct sched_entity *rightmost, *se = &p->se;
        struct rb_node *parent;

        /*
         * Are we the only task in the tree?
         */
        if (unlikely(cfs_rq->nr_running == 1))
                return;

        if (likely(!sysctl_sched_compat_yield)) {
                __update_rq_clock(rq);
                /*
                 * Dequeue and enqueue the task to update its
                 * position within the tree:
                 */
                dequeue_entity(cfs_rq, &p->se, 0);
                enqueue_entity(cfs_rq, &p->se, 0);

                return;
        }
        /*
         * Find the rightmost entry in the rbtree:
         */
        do {
                parent = *link;
                link = &parent->rb_right;
        } while (*link);

        rightmost = rb_entry(parent, struct sched_entity, run_node);
        /*
         * Already in the rightmost position?
         */
        if (unlikely(rightmost == se))
                return;

        /*
         * Minimally necessary key value to be last in the tree:
         */
        se->fair_key = rightmost->fair_key + 1;

        if (cfs_rq->rb_leftmost == &se->run_node)
                cfs_rq->rb_leftmost = rb_next(&se->run_node);
        /*
         * Relink the task to the rightmost position:
         */
        rb_erase(&se->run_node, &cfs_rq->tasks_timeline);
        rb_link_node(&se->run_node, parent, link);
        rb_insert_color(&se->run_node, &cfs_rq->tasks_timeline);
}

/*
 * Preempt the current task with a newly woken task if needed:
 */
static void check_preempt_curr_fair(struct rq *rq, struct task_struct *p)
{
        struct task_struct *curr = rq->curr;
        struct cfs_rq *cfs_rq = task_cfs_rq(curr);
        unsigned long gran;

        if (unlikely(rt_prio(p->prio))) {
                update_rq_clock(rq);
                update_curr(cfs_rq);
                resched_task(curr);
                return;
        }

        gran = sysctl_sched_wakeup_granularity;
        /*
         * Batch tasks prefer throughput over latency:
         */
        if (unlikely(p->policy == SCHED_BATCH))
                gran = sysctl_sched_batch_wakeup_granularity;

        if (is_same_group(curr, p))
                __check_preempt_curr_fair(cfs_rq, &p->se, &curr->se, gran);
}

static struct task_struct *pick_next_task_fair(struct rq *rq)
{
        struct cfs_rq *cfs_rq = &rq->cfs;
        struct sched_entity *se;

        if (unlikely(!cfs_rq->nr_running))
                return NULL;

        do {
                se = pick_next_entity(cfs_rq);
                cfs_rq = group_cfs_rq(se);
        } while (cfs_rq);

        return task_of(se);
}

/*
 * Account for a descheduled task:
 */
static void put_prev_task_fair(struct rq *rq, struct task_struct *prev)
{
        struct sched_entity *se = &prev->se;
        struct cfs_rq *cfs_rq;

        for_each_sched_entity(se) {
                cfs_rq = cfs_rq_of(se);
                put_prev_entity(cfs_rq, se);
        }
}

/**************************************************
 * Fair scheduling class load-balancing methods:
 */

/*
 * Load-balancing iterator. Note: while the runqueue stays locked
 * during the whole iteration, the current task might be
 * dequeued so the iterator has to be dequeue-safe. Here we
 * achieve that by always pre-iterating before returning
 * the current task:
 */
static inline struct task_struct *
__load_balance_iterator(struct cfs_rq *cfs_rq, struct rb_node *curr)
{
        struct task_struct *p;

        if (!curr)
                return NULL;

        p = rb_entry(curr, struct task_struct, se.run_node);
        cfs_rq->rb_load_balance_curr = rb_next(curr);

        return p;
}

static struct task_struct *load_balance_start_fair(void *arg)
{
        struct cfs_rq *cfs_rq = arg;

        return __load_balance_iterator(cfs_rq, first_fair(cfs_rq));
}

static struct task_struct *load_balance_next_fair(void *arg)
{
        struct cfs_rq *cfs_rq = arg;

        return __load_balance_iterator(cfs_rq, cfs_rq->rb_load_balance_curr);
}

#ifdef CONFIG_FAIR_GROUP_SCHED
static int cfs_rq_best_prio(struct cfs_rq *cfs_rq)
{
        struct sched_entity *curr;
        struct task_struct *p;

        if (!cfs_rq->nr_running)
                return MAX_PRIO;

        curr = __pick_next_entity(cfs_rq);
        p = task_of(curr);

        return p->prio;
}
#endif

static unsigned long
load_balance_fair(struct rq *this_rq, int this_cpu, struct rq *busiest,
                  unsigned long max_nr_move, unsigned long max_load_move,
                  struct sched_domain *sd, enum cpu_idle_type idle,
                  int *all_pinned, int *this_best_prio)
{
        struct cfs_rq *busy_cfs_rq;
        unsigned long load_moved, total_nr_moved = 0, nr_moved;
        long rem_load_move = max_load_move;
        struct rq_iterator cfs_rq_iterator;

        cfs_rq_iterator.start = load_balance_start_fair;
        cfs_rq_iterator.next = load_balance_next_fair;

        for_each_leaf_cfs_rq(busiest, busy_cfs_rq) {
#ifdef CONFIG_FAIR_GROUP_SCHED
                struct cfs_rq *this_cfs_rq;
                long imbalance;
                unsigned long maxload;

                this_cfs_rq = cpu_cfs_rq(busy_cfs_rq, this_cpu);

                imbalance = busy_cfs_rq->load.weight - this_cfs_rq->load.weight;
                /* Don't pull if this_cfs_rq has more load than busy_cfs_rq */
                if (imbalance <= 0)
                        continue;

                /* Don't pull more than imbalance/2 */
                imbalance /= 2;
                maxload = min(rem_load_move, imbalance);

                *this_best_prio = cfs_rq_best_prio(this_cfs_rq);
#else
# define maxload rem_load_move
#endif
                /* pass busy_cfs_rq argument into
                 * load_balance_[start|next]_fair iterators
                 */
                cfs_rq_iterator.arg = busy_cfs_rq;
                nr_moved = balance_tasks(this_rq, this_cpu, busiest,
                                max_nr_move, maxload, sd, idle, all_pinned,
                                &load_moved, this_best_prio, &cfs_rq_iterator);

                total_nr_moved += nr_moved;
                max_nr_move -= nr_moved;
                rem_load_move -= load_moved;

                if (max_nr_move <= 0 || rem_load_move <= 0)
                        break;
        }

        return max_load_move - rem_load_move;
}

/*
 * scheduler tick hitting a task of our scheduling class:
 */
static void task_tick_fair(struct rq *rq, struct task_struct *curr)
{
        struct cfs_rq *cfs_rq;
        struct sched_entity *se = &curr->se;

        for_each_sched_entity(se) {
                cfs_rq = cfs_rq_of(se);
                entity_tick(cfs_rq, se);
        }
}

/*
 * Share the fairness runtime between parent and child, thus the
 * total amount of pressure for CPU stays equal - new tasks
 * get a chance to run but frequent forkers are not allowed to
 * monopolize the CPU. Note: the parent runqueue is locked,
 * the child is not running yet.
 */
static void task_new_fair(struct rq *rq, struct task_struct *p)
{
        struct cfs_rq *cfs_rq = task_cfs_rq(p);
        struct sched_entity *se = &p->se, *curr = cfs_rq_curr(cfs_rq);

        sched_info_queued(p);

        update_curr(cfs_rq);
        update_stats_enqueue(cfs_rq, se);
        /*
         * Child runs first: we let it run before the parent
         * until it reschedules once. We set up the key so that
         * it will preempt the parent:
         */
        se->fair_key = curr->fair_key -
                niced_granularity(curr, sched_granularity(cfs_rq)) - 1;
        /*
         * The first wait is dominated by the child-runs-first logic,
         * so do not credit it with that waiting time yet:
         */
        if (sysctl_sched_features & SCHED_FEAT_SKIP_INITIAL)
                se->wait_start_fair = 0;

        /*
         * The statistical average of wait_runtime is about
         * -granularity/2, so initialize the task with that:
         */
        if (sysctl_sched_features & SCHED_FEAT_START_DEBIT)
                se->wait_runtime = -(sched_granularity(cfs_rq) / 2);

        __enqueue_entity(cfs_rq, se);
}

#ifdef CONFIG_FAIR_GROUP_SCHED
/* Account for a task changing its policy or group.
 *
 * This routine is mostly called to set cfs_rq->curr field when a task
 * migrates between groups/classes.
 */
static void set_curr_task_fair(struct rq *rq)
{
        struct sched_entity *se = &rq->curr->se;

        for_each_sched_entity(se)
                set_next_entity(cfs_rq_of(se), se);
}
#else
static void set_curr_task_fair(struct rq *rq)
{
}
#endif

/*
 * All the scheduling class methods:
 */
struct sched_class fair_sched_class __read_mostly = {
        .enqueue_task           = enqueue_task_fair,
        .dequeue_task           = dequeue_task_fair,
        .yield_task             = yield_task_fair,

        .check_preempt_curr     = check_preempt_curr_fair,

        .pick_next_task         = pick_next_task_fair,
        .put_prev_task          = put_prev_task_fair,

        .load_balance           = load_balance_fair,

        .set_curr_task          = set_curr_task_fair,
        .task_tick              = task_tick_fair,
        .task_new               = task_new_fair,
};

#ifdef CONFIG_SCHED_DEBUG
static void print_cfs_stats(struct seq_file *m, int cpu)
{
        struct cfs_rq *cfs_rq;

        for_each_leaf_cfs_rq(cpu_rq(cpu), cfs_rq)
                print_cfs_rq(m, cpu, cfs_rq);
}
#endif