sched: default to more agressive yield for SCHED_BATCH tasks

[GitHub/mt8127/android_kernel_alcatel_ttab.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 * (1 + ilog(ncpus)), units: nanoseconds)
 *
 * NOTE: this latency value is not the same as the concept of
 * 'timeslice length' - timeslices in CFS are of variable length
 * and have no persistent notion like in traditional, time-slice
 * based scheduling concepts.
 *
 * (to see the precise effective timeslice length of your workload,
 *  run vmstat and monitor the context-switches (cs) field)
 */
unsigned int sysctl_sched_latency = 20000000ULL;

/*
 * Minimal preemption granularity for CPU-bound tasks:
 * (default: 4 msec * (1 + ilog(ncpus)), units: nanoseconds)
 */
unsigned int sysctl_sched_min_granularity = 4000000ULL;

/*
 * is kept at sysctl_sched_latency / sysctl_sched_min_granularity
 */
static unsigned int sched_nr_latency = 5;

/*
 * After fork, child runs first. (default) If set to 0 then
 * parent will (try to) run first.
 */
const_debug unsigned int sysctl_sched_child_runs_first = 1;

/*
 * 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: 10 msec * (1 + ilog(ncpus)), 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 = 10000000UL;

/*
 * SCHED_OTHER wake-up granularity.
 * (default: 10 msec * (1 + ilog(ncpus)), 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 = 10000000UL;

const_debug unsigned int sysctl_sched_migration_cost = 500000UL;

/**************************************************************
 * 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;
}

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

#else   /* CONFIG_FAIR_GROUP_SCHED */

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

#define entity_is_task(se)      1

#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:
 */

static inline u64 max_vruntime(u64 min_vruntime, u64 vruntime)
{
        s64 delta = (s64)(vruntime - min_vruntime);
        if (delta > 0)
                min_vruntime = vruntime;

        return min_vruntime;
}

static inline u64 min_vruntime(u64 min_vruntime, u64 vruntime)
{
        s64 delta = (s64)(vruntime - min_vruntime);
        if (delta < 0)
                min_vruntime = vruntime;

        return min_vruntime;
}

static inline s64 entity_key(struct cfs_rq *cfs_rq, struct sched_entity *se)
{
        return se->vruntime - cfs_rq->min_vruntime;
}

/*
 * Enqueue an entity into the rb-tree:
 */
static 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 = entity_key(cfs_rq, se);
        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 < entity_key(cfs_rq, entry)) {
                        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);
}

static 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);
}

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);
}

static inline struct sched_entity *__pick_last_entity(struct cfs_rq *cfs_rq)
{
        struct rb_node **link = &cfs_rq->tasks_timeline.rb_node;
        struct sched_entity *se = NULL;
        struct rb_node *parent;

        while (*link) {
                parent = *link;
                se = rb_entry(parent, struct sched_entity, run_node);
                link = &parent->rb_right;
        }

        return se;
}

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

#ifdef CONFIG_SCHED_DEBUG
int sched_nr_latency_handler(struct ctl_table *table, int write,
                struct file *filp, void __user *buffer, size_t *lenp,
                loff_t *ppos)
{
        int ret = proc_dointvec_minmax(table, write, filp, buffer, lenp, ppos);

        if (ret || !write)
                return ret;

        sched_nr_latency = DIV_ROUND_UP(sysctl_sched_latency,
                                        sysctl_sched_min_granularity);

        return 0;
}
#endif

/*
 * The idea is to set a period in which each task runs once.
 *
 * When there are too many tasks (sysctl_sched_nr_latency) we have to stretch
 * this period because otherwise the slices get too small.
 *
 * p = (nr <= nl) ? l : l*nr/nl
 */
static u64 __sched_period(unsigned long nr_running)
{
        u64 period = sysctl_sched_latency;
        unsigned long nr_latency = sched_nr_latency;

        if (unlikely(nr_running > nr_latency)) {
                period *= nr_running;
                do_div(period, nr_latency);
        }

        return period;
}

/*
 * We calculate the wall-time slice from the period by taking a part
 * proportional to the weight.
 *
 * s = p*w/rw
 */
static u64 sched_slice(struct cfs_rq *cfs_rq, struct sched_entity *se)
{
        u64 slice = __sched_period(cfs_rq->nr_running);

        slice *= se->load.weight;
        do_div(slice, cfs_rq->load.weight);

        return slice;
}

/*
 * We calculate the vruntime slice.
 *
 * vs = s/w = p/rw
 */
static u64 __sched_vslice(unsigned long rq_weight, unsigned long nr_running)
{
        u64 vslice = __sched_period(nr_running);

        vslice *= NICE_0_LOAD;
        do_div(vslice, rq_weight);

        return vslice;
}

static u64 sched_vslice(struct cfs_rq *cfs_rq)
{
        return __sched_vslice(cfs_rq->load.weight, cfs_rq->nr_running);
}

static u64 sched_vslice_add(struct cfs_rq *cfs_rq, struct sched_entity *se)
{
        return __sched_vslice(cfs_rq->load.weight + se->load.weight,
                        cfs_rq->nr_running + 1);
}

/*
 * 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_exec)
{
        unsigned long delta_exec_weighted;
        u64 vruntime;

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

        curr->sum_exec_runtime += delta_exec;
        schedstat_add(cfs_rq, exec_clock, delta_exec);
        delta_exec_weighted = delta_exec;
        if (unlikely(curr->load.weight != NICE_0_LOAD)) {
                delta_exec_weighted = calc_delta_fair(delta_exec_weighted,
                                                        &curr->load);
        }
        curr->vruntime += delta_exec_weighted;

        /*
         * maintain cfs_rq->min_vruntime to be a monotonic increasing
         * value tracking the leftmost vruntime in the tree.
         */
        if (first_fair(cfs_rq)) {
                vruntime = min_vruntime(curr->vruntime,
                                __pick_next_entity(cfs_rq)->vruntime);
        } else
                vruntime = curr->vruntime;

        cfs_rq->min_vruntime =
                max_vruntime(cfs_rq->min_vruntime, vruntime);
}

static void update_curr(struct cfs_rq *cfs_rq)
{
        struct sched_entity *curr = cfs_rq->curr;
        u64 now = rq_of(cfs_rq)->clock;
        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)(now - curr->exec_start);

        __update_curr(cfs_rq, curr, delta_exec);
        curr->exec_start = now;

        if (entity_is_task(curr)) {
                struct task_struct *curtask = task_of(curr);

                cpuacct_charge(curtask, delta_exec);
        }
}

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

/*
 * Task is being enqueued - update stats:
 */
static void update_stats_enqueue(struct cfs_rq *cfs_rq, struct sched_entity *se)
{
        /*
         * Are we enqueueing a waiting task? (for current tasks
         * a dequeue/enqueue event is a NOP)
         */
        if (se != cfs_rq->curr)
                update_stats_wait_start(cfs_rq, se);
}

static void
update_stats_wait_end(struct cfs_rq *cfs_rq, struct sched_entity *se)
{
        schedstat_set(se->wait_max, max(se->wait_max,
                        rq_of(cfs_rq)->clock - se->wait_start));
        schedstat_set(se->wait_start, 0);
}

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

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

static void
account_entity_enqueue(struct cfs_rq *cfs_rq, struct sched_entity *se)
{
        update_load_add(&cfs_rq->load, se->load.weight);
        cfs_rq->nr_running++;
        se->on_rq = 1;
}

static void
account_entity_dequeue(struct cfs_rq *cfs_rq, struct sched_entity *se)
{
        update_load_sub(&cfs_rq->load, se->load.weight);
        cfs_rq->nr_running--;
        se->on_rq = 0;
}

static void enqueue_sleeper(struct cfs_rq *cfs_rq, struct sched_entity *se)
{
#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;

                /*
                 * Blocking time is in units of nanosecs, so shift by 20 to
                 * get a milliseconds-range estimation of the amount of
                 * time that the task spent sleeping:
                 */
                if (unlikely(prof_on == SLEEP_PROFILING)) {
                        struct task_struct *tsk = task_of(se);

                        profile_hits(SLEEP_PROFILING, (void *)get_wchan(tsk),
                                     delta >> 20);
                }
        }
#endif
}

static void check_spread(struct cfs_rq *cfs_rq, struct sched_entity *se)
{
#ifdef CONFIG_SCHED_DEBUG
        s64 d = se->vruntime - cfs_rq->min_vruntime;

        if (d < 0)
                d = -d;

        if (d > 3*sysctl_sched_latency)
                schedstat_inc(cfs_rq, nr_spread_over);
#endif
}

static void
place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial)
{
        u64 vruntime;

        vruntime = cfs_rq->min_vruntime;

        if (sched_feat(TREE_AVG)) {
                struct sched_entity *last = __pick_last_entity(cfs_rq);
                if (last) {
                        vruntime += last->vruntime;
                        vruntime >>= 1;
                }
        } else if (sched_feat(APPROX_AVG) && cfs_rq->nr_running)
                vruntime += sched_vslice(cfs_rq)/2;

        /*
         * The 'current' period is already promised to the current tasks,
         * however the extra weight of the new task will slow them down a
         * little, place the new task so that it fits in the slot that
         * stays open at the end.
         */
        if (initial && sched_feat(START_DEBIT))
                vruntime += sched_vslice_add(cfs_rq, se);

        if (!initial) {
                /* sleeps upto a single latency don't count. */
                if (sched_feat(NEW_FAIR_SLEEPERS) && entity_is_task(se) &&
                                task_of(se)->policy != SCHED_BATCH)
                        vruntime -= sysctl_sched_latency;

                /* ensure we never gain time by being placed backwards. */
                vruntime = max_vruntime(se->vruntime, vruntime);
        }

        se->vruntime = vruntime;
}

static void
enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int wakeup)
{
        /*
         * Update run-time statistics of the 'current'.
         */
        update_curr(cfs_rq);

        if (wakeup) {
                place_entity(cfs_rq, se, 0);
                enqueue_sleeper(cfs_rq, se);
        }

        update_stats_enqueue(cfs_rq, se);
        check_spread(cfs_rq, se);
        if (se != cfs_rq->curr)
                __enqueue_entity(cfs_rq, se);
        account_entity_enqueue(cfs_rq, se);
}

static void
dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int sleep)
{
        /*
         * Update run-time statistics of the 'current'.
         */
        update_curr(cfs_rq);

        update_stats_dequeue(cfs_rq, se);
        if (sleep) {
#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
        }

        if (se != cfs_rq->curr)
                __dequeue_entity(cfs_rq, se);
        account_entity_dequeue(cfs_rq, se);
}

/*
 * Preempt the current task with a newly woken task if needed:
 */
static void
check_preempt_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr)
{
        unsigned long ideal_runtime, delta_exec;

        ideal_runtime = sched_slice(cfs_rq, curr);
        delta_exec = curr->sum_exec_runtime - curr->prev_sum_exec_runtime;
        if (delta_exec > ideal_runtime)
                resched_task(rq_of(cfs_rq)->curr);
}

static void
set_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *se)
{
        /* 'current' is not kept within the tree. */
        if (se->on_rq) {
                /*
                 * 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.
                 */
                update_stats_wait_end(cfs_rq, se);
                __dequeue_entity(cfs_rq, se);
        }

        update_stats_curr_start(cfs_rq, se);
        cfs_rq->curr = se;
#ifdef CONFIG_SCHEDSTATS
        /*
         * Track our maximum slice length, if the CPU's load is at
         * least twice that of our own weight (i.e. dont track it
         * when there are only lesser-weight tasks around):
         */
        if (rq_of(cfs_rq)->load.weight >= 2*se->load.weight) {
                se->slice_max = max(se->slice_max,
                        se->sum_exec_runtime - se->prev_sum_exec_runtime);
        }
#endif
        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 = NULL;

        if (first_fair(cfs_rq)) {
                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);

        check_spread(cfs_rq, prev);
        if (prev->on_rq) {
                update_stats_wait_start(cfs_rq, prev);
                /* Put 'current' back into the tree. */
                __enqueue_entity(cfs_rq, prev);
        }
        cfs_rq->curr = NULL;
}

static void entity_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr)
{
        /*
         * Update run-time statistics of the 'current'.
         */
        update_curr(cfs_rq);

        if (cfs_rq->nr_running > 1 || !sched_feat(WAKEUP_PREEMPT))
                check_preempt_tick(cfs_rq, curr);
}

/**************************************************
 * 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)
{
        return cfs_rq->tg->cfs_rq[this_cpu];
}

/* 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) entities belong to the same group ? */
static inline int
is_same_group(struct sched_entity *se, struct sched_entity *pse)
{
        if (se->cfs_rq == pse->cfs_rq)
                return 1;

        return 0;
}

static inline struct sched_entity *parent_entity(struct sched_entity *se)
{
        return se->parent;
}

#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 sched_entity *se, struct sched_entity *pse)
{
        return 1;
}

static inline struct sched_entity *parent_entity(struct sched_entity *se)
{
        return NULL;
}

#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);
                wakeup = 1;
        }
}

/*
 * 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;
                sleep = 1;
        }
}

/*
 * 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 *curr = rq->curr;
        struct cfs_rq *cfs_rq = task_cfs_rq(curr);
        struct sched_entity *rightmost, *se = &curr->se;

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

        if (likely(!sysctl_sched_compat_yield) && curr->policy != SCHED_BATCH) {
                __update_rq_clock(rq);
                /*
                 * Update run-time statistics of the 'current'.
                 */
                update_curr(cfs_rq);

                return;
        }
        /*
         * Find the rightmost entry in the rbtree:
         */
        rightmost = __pick_last_entity(cfs_rq);
        /*
         * Already in the rightmost position?
         */
        if (unlikely(rightmost->vruntime < se->vruntime))
                return;

        /*
         * Minimally necessary key value to be last in the tree:
         * Upon rescheduling, sched_class::put_prev_task() will place
         * 'current' within the tree based on its new key value.
         */
        se->vruntime = rightmost->vruntime + 1;
}

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

        if (unlikely(rt_prio(p->prio))) {
                update_rq_clock(rq);
                update_curr(cfs_rq);
                resched_task(curr);
                return;
        }
        /*
         * Batch tasks do not preempt (their preemption is driven by
         * the tick):
         */
        if (unlikely(p->policy == SCHED_BATCH))
                return;

        if (!sched_feat(WAKEUP_PREEMPT))
                return;

        while (!is_same_group(se, pse)) {
                se = parent_entity(se);
                pse = parent_entity(pse);
        }

        gran = sysctl_sched_wakeup_granularity;
        if (unlikely(se->load.weight != NICE_0_LOAD))
                gran = calc_delta_fair(gran, &se->load);

        if (pse->vruntime + gran < se->vruntime)
                resched_task(curr);
}

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);
        }
}

#ifdef CONFIG_SMP
/**************************************************
 * 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 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 = cfs_rq->curr;
        if (!curr)
                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_load_move,
                  struct sched_domain *sd, enum cpu_idle_type idle,
                  int *all_pinned, int *this_best_prio)
{
        struct cfs_rq *busy_cfs_rq;
        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;
                rem_load_move -= balance_tasks(this_rq, this_cpu, busiest,
                                               maxload, sd, idle, all_pinned,
                                               this_best_prio,
                                               &cfs_rq_iterator);

                if (rem_load_move <= 0)
                        break;
        }

        return max_load_move - rem_load_move;
}

static int
move_one_task_fair(struct rq *this_rq, int this_cpu, struct rq *busiest,
                   struct sched_domain *sd, enum cpu_idle_type idle)
{
        struct cfs_rq *busy_cfs_rq;
        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) {
                /*
                 * pass busy_cfs_rq argument into
                 * load_balance_[start|next]_fair iterators
                 */
                cfs_rq_iterator.arg = busy_cfs_rq;
                if (iter_move_one_task(this_rq, this_cpu, busiest, sd, idle,
                                       &cfs_rq_iterator))
                    return 1;
        }

        return 0;
}
#endif

/*
 * 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);
        }
}

#define swap(a, b) do { typeof(a) tmp = (a); (a) = (b); (b) = tmp; } while (0)

/*
 * 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;
        int this_cpu = smp_processor_id();

        sched_info_queued(p);

        update_curr(cfs_rq);
        place_entity(cfs_rq, se, 1);

        /* 'curr' will be NULL if the child belongs to a different group */
        if (sysctl_sched_child_runs_first && this_cpu == task_cpu(p) &&
                        curr && curr->vruntime < se->vruntime) {
                /*
                 * Upon rescheduling, sched_class::put_prev_task() will place
                 * 'current' within the tree based on its new key value.
                 */
                swap(curr->vruntime, se->vruntime);
        }

        enqueue_task_fair(rq, p, 0);
        resched_task(rq->curr);
}

/* 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);
}

/*
 * All the scheduling class methods:
 */
static const struct sched_class fair_sched_class = {
        .next                   = &idle_sched_class,
        .enqueue_task           = enqueue_task_fair,
        .dequeue_task           = dequeue_task_fair,
        .yield_task             = yield_task_fair,

        .check_preempt_curr     = check_preempt_wakeup,

        .pick_next_task         = pick_next_task_fair,
        .put_prev_task          = put_prev_task_fair,

#ifdef CONFIG_SMP
        .load_balance           = load_balance_fair,
        .move_one_task          = move_one_task_fair,
#endif

        .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;

#ifdef CONFIG_FAIR_GROUP_SCHED
        print_cfs_rq(m, cpu, &cpu_rq(cpu)->cfs);
#endif
        for_each_leaf_cfs_rq(cpu_rq(cpu), cfs_rq)
                print_cfs_rq(m, cpu, cfs_rq);
}
#endif