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    Subject[PATCH 3/5] sched: replace migration_thread with cpu_stop
    Date
    Currently migration_thread is serving three purposes - migration
    pusher, context to execute active_load_balance() and forced context
    switcher for expedited RCU synchronize_sched. All three roles are
    hardcoded into migration_thread() and determining which job is
    scheduled is slightly messy.

    This patch kills migration_thread and replaces all three uses with
    cpu_stop. The three different roles of migration_thread() are
    splitted into three separate cpu_stop callbacks -
    migration_cpu_stop(), active_load_balance_cpu_stop() and
    synchronize_sched_expedited_cpu_stop() - and each use case now simply
    asks cpu_stop to execute the callback as necessary.

    synchronize_sched_expedited() was implemented with private
    preallocated resources and custom multi-cpu queueing and waiting
    logic, both of which are provided by cpu_stop.
    synchronize_sched_expedited_count is made atomic and all other shared
    resources along with the mutex are dropped.

    synchronize_sched_expedited() also implemented a check to detect cases
    where not all the callback got executed on their assigned cpus and
    fall back to synchronize_sched(). If called with cpu hotplug blocked,
    cpu_stop already guarantees that and the condition cannot happen;
    otherwise, stop_machine() would break. However, this patch preserves
    the paranoid check using a cpumask to record on which cpus the stopper
    ran so that it can serve as a bisection point if something actually
    goes wrong theree.

    Because the internal execution state is no longer visible,
    rcu_expedited_torture_stats() is removed.

    This patch also renames cpu_stop threads to from "stopper/%d" to
    "migration/%d". The names of these threads ultimately don't matter
    and there's no reason to make unnecessary userland visible changes.

    With this patch applied, stop_machine() and sched now share the same
    resources. stop_machine() is faster without wasting any resources and
    sched migration users are much cleaner.

    Signed-off-by: Tejun Heo <tj@kernel.org>
    Acked-by: Peter Zijlstra <peterz@infradead.org>
    Cc: Ingo Molnar <mingo@elte.hu>
    Cc: Dipankar Sarma <dipankar@in.ibm.com>
    Cc: Josh Triplett <josh@freedesktop.org>
    Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Cc: Oleg Nesterov <oleg@redhat.com>
    Cc: Dimitri Sivanich <sivanich@sgi.com>
    ---
    Documentation/RCU/torture.txt | 10 --
    include/linux/rcutiny.h | 2 -
    include/linux/rcutree.h | 1 -
    kernel/rcutorture.c | 2 +-
    kernel/sched.c | 315 ++++++++++++-----------------------------
    kernel/sched_fair.c | 48 +++++--
    kernel/stop_machine.c | 2 +-
    7 files changed, 127 insertions(+), 253 deletions(-)

    diff --git a/Documentation/RCU/torture.txt b/Documentation/RCU/torture.txt
    index 0e50bc2..5d90167 100644
    --- a/Documentation/RCU/torture.txt
    +++ b/Documentation/RCU/torture.txt
    @@ -182,16 +182,6 @@ Similarly, sched_expedited RCU provides the following:
    sched_expedited-torture: Reader Pipe: 12660320201 95875 0 0 0 0 0 0 0 0 0
    sched_expedited-torture: Reader Batch: 12660424885 0 0 0 0 0 0 0 0 0 0
    sched_expedited-torture: Free-Block Circulation: 1090795 1090795 1090794 1090793 1090792 1090791 1090790 1090789 1090788 1090787 0
    - state: -1 / 0:0 3:0 4:0
    -
    -As before, the first four lines are similar to those for RCU.
    -The last line shows the task-migration state. The first number is
    --1 if synchronize_sched_expedited() is idle, -2 if in the process of
    -posting wakeups to the migration kthreads, and N when waiting on CPU N.
    -Each of the colon-separated fields following the "/" is a CPU:state pair.
    -Valid states are "0" for idle, "1" for waiting for quiescent state,
    -"2" for passed through quiescent state, and "3" when a race with a
    -CPU-hotplug event forces use of the synchronize_sched() primitive.


    USAGE
    diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
    index a519587..0006b2d 100644
    --- a/include/linux/rcutiny.h
    +++ b/include/linux/rcutiny.h
    @@ -60,8 +60,6 @@ static inline long rcu_batches_completed_bh(void)
    return 0;
    }

    -extern int rcu_expedited_torture_stats(char *page);
    -
    static inline void rcu_force_quiescent_state(void)
    {
    }
    diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h
    index 42cc3a0..24e467e 100644
    --- a/include/linux/rcutree.h
    +++ b/include/linux/rcutree.h
    @@ -35,7 +35,6 @@ struct notifier_block;
    extern void rcu_sched_qs(int cpu);
    extern void rcu_bh_qs(int cpu);
    extern int rcu_needs_cpu(int cpu);
    -extern int rcu_expedited_torture_stats(char *page);

    #ifdef CONFIG_TREE_PREEMPT_RCU

    diff --git a/kernel/rcutorture.c b/kernel/rcutorture.c
    index 58df55b..2b676f3 100644
    --- a/kernel/rcutorture.c
    +++ b/kernel/rcutorture.c
    @@ -669,7 +669,7 @@ static struct rcu_torture_ops sched_expedited_ops = {
    .sync = synchronize_sched_expedited,
    .cb_barrier = NULL,
    .fqs = rcu_sched_force_quiescent_state,
    - .stats = rcu_expedited_torture_stats,
    + .stats = NULL,
    .irq_capable = 1,
    .name = "sched_expedited"
    };
    diff --git a/kernel/sched.c b/kernel/sched.c
    index 4956ed0..f1d577a 100644
    --- a/kernel/sched.c
    +++ b/kernel/sched.c
    @@ -55,9 +55,9 @@
    #include <linux/cpu.h>
    #include <linux/cpuset.h>
    #include <linux/percpu.h>
    -#include <linux/kthread.h>
    #include <linux/proc_fs.h>
    #include <linux/seq_file.h>
    +#include <linux/stop_machine.h>
    #include <linux/sysctl.h>
    #include <linux/syscalls.h>
    #include <linux/times.h>
    @@ -539,15 +539,13 @@ struct rq {
    int post_schedule;
    int active_balance;
    int push_cpu;
    + struct cpu_stop_work active_balance_work;
    /* cpu of this runqueue: */
    int cpu;
    int online;

    unsigned long avg_load_per_task;

    - struct task_struct *migration_thread;
    - struct list_head migration_queue;
    -
    u64 rt_avg;
    u64 age_stamp;
    u64 idle_stamp;
    @@ -2037,21 +2035,18 @@ void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
    __set_task_cpu(p, new_cpu);
    }

    -struct migration_req {
    - struct list_head list;
    -
    +struct migration_arg {
    struct task_struct *task;
    int dest_cpu;
    -
    - struct completion done;
    };

    +static int migration_cpu_stop(void *data);
    +
    /*
    * The task's runqueue lock must be held.
    * Returns true if you have to wait for migration thread.
    */
    -static int
    -migrate_task(struct task_struct *p, int dest_cpu, struct migration_req *req)
    +static bool migrate_task(struct task_struct *p, int dest_cpu)
    {
    struct rq *rq = task_rq(p);

    @@ -2059,15 +2054,7 @@ migrate_task(struct task_struct *p, int dest_cpu, struct migration_req *req)
    * If the task is not on a runqueue (and not running), then
    * the next wake-up will properly place the task.
    */
    - if (!p->se.on_rq && !task_running(rq, p))
    - return 0;
    -
    - init_completion(&req->done);
    - req->task = p;
    - req->dest_cpu = dest_cpu;
    - list_add(&req->list, &rq->migration_queue);
    -
    - return 1;
    + return p->se.on_rq || task_running(rq, p);
    }

    /*
    @@ -3110,7 +3097,6 @@ static void update_cpu_load(struct rq *this_rq)
    void sched_exec(void)
    {
    struct task_struct *p = current;
    - struct migration_req req;
    unsigned long flags;
    struct rq *rq;
    int dest_cpu;
    @@ -3124,17 +3110,11 @@ void sched_exec(void)
    * select_task_rq() can race against ->cpus_allowed
    */
    if (cpumask_test_cpu(dest_cpu, &p->cpus_allowed) &&
    - likely(cpu_active(dest_cpu)) &&
    - migrate_task(p, dest_cpu, &req)) {
    - /* Need to wait for migration thread (might exit: take ref). */
    - struct task_struct *mt = rq->migration_thread;
    + likely(cpu_active(dest_cpu)) && migrate_task(p, dest_cpu)) {
    + struct migration_arg arg = { p, dest_cpu };

    - get_task_struct(mt);
    task_rq_unlock(rq, &flags);
    - wake_up_process(mt);
    - put_task_struct(mt);
    - wait_for_completion(&req.done);
    -
    + stop_one_cpu(cpu_of(rq), migration_cpu_stop, &arg);
    return;
    }
    unlock:
    @@ -5290,17 +5270,15 @@ static inline void sched_init_granularity(void)
    /*
    * This is how migration works:
    *
    - * 1) we queue a struct migration_req structure in the source CPU's
    - * runqueue and wake up that CPU's migration thread.
    - * 2) we down() the locked semaphore => thread blocks.
    - * 3) migration thread wakes up (implicitly it forces the migrated
    - * thread off the CPU)
    - * 4) it gets the migration request and checks whether the migrated
    - * task is still in the wrong runqueue.
    - * 5) if it's in the wrong runqueue then the migration thread removes
    + * 1) we invoke migration_cpu_stop() on the target CPU using
    + * stop_one_cpu().
    + * 2) stopper starts to run (implicitly forcing the migrated thread
    + * off the CPU)
    + * 3) it checks whether the migrated task is still in the wrong runqueue.
    + * 4) if it's in the wrong runqueue then the migration thread removes
    * it and puts it into the right queue.
    - * 6) migration thread up()s the semaphore.
    - * 7) we wake up and the migration is done.
    + * 5) stopper completes and stop_one_cpu() returns and the migration
    + * is done.
    */

    /*
    @@ -5314,9 +5292,9 @@ static inline void sched_init_granularity(void)
    */
    int set_cpus_allowed_ptr(struct task_struct *p, const struct cpumask *new_mask)
    {
    - struct migration_req req;
    unsigned long flags;
    struct rq *rq;
    + unsigned int dest_cpu;
    int ret = 0;

    /*
    @@ -5354,15 +5332,12 @@ again:
    if (cpumask_test_cpu(task_cpu(p), new_mask))
    goto out;

    - if (migrate_task(p, cpumask_any_and(cpu_active_mask, new_mask), &req)) {
    + dest_cpu = cpumask_any_and(cpu_active_mask, new_mask);
    + if (migrate_task(p, dest_cpu)) {
    + struct migration_arg arg = { p, dest_cpu };
    /* Need help from migration thread: drop lock and wait. */
    - struct task_struct *mt = rq->migration_thread;
    -
    - get_task_struct(mt);
    task_rq_unlock(rq, &flags);
    - wake_up_process(mt);
    - put_task_struct(mt);
    - wait_for_completion(&req.done);
    + stop_one_cpu(cpu_of(rq), migration_cpu_stop, &arg);
    tlb_migrate_finish(p->mm);
    return 0;
    }
    @@ -5420,70 +5395,22 @@ fail:
    return ret;
    }

    -#define RCU_MIGRATION_IDLE 0
    -#define RCU_MIGRATION_NEED_QS 1
    -#define RCU_MIGRATION_GOT_QS 2
    -#define RCU_MIGRATION_MUST_SYNC 3
    -
    /*
    - * migration_thread - this is a highprio system thread that performs
    - * thread migration by bumping thread off CPU then 'pushing' onto
    - * another runqueue.
    + * migration_cpu_stop - this will be executed by a highprio stopper thread
    + * and performs thread migration by bumping thread off CPU then
    + * 'pushing' onto another runqueue.
    */
    -static int migration_thread(void *data)
    +static int migration_cpu_stop(void *data)
    {
    - int badcpu;
    - int cpu = (long)data;
    - struct rq *rq;
    -
    - rq = cpu_rq(cpu);
    - BUG_ON(rq->migration_thread != current);
    -
    - set_current_state(TASK_INTERRUPTIBLE);
    - while (!kthread_should_stop()) {
    - struct migration_req *req;
    - struct list_head *head;
    -
    - raw_spin_lock_irq(&rq->lock);
    -
    - if (cpu_is_offline(cpu)) {
    - raw_spin_unlock_irq(&rq->lock);
    - break;
    - }
    -
    - if (rq->active_balance) {
    - active_load_balance(rq, cpu);
    - rq->active_balance = 0;
    - }
    -
    - head = &rq->migration_queue;
    -
    - if (list_empty(head)) {
    - raw_spin_unlock_irq(&rq->lock);
    - schedule();
    - set_current_state(TASK_INTERRUPTIBLE);
    - continue;
    - }
    - req = list_entry(head->next, struct migration_req, list);
    - list_del_init(head->next);
    -
    - if (req->task != NULL) {
    - raw_spin_unlock(&rq->lock);
    - __migrate_task(req->task, cpu, req->dest_cpu);
    - } else if (likely(cpu == (badcpu = smp_processor_id()))) {
    - req->dest_cpu = RCU_MIGRATION_GOT_QS;
    - raw_spin_unlock(&rq->lock);
    - } else {
    - req->dest_cpu = RCU_MIGRATION_MUST_SYNC;
    - raw_spin_unlock(&rq->lock);
    - WARN_ONCE(1, "migration_thread() on CPU %d, expected %d\n", badcpu, cpu);
    - }
    - local_irq_enable();
    -
    - complete(&req->done);
    - }
    - __set_current_state(TASK_RUNNING);
    + struct migration_arg *arg = data;

    + /*
    + * The original target cpu might have gone down and we might
    + * be on another cpu but it doesn't matter.
    + */
    + local_irq_disable();
    + __migrate_task(arg->task, raw_smp_processor_id(), arg->dest_cpu);
    + local_irq_enable();
    return 0;
    }

    @@ -5850,35 +5777,20 @@ static void set_rq_offline(struct rq *rq)
    static int __cpuinit
    migration_call(struct notifier_block *nfb, unsigned long action, void *hcpu)
    {
    - struct task_struct *p;
    int cpu = (long)hcpu;
    unsigned long flags;
    - struct rq *rq;
    + struct rq *rq = cpu_rq(cpu);

    switch (action) {

    case CPU_UP_PREPARE:
    case CPU_UP_PREPARE_FROZEN:
    - p = kthread_create(migration_thread, hcpu, "migration/%d", cpu);
    - if (IS_ERR(p))
    - return NOTIFY_BAD;
    - kthread_bind(p, cpu);
    - /* Must be high prio: stop_machine expects to yield to it. */
    - rq = task_rq_lock(p, &flags);
    - __setscheduler(rq, p, SCHED_FIFO, MAX_RT_PRIO-1);
    - task_rq_unlock(rq, &flags);
    - get_task_struct(p);
    - cpu_rq(cpu)->migration_thread = p;
    rq->calc_load_update = calc_load_update;
    break;

    case CPU_ONLINE:
    case CPU_ONLINE_FROZEN:
    - /* Strictly unnecessary, as first user will wake it. */
    - wake_up_process(cpu_rq(cpu)->migration_thread);
    -
    /* Update our root-domain */
    - rq = cpu_rq(cpu);
    raw_spin_lock_irqsave(&rq->lock, flags);
    if (rq->rd) {
    BUG_ON(!cpumask_test_cpu(cpu, rq->rd->span));
    @@ -5889,25 +5801,9 @@ migration_call(struct notifier_block *nfb, unsigned long action, void *hcpu)
    break;

    #ifdef CONFIG_HOTPLUG_CPU
    - case CPU_UP_CANCELED:
    - case CPU_UP_CANCELED_FROZEN:
    - if (!cpu_rq(cpu)->migration_thread)
    - break;
    - /* Unbind it from offline cpu so it can run. Fall thru. */
    - kthread_bind(cpu_rq(cpu)->migration_thread,
    - cpumask_any(cpu_online_mask));
    - kthread_stop(cpu_rq(cpu)->migration_thread);
    - put_task_struct(cpu_rq(cpu)->migration_thread);
    - cpu_rq(cpu)->migration_thread = NULL;
    - break;
    -
    case CPU_DEAD:
    case CPU_DEAD_FROZEN:
    migrate_live_tasks(cpu);
    - rq = cpu_rq(cpu);
    - kthread_stop(rq->migration_thread);
    - put_task_struct(rq->migration_thread);
    - rq->migration_thread = NULL;
    /* Idle task back to normal (off runqueue, low prio) */
    raw_spin_lock_irq(&rq->lock);
    deactivate_task(rq, rq->idle, 0);
    @@ -5918,29 +5814,11 @@ migration_call(struct notifier_block *nfb, unsigned long action, void *hcpu)
    migrate_nr_uninterruptible(rq);
    BUG_ON(rq->nr_running != 0);
    calc_global_load_remove(rq);
    - /*
    - * No need to migrate the tasks: it was best-effort if
    - * they didn't take sched_hotcpu_mutex. Just wake up
    - * the requestors.
    - */
    - raw_spin_lock_irq(&rq->lock);
    - while (!list_empty(&rq->migration_queue)) {
    - struct migration_req *req;
    -
    - req = list_entry(rq->migration_queue.next,
    - struct migration_req, list);
    - list_del_init(&req->list);
    - raw_spin_unlock_irq(&rq->lock);
    - complete(&req->done);
    - raw_spin_lock_irq(&rq->lock);
    - }
    - raw_spin_unlock_irq(&rq->lock);
    break;

    case CPU_DYING:
    case CPU_DYING_FROZEN:
    /* Update our root-domain */
    - rq = cpu_rq(cpu);
    raw_spin_lock_irqsave(&rq->lock, flags);
    if (rq->rd) {
    BUG_ON(!cpumask_test_cpu(cpu, rq->rd->span));
    @@ -7757,10 +7635,8 @@ void __init sched_init(void)
    rq->push_cpu = 0;
    rq->cpu = i;
    rq->online = 0;
    - rq->migration_thread = NULL;
    rq->idle_stamp = 0;
    rq->avg_idle = 2*sysctl_sched_migration_cost;
    - INIT_LIST_HEAD(&rq->migration_queue);
    rq_attach_root(rq, &def_root_domain);
    #endif
    init_rq_hrtick(rq);
    @@ -9054,43 +8930,39 @@ struct cgroup_subsys cpuacct_subsys = {

    #ifndef CONFIG_SMP

    -int rcu_expedited_torture_stats(char *page)
    -{
    - return 0;
    -}
    -EXPORT_SYMBOL_GPL(rcu_expedited_torture_stats);
    -
    void synchronize_sched_expedited(void)
    {
    + /*
    + * There must be a full memory barrier on each affected CPU
    + * between the time that try_stop_cpus() is called and the
    + * time that it returns.
    + *
    + * In the current initial implementation of cpu_stop, the
    + * above condition is already met when the control reaches
    + * this point and the following smp_mb() is not strictly
    + * necessary. Do smp_mb() anyway for documentation and
    + * robustness against future implementation changes.
    + */
    + smp_mb();
    }
    EXPORT_SYMBOL_GPL(synchronize_sched_expedited);

    #else /* #ifndef CONFIG_SMP */

    -static DEFINE_PER_CPU(struct migration_req, rcu_migration_req);
    -static DEFINE_MUTEX(rcu_sched_expedited_mutex);
    -
    -#define RCU_EXPEDITED_STATE_POST -2
    -#define RCU_EXPEDITED_STATE_IDLE -1
    +static atomic_t synchronize_sched_expedited_count = ATOMIC_INIT(0);

    -static int rcu_expedited_state = RCU_EXPEDITED_STATE_IDLE;
    -
    -int rcu_expedited_torture_stats(char *page)
    +static int synchronize_sched_expedited_cpu_stop(void *data)
    {
    - int cnt = 0;
    - int cpu;
    + static DEFINE_SPINLOCK(done_mask_lock);
    + struct cpumask *done_mask = data;

    - cnt += sprintf(&page[cnt], "state: %d /", rcu_expedited_state);
    - for_each_online_cpu(cpu) {
    - cnt += sprintf(&page[cnt], " %d:%d",
    - cpu, per_cpu(rcu_migration_req, cpu).dest_cpu);
    + if (done_mask) {
    + spin_lock(&done_mask_lock);
    + cpumask_set_cpu(smp_processor_id(), done_mask);
    + spin_unlock(&done_mask_lock);
    }
    - cnt += sprintf(&page[cnt], "\n");
    - return cnt;
    + return 0;
    }
    -EXPORT_SYMBOL_GPL(rcu_expedited_torture_stats);
    -
    -static long synchronize_sched_expedited_count;

    /*
    * Wait for an rcu-sched grace period to elapse, but use "big hammer"
    @@ -9104,60 +8976,55 @@ static long synchronize_sched_expedited_count;
    */
    void synchronize_sched_expedited(void)
    {
    - int cpu;
    - unsigned long flags;
    - bool need_full_sync = 0;
    - struct rq *rq;
    - struct migration_req *req;
    - long snap;
    - int trycount = 0;
    + cpumask_var_t done_mask_var;
    + struct cpumask *done_mask = NULL;
    + int snap, trycount = 0;
    +
    + /*
    + * done_mask is used to check that all cpus actually have
    + * finished running the stopper, which is guaranteed by
    + * stop_cpus() if it's called with cpu hotplug blocked. Keep
    + * the paranoia for now but it's best effort if cpumask is off
    + * stack.
    + */
    + if (zalloc_cpumask_var(&done_mask_var, GFP_ATOMIC))
    + done_mask = done_mask_var;

    smp_mb(); /* ensure prior mod happens before capturing snap. */
    - snap = ACCESS_ONCE(synchronize_sched_expedited_count) + 1;
    + snap = atomic_read(&synchronize_sched_expedited_count) + 1;
    get_online_cpus();
    - while (!mutex_trylock(&rcu_sched_expedited_mutex)) {
    + while (try_stop_cpus(cpu_online_mask,
    + synchronize_sched_expedited_cpu_stop,
    + done_mask) == -EAGAIN) {
    put_online_cpus();
    if (trycount++ < 10)
    udelay(trycount * num_online_cpus());
    else {
    synchronize_sched();
    - return;
    + goto free_out;
    }
    - if (ACCESS_ONCE(synchronize_sched_expedited_count) - snap > 0) {
    + if (atomic_read(&synchronize_sched_expedited_count) - snap > 0) {
    smp_mb(); /* ensure test happens before caller kfree */
    - return;
    + goto free_out;
    }
    get_online_cpus();
    }
    - rcu_expedited_state = RCU_EXPEDITED_STATE_POST;
    - for_each_online_cpu(cpu) {
    - rq = cpu_rq(cpu);
    - req = &per_cpu(rcu_migration_req, cpu);
    - init_completion(&req->done);
    - req->task = NULL;
    - req->dest_cpu = RCU_MIGRATION_NEED_QS;
    - raw_spin_lock_irqsave(&rq->lock, flags);
    - list_add(&req->list, &rq->migration_queue);
    - raw_spin_unlock_irqrestore(&rq->lock, flags);
    - wake_up_process(rq->migration_thread);
    - }
    - for_each_online_cpu(cpu) {
    - rcu_expedited_state = cpu;
    - req = &per_cpu(rcu_migration_req, cpu);
    - rq = cpu_rq(cpu);
    - wait_for_completion(&req->done);
    - raw_spin_lock_irqsave(&rq->lock, flags);
    - if (unlikely(req->dest_cpu == RCU_MIGRATION_MUST_SYNC))
    - need_full_sync = 1;
    - req->dest_cpu = RCU_MIGRATION_IDLE;
    - raw_spin_unlock_irqrestore(&rq->lock, flags);
    - }
    - rcu_expedited_state = RCU_EXPEDITED_STATE_IDLE;
    - synchronize_sched_expedited_count++;
    - mutex_unlock(&rcu_sched_expedited_mutex);
    + atomic_inc(&synchronize_sched_expedited_count);
    + if (done_mask)
    + cpumask_xor(done_mask, done_mask, cpu_online_mask);
    put_online_cpus();
    - if (need_full_sync)
    +
    + /* paranoia - this can't happen */
    + if (done_mask && cpumask_weight(done_mask)) {
    + char buf[80];
    +
    + cpulist_scnprintf(buf, sizeof(buf), done_mask);
    + WARN_ONCE(1, "synchronize_sched_expedited: cpu online and done masks disagree on %d cpus: %s\n",
    + cpumask_weight(done_mask), buf);
    synchronize_sched();
    + }
    +free_out:
    + free_cpumask_var(done_mask_var);
    }
    EXPORT_SYMBOL_GPL(synchronize_sched_expedited);

    diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
    index cbd8b8a..217e4a9 100644
    --- a/kernel/sched_fair.c
    +++ b/kernel/sched_fair.c
    @@ -2798,6 +2798,8 @@ static int need_active_balance(struct sched_domain *sd, int sd_idle, int idle)
    return unlikely(sd->nr_balance_failed > sd->cache_nice_tries+2);
    }

    +static int active_load_balance_cpu_stop(void *data);
    +
    /*
    * Check this_cpu to ensure it is balanced within domain. Attempt to move
    * tasks if there is an imbalance.
    @@ -2887,8 +2889,9 @@ redo:
    if (need_active_balance(sd, sd_idle, idle)) {
    raw_spin_lock_irqsave(&busiest->lock, flags);

    - /* don't kick the migration_thread, if the curr
    - * task on busiest cpu can't be moved to this_cpu
    + /* don't kick the active_load_balance_cpu_stop,
    + * if the curr task on busiest cpu can't be
    + * moved to this_cpu
    */
    if (!cpumask_test_cpu(this_cpu,
    &busiest->curr->cpus_allowed)) {
    @@ -2898,14 +2901,22 @@ redo:
    goto out_one_pinned;
    }

    + /*
    + * ->active_balance synchronizes accesses to
    + * ->active_balance_work. Once set, it's cleared
    + * only after active load balance is finished.
    + */
    if (!busiest->active_balance) {
    busiest->active_balance = 1;
    busiest->push_cpu = this_cpu;
    active_balance = 1;
    }
    raw_spin_unlock_irqrestore(&busiest->lock, flags);
    +
    if (active_balance)
    - wake_up_process(busiest->migration_thread);
    + stop_one_cpu_nowait(cpu_of(busiest),
    + active_load_balance_cpu_stop, busiest,
    + &busiest->active_balance_work);

    /*
    * We've kicked active balancing, reset the failure
    @@ -3012,24 +3023,29 @@ static void idle_balance(int this_cpu, struct rq *this_rq)
    }

    /*
    - * active_load_balance is run by migration threads. It pushes running tasks
    - * off the busiest CPU onto idle CPUs. It requires at least 1 task to be
    - * running on each physical CPU where possible, and avoids physical /
    - * logical imbalances.
    - *
    - * Called with busiest_rq locked.
    + * active_load_balance_cpu_stop is run by cpu stopper. It pushes
    + * running tasks off the busiest CPU onto idle CPUs. It requires at
    + * least 1 task to be running on each physical CPU where possible, and
    + * avoids physical / logical imbalances.
    */
    -static void active_load_balance(struct rq *busiest_rq, int busiest_cpu)
    +static int active_load_balance_cpu_stop(void *data)
    {
    + struct rq *busiest_rq = data;
    + int busiest_cpu = cpu_of(busiest_rq);
    int target_cpu = busiest_rq->push_cpu;
    + struct rq *target_rq = cpu_rq(target_cpu);
    struct sched_domain *sd;
    - struct rq *target_rq;
    +
    + raw_spin_lock_irq(&busiest_rq->lock);
    +
    + /* make sure the requested cpu hasn't gone down in the meantime */
    + if (unlikely(busiest_cpu != smp_processor_id() ||
    + !busiest_rq->active_balance))
    + goto out_unlock;

    /* Is there any task to move? */
    if (busiest_rq->nr_running <= 1)
    - return;
    -
    - target_rq = cpu_rq(target_cpu);
    + goto out_unlock;

    /*
    * This condition is "impossible", if it occurs
    @@ -3058,6 +3074,10 @@ static void active_load_balance(struct rq *busiest_rq, int busiest_cpu)
    schedstat_inc(sd, alb_failed);
    }
    double_unlock_balance(busiest_rq, target_rq);
    +out_unlock:
    + busiest_rq->active_balance = 0;
    + raw_spin_unlock_irq(&busiest_rq->lock);
    + return 0;
    }

    #ifdef CONFIG_NO_HZ
    diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
    index 884c7a1..5b20141 100644
    --- a/kernel/stop_machine.c
    +++ b/kernel/stop_machine.c
    @@ -301,7 +301,7 @@ static int __cpuinit cpu_stop_cpu_callback(struct notifier_block *nfb,
    case CPU_UP_PREPARE:
    BUG_ON(stopper->thread || stopper->enabled ||
    !list_empty(&stopper->works));
    - p = kthread_create(cpu_stopper_thread, stopper, "stopper/%d",
    + p = kthread_create(cpu_stopper_thread, stopper, "migration/%d",
    cpu);
    if (IS_ERR(p))
    return NOTIFY_BAD;
    --
    1.6.4.2


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