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    Subject[RFC][PATCH 11/18] sched: Add p->pi_lock to task_rq_lock()
    In order to be able to call set_task_cpu() while either holding
    p->pi_lock or task_rq(p)->lock we need to hold both locks in order to
    stabilize task_rq().

    This makes task_rq_lock() acquire both locks, and have
    __task_rq_lock() validate that p->pi_lock is held. This increases the
    locking overhead for most scheduler syscalls but allows reduction of
    rq->lock contention for some scheduler hot paths (ttwu).

    Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
    ---
    kernel/sched.c | 81 ++++++++++++++++++++++++++-------------------------------
    1 file changed, 37 insertions(+), 44 deletions(-)

    Index: linux-2.6/kernel/sched.c
    ===================================================================
    --- linux-2.6.orig/kernel/sched.c
    +++ linux-2.6/kernel/sched.c
    @@ -602,7 +602,7 @@ static inline int cpu_of(struct rq *rq)
    * Return the group to which this tasks belongs.
    *
    * We use task_subsys_state_check() and extend the RCU verification
    - * with lockdep_is_held(&task_rq(p)->lock) because cpu_cgroup_attach()
    + * with lockdep_is_held(&p->pi_lock) because cpu_cgroup_attach()
    * holds that lock for each task it moves into the cgroup. Therefore
    * by holding that lock, we pin the task to the current cgroup.
    */
    @@ -612,7 +612,7 @@ static inline struct task_group *task_gr
    struct cgroup_subsys_state *css;

    css = task_subsys_state_check(p, cpu_cgroup_subsys_id,
    - lockdep_is_held(&task_rq(p)->lock));
    + lockdep_is_held(&p->pi_lock));
    tg = container_of(css, struct task_group, css);

    return autogroup_task_group(p, tg);
    @@ -928,23 +928,15 @@ static inline void finish_lock_switch(st
    #endif /* __ARCH_WANT_UNLOCKED_CTXSW */

    /*
    - * Check whether the task is waking, we use this to synchronize ->cpus_allowed
    - * against ttwu().
    - */
    -static inline int task_is_waking(struct task_struct *p)
    -{
    - return unlikely(p->state == TASK_WAKING);
    -}
    -
    -/*
    - * __task_rq_lock - lock the runqueue a given task resides on.
    - * Must be called interrupts disabled.
    + * __task_rq_lock - lock the rq @p resides on.
    */
    static inline struct rq *__task_rq_lock(struct task_struct *p)
    __acquires(rq->lock)
    {
    struct rq *rq;

    + lockdep_assert_held(&p->pi_lock);
    +
    for (;;) {
    rq = task_rq(p);
    raw_spin_lock(&rq->lock);
    @@ -955,22 +947,22 @@ static inline struct rq *__task_rq_lock(
    }

    /*
    - * task_rq_lock - lock the runqueue a given task resides on and disable
    - * interrupts. Note the ordering: we can safely lookup the task_rq without
    - * explicitly disabling preemption.
    + * task_rq_lock - lock p->pi_lock and lock the rq @p resides on.
    */
    static struct rq *task_rq_lock(struct task_struct *p, unsigned long *flags)
    + __acquires(p->pi_lock)
    __acquires(rq->lock)
    {
    struct rq *rq;

    for (;;) {
    - local_irq_save(*flags);
    + raw_spin_lock_irqsave(&p->pi_lock, *flags);
    rq = task_rq(p);
    raw_spin_lock(&rq->lock);
    if (likely(rq == task_rq(p)))
    return rq;
    - raw_spin_unlock_irqrestore(&rq->lock, *flags);
    + raw_spin_unlock(&rq->lock);
    + raw_spin_unlock_irqrestore(&p->pi_lock, *flags);
    }
    }

    @@ -980,10 +972,13 @@ static void __task_rq_unlock(struct rq *
    raw_spin_unlock(&rq->lock);
    }

    -static inline void task_rq_unlock(struct rq *rq, unsigned long *flags)
    +static inline void
    +task_rq_unlock(struct rq *rq, struct task_struct *p, unsigned long *flags)
    __releases(rq->lock)
    + __releases(p->pi_lock)
    {
    - raw_spin_unlock_irqrestore(&rq->lock, *flags);
    + raw_spin_unlock(&rq->lock);
    + raw_spin_unlock_irqrestore(&p->pi_lock, *flags);
    }

    /*
    @@ -2115,6 +2110,11 @@ void set_task_cpu(struct task_struct *p,
    */
    WARN_ON_ONCE(p->state != TASK_RUNNING && p->state != TASK_WAKING &&
    !(task_thread_info(p)->preempt_count & PREEMPT_ACTIVE));
    +
    +#ifdef CONFIG_LOCKDEP
    + WARN_ON_ONCE(debug_locks && !(lockdep_is_held(&p->pi_lock) ||
    + lockdep_is_held(&task_rq(p)->lock)));
    +#endif
    #endif

    trace_sched_migrate_task(p, new_cpu);
    @@ -2210,7 +2210,7 @@ unsigned long wait_task_inactive(struct
    ncsw = 0;
    if (!match_state || p->state == match_state)
    ncsw = p->nvcsw | LONG_MIN; /* sets MSB */
    - task_rq_unlock(rq, &flags);
    + task_rq_unlock(rq, p, &flags);

    /*
    * If it changed from the expected state, bail out now.
    @@ -2596,6 +2596,7 @@ static void __sched_fork(struct task_str
    */
    void sched_fork(struct task_struct *p, int clone_flags)
    {
    + unsigned long flags;
    int cpu = get_cpu();

    __sched_fork(p);
    @@ -2646,9 +2647,9 @@ void sched_fork(struct task_struct *p, i
    *
    * Silence PROVE_RCU.
    */
    - rcu_read_lock();
    + raw_spin_lock_irqsave(&p->pi_lock, flags);
    set_task_cpu(p, cpu);
    - rcu_read_unlock();
    + raw_spin_unlock_irqrestore(&p->pi_lock, flags);

    #if defined(CONFIG_SCHEDSTATS) || defined(CONFIG_TASK_DELAY_ACCT)
    if (likely(sched_info_on()))
    @@ -3472,7 +3473,7 @@ unsigned long long task_delta_exec(struc

    rq = task_rq_lock(p, &flags);
    ns = do_task_delta_exec(p, rq);
    - task_rq_unlock(rq, &flags);
    + task_rq_unlock(rq, p, &flags);

    return ns;
    }
    @@ -3490,7 +3491,7 @@ unsigned long long task_sched_runtime(st

    rq = task_rq_lock(p, &flags);
    ns = p->se.sum_exec_runtime + do_task_delta_exec(p, rq);
    - task_rq_unlock(rq, &flags);
    + task_rq_unlock(rq, p, &flags);

    return ns;
    }
    @@ -3514,7 +3515,7 @@ unsigned long long thread_group_sched_ru
    rq = task_rq_lock(p, &flags);
    thread_group_cputime(p, &totals);
    ns = totals.sum_exec_runtime + do_task_delta_exec(p, rq);
    - task_rq_unlock(rq, &flags);
    + task_rq_unlock(rq, p, &flags);

    return ns;
    }
    @@ -4538,16 +4539,13 @@ EXPORT_SYMBOL(sleep_on_timeout);
    */
    void rt_mutex_setprio(struct task_struct *p, int prio)
    {
    - unsigned long flags;
    int oldprio, on_rq, running;
    struct rq *rq;
    const struct sched_class *prev_class;

    BUG_ON(prio < 0 || prio > MAX_PRIO);

    - lockdep_assert_held(&p->pi_lock);
    -
    - rq = task_rq_lock(p, &flags);
    + rq = __task_rq_lock(p);

    trace_sched_pi_setprio(p, prio);
    oldprio = p->prio;
    @@ -4573,7 +4571,7 @@ void rt_mutex_setprio(struct task_struct

    check_class_changed(rq, p, prev_class, oldprio, running);
    }
    - task_rq_unlock(rq, &flags);
    + __task_rq_unlock(rq);
    }

    #endif
    @@ -4621,7 +4619,7 @@ void set_user_nice(struct task_struct *p
    resched_task(rq->curr);
    }
    out_unlock:
    - task_rq_unlock(rq, &flags);
    + task_rq_unlock(rq, p, &flags);
    }
    EXPORT_SYMBOL(set_user_nice);

    @@ -4843,13 +4841,11 @@ static int __sched_setscheduler(struct t
    /*
    * make sure no PI-waiters arrive (or leave) while we are
    * changing the priority of the task:
    - */
    - raw_spin_lock_irqsave(&p->pi_lock, flags);
    - /*
    + *
    * To be able to change p->policy safely, the apropriate
    * runqueue lock must be held.
    */
    - rq = __task_rq_lock(p);
    + rq = task_rq_lock(p, &flags);

    /*
    * Changing the policy of the stop threads its a very bad idea
    @@ -4902,8 +4898,7 @@ static int __sched_setscheduler(struct t

    check_class_changed(rq, p, prev_class, oldprio, running);
    }
    - __task_rq_unlock(rq);
    - raw_spin_unlock_irqrestore(&p->pi_lock, flags);
    + task_rq_unlock(rq, p, &flags);

    rt_mutex_adjust_pi(p);

    @@ -5432,7 +5427,7 @@ SYSCALL_DEFINE2(sched_rr_get_interval, p

    rq = task_rq_lock(p, &flags);
    time_slice = p->sched_class->get_rr_interval(rq, p);
    - task_rq_unlock(rq, &flags);
    + task_rq_unlock(rq, p, &flags);

    rcu_read_unlock();
    jiffies_to_timespec(time_slice, &t);
    @@ -5655,8 +5650,7 @@ int set_cpus_allowed_ptr(struct task_str
    unsigned int dest_cpu;
    int ret = 0;

    - raw_spin_lock_irqsave(&p->pi_lock, flags);
    - rq = __task_rq_lock(p);
    + rq = task_rq_lock(p, &flags);

    if (!cpumask_intersects(new_mask, cpu_active_mask)) {
    ret = -EINVAL;
    @@ -5691,8 +5685,7 @@ int set_cpus_allowed_ptr(struct task_str
    return 0;
    }
    out:
    - __task_rq_unlock(rq);
    - raw_spin_unlock_irqrestore(&p->pi_lock, flags);
    + task_rq_unlock(rq, p, &flags);

    return ret;
    }
    @@ -8463,7 +8456,7 @@ void sched_move_task(struct task_struct
    if (on_rq)
    enqueue_task(rq, tsk, 0);

    - task_rq_unlock(rq, &flags);
    + task_rq_unlock(rq, tsk, &flags);
    }
    #endif /* CONFIG_CGROUP_SCHED */




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