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    Subject[PATCH 1/4] sched: Optimize fork() paths
    The task_fork_fair() callback already calls __set_task_cpu() and takes
    rq->lock.

    If we move the sched_class::task_fork callback in sched_fork() under
    the existing p->pi_lock, right after its set_task_cpu() call, we can
    avoid doing two such calls and omit the IRQ disabling on the rq->lock.

    Change to __set_task_cpu() to skip the migration bits, this is a new
    task, not a migration.

    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    ---
    kernel/sched/core.c | 7 +++----
    kernel/sched/fair.c | 27 ++++++---------------------
    2 files changed, 9 insertions(+), 25 deletions(-)

    --- a/kernel/sched/core.c
    +++ b/kernel/sched/core.c
    @@ -2381,9 +2381,6 @@ int sched_fork(unsigned long clone_flags
    p->sched_class = &fair_sched_class;
    }

    - if (p->sched_class->task_fork)
    - p->sched_class->task_fork(p);
    -
    /*
    * The child is not yet in the pid-hash so no cgroup attach races,
    * and the cgroup is pinned to this child due to cgroup_fork()
    @@ -2392,7 +2389,9 @@ int sched_fork(unsigned long clone_flags
    * Silence PROVE_RCU.
    */
    raw_spin_lock_irqsave(&p->pi_lock, flags);
    - set_task_cpu(p, cpu);
    + __set_task_cpu(p, cpu);
    + if (p->sched_class->task_fork)
    + p->sched_class->task_fork(p);
    raw_spin_unlock_irqrestore(&p->pi_lock, flags);

    #ifdef CONFIG_SCHED_INFO
    --- a/kernel/sched/fair.c
    +++ b/kernel/sched/fair.c
    @@ -4418,7 +4418,7 @@ enqueue_task_fair(struct rq *rq, struct
    *
    * note: in the case of encountering a throttled cfs_rq we will
    * post the final h_nr_running increment below.
    - */
    + */
    if (cfs_rq_throttled(cfs_rq))
    break;
    cfs_rq->h_nr_running++;
    @@ -8255,31 +8255,17 @@ static void task_fork_fair(struct task_s
    {
    struct cfs_rq *cfs_rq;
    struct sched_entity *se = &p->se, *curr;
    - int this_cpu = smp_processor_id();
    struct rq *rq = this_rq();
    - unsigned long flags;
    -
    - raw_spin_lock_irqsave(&rq->lock, flags);

    + raw_spin_lock(&rq->lock);
    update_rq_clock(rq);

    cfs_rq = task_cfs_rq(current);
    curr = cfs_rq->curr;
    -
    - /*
    - * Not only the cpu but also the task_group of the parent might have
    - * been changed after parent->se.parent,cfs_rq were copied to
    - * child->se.parent,cfs_rq. So call __set_task_cpu() to make those
    - * of child point to valid ones.
    - */
    - rcu_read_lock();
    - __set_task_cpu(p, this_cpu);
    - rcu_read_unlock();
    -
    - update_curr(cfs_rq);
    -
    - if (curr)
    + if (curr) {
    + update_curr(cfs_rq);
    se->vruntime = curr->vruntime;
    + }
    place_entity(cfs_rq, se, 1);

    if (sysctl_sched_child_runs_first && curr && entity_before(curr, se)) {
    @@ -8292,8 +8278,7 @@ static void task_fork_fair(struct task_s
    }

    se->vruntime -= cfs_rq->min_vruntime;
    -
    - raw_spin_unlock_irqrestore(&rq->lock, flags);
    + raw_spin_unlock(&rq->lock);
    }

    /*

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