linux-kernel - Re: [PATCH] sched: Do not stop cpu in set_cpus_allowed

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Date:	Mon, 15 Sep 2014 15:18:02 +0530
From:	Preeti Murthy <preeti.lkml@...il.com>
To:	Kirill Tkhai <ktkhai@...allels.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, Kirill Tkhai <tkhai@...dex.ru>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
Subject: Re: [PATCH] sched: Do not stop cpu in set_cpus_allowed_ptr() if task
 is not running

Hi Kirill,

Which tree is this patch based on? __migrate_task() does a
double_rq_lock/unlock() today in mainline, doesn't it? I don't
however see that in your patch.

Regards
Preeti U Murthy

On Fri, Sep 12, 2014 at 4:33 PM, Kirill Tkhai <ktkhai@...allels.com> wrote:
>
> If a task is queued but not running on it rq, we can simply migrate
> it without migration thread and switching of context.
>
> Signed-off-by: Kirill Tkhai <ktkhai@...allels.com>
> ---
>  kernel/sched/core.c |   47 ++++++++++++++++++++++++++++++++---------------
>  1 file changed, 32 insertions(+), 15 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index d4399b4..dbbba26 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4594,6 +4594,33 @@ void init_idle(struct task_struct *idle, int cpu)
>  }
>
>  #ifdef CONFIG_SMP
> +/*
> + * move_queued_task - move a queued task to new rq.
> + *
> + * Returns (locked) new rq. Old rq's lock is released.
> + */
> +static struct rq *move_queued_task(struct task_struct *p, int new_cpu)
> +{
> +       struct rq *rq = task_rq(p);
> +
> +       lockdep_assert_held(&rq->lock);
> +
> +       dequeue_task(rq, p, 0);
> +       p->on_rq = TASK_ON_RQ_MIGRATING;
> +       set_task_cpu(p, new_cpu);
> +       raw_spin_unlock(&rq->lock);
> +
> +       rq = cpu_rq(new_cpu);
> +
> +       raw_spin_lock(&rq->lock);
> +       BUG_ON(task_cpu(p) != new_cpu);
> +       p->on_rq = TASK_ON_RQ_QUEUED;
> +       enqueue_task(rq, p, 0);
> +       check_preempt_curr(rq, p, 0);
> +
> +       return rq;
> +}
> +
>  void do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask)
>  {
>         if (p->sched_class && p->sched_class->set_cpus_allowed)
> @@ -4650,14 +4677,15 @@ int set_cpus_allowed_ptr(struct task_struct *p, const struct cpumask *new_mask)
>                 goto out;
>
>         dest_cpu = cpumask_any_and(cpu_active_mask, new_mask);
> -       if (task_on_rq_queued(p) || p->state == TASK_WAKING) {
> +       if (task_running(rq, p) || p->state == TASK_WAKING) {
>                 struct migration_arg arg = { p, dest_cpu };
>                 /* Need help from migration thread: drop lock and wait. */
>                 task_rq_unlock(rq, p, &flags);
>                 stop_one_cpu(cpu_of(rq), migration_cpu_stop, &arg);
>                 tlb_migrate_finish(p->mm);
>                 return 0;
> -       }
> +       } else if (task_on_rq_queued(p))
> +               rq = move_queued_task(p, dest_cpu);
>  out:
>         task_rq_unlock(rq, p, &flags);
>
> @@ -4700,19 +4728,8 @@ static int __migrate_task(struct task_struct *p, int src_cpu, int dest_cpu)
>          * If we're not on a rq, the next wake-up will ensure we're
>          * placed properly.
>          */
> -       if (task_on_rq_queued(p)) {
> -               dequeue_task(rq, p, 0);
> -               p->on_rq = TASK_ON_RQ_MIGRATING;
> -               set_task_cpu(p, dest_cpu);
> -               raw_spin_unlock(&rq->lock);
> -
> -               rq = cpu_rq(dest_cpu);
> -               raw_spin_lock(&rq->lock);
> -               BUG_ON(task_rq(p) != rq);
> -               p->on_rq = TASK_ON_RQ_QUEUED;
> -               enqueue_task(rq, p, 0);
> -               check_preempt_curr(rq, p, 0);
> -       }
> +       if (task_on_rq_queued(p))
> +               rq = move_queued_task(p, dest_cpu);
>  done:
>         ret = 1;
>  fail:
>
>
>
> --
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