Re: [RFC -v3 PATCH 2/3] sched: add yield_to function

Hillf Danton
2011-01-06T14:33:38Z

On Thu, Jan 6, 2011 at 12:57 AM, Mike Galbraith <efault@gmx.de> wrote:
> sched: Add yield_to(task, preempt) functionality.
>
> Currently only implemented for fair class tasks.
>
> Add a yield_to_task method() to the fair scheduling class. allowing the
> caller of yield_to() to accelerate another thread in it's thread group,
> task group, and sched class toward either it's cpu, or potentially the
> caller's own cpu if the 'preempt' argument is also passed.
>
> Implemented via a scheduler hint, using cfs_rq->next to encourage the
> target being selected.
>
> Signed-off-by: Rik van Riel <riel@redhat.com>
> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
> Signed-off-by: Mike Galbraith <efault@gmx.de>
>
> ---
>  include/linux/sched.h |    1
>  kernel/sched.c        |   56 ++++++++++++++++++++++++++++++++++++++++++++++++++
>  kernel/sched_fair.c   |   52 ++++++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 109 insertions(+)
>
> Index: linux-2.6/include/linux/sched.h
> ===================================================================
> --- linux-2.6.orig/include/linux/sched.h
> +++ linux-2.6/include/linux/sched.h
> @@ -1056,6 +1056,7 @@ struct sched_class {
>        void (*enqueue_task) (struct rq *rq, struct task_struct *p, int flags);
>        void (*dequeue_task) (struct rq *rq, struct task_struct *p, int flags);
>        void (*yield_task) (struct rq *rq);
> +       int (*yield_to_task) (struct task_struct *p, int preempt);
>
>        void (*check_preempt_curr) (struct rq *rq, struct task_struct *p, int flags);
>
> Index: linux-2.6/kernel/sched.c
> ===================================================================
> --- linux-2.6.orig/kernel/sched.c
> +++ linux-2.6/kernel/sched.c
> @@ -5327,6 +5327,62 @@ void __sched yield(void)
>  }
>  EXPORT_SYMBOL(yield);
>
> +/**
> + * yield_to - yield the current processor to another thread in
> + * your thread group, or accelerate that thread toward the
> + * processor it's on.
> + *
> + * It's the caller's job to ensure that the target task struct
> + * can't go away on us before we can do any checks.
> + */
> +void __sched yield_to(struct task_struct *p, int preempt)
> +{
> +       struct task_struct *curr = current;
> +       struct rq *rq, *p_rq;
> +       unsigned long flags;
> +       int yield = 0;
> +
> +       local_irq_save(flags);
> +       rq = this_rq();
> +
> +again:
> +       p_rq = task_rq(p);
> +       double_rq_lock(rq, p_rq);
> +       while (task_rq(p) != p_rq) {
> +               double_rq_unlock(rq, p_rq);
> +               goto again;
> +       }
> +
> +       if (!curr->sched_class->yield_to_task)
> +               goto out;
> +
> +       if (curr->sched_class != p->sched_class)
> +               goto out;
> +

to be clearer?
if (task_running(p_rq, p) || p->state != TASK_RUNNING)

> +               goto out;
> +
> +       if (!same_thread_group(p, curr))
> +               goto out;