Re: [PATCH -v8a 4/7] sched: Add yield_to(task, preempt) functionality

Peter Zijlstra
2011-02-01T15:51:16Z

On Tue, 2011-02-01 at 09:50 -0500, Rik van Riel wrote:

> +/**
> + * 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.
> + *
> + * Returns true if we indeed boosted the target task.
> + */
> +bool __sched yield_to(struct task_struct *p, bool preempt)
> +{
> + struct task_struct *curr = current;
> + struct rq *rq, *p_rq;
> + unsigned long flags;
> + bool yielded = 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;
> +
> + if (task_running(p_rq, p) || p->state)
> + goto out;
> +
> + yielded = curr->sched_class->yield_to_task(rq, p, preempt);
> +
> + if (yielded) {
> + schedstat_inc(rq, yld_count);
> + current->sched_class->yield_task(rq);
> + }

We can avoid this second indirect function call by

> +
> +out:
> + double_rq_unlock(rq, p_rq);
> + local_irq_restore(flags);
> +
> + if (yielded)
> + schedule();
> +
> + return yielded;
> +}
> +EXPORT_SYMBOL_GPL(yield_to);

> +static bool yield_to_task_fair(struct rq *rq, struct task_struct *p, bool preempt)
> +{
> + struct sched_entity *se = &p->se;
> +
> + if (!se->on_rq)
> + return false;
> +
> + /* Tell the scheduler that we'd really like pse to run next. */
> + set_next_buddy(se);
> +
> + /* Make p's CPU reschedule; pick_next_entity takes care of fairness. */
> + if (preempt)
> + resched_task(rq->curr);

calling: yield_task_fair(rq); here.

> + return true;
> +}

I'll make that change on commit.