Re: [PATCH V3 RFC 1/2] sched: Bail out of yield_to when source and target runqueue has one task

[Andrew Jones <drjones@redhat.com>]

On Mon, Nov 26, 2012 at 05:37:54PM +0530, Raghavendra K T wrote:
> From: Peter Zijlstra <peterz@infradead.org>
> 
> In case of undercomitted scenarios, especially in large guests
> yield_to overhead is significantly high. when run queue length of
> source and target is one, take an opportunity to bail out and return
> -ESRCH. This return condition can be further exploited to quickly come
> out of PLE handler.
> 
> (History: Raghavendra initially worked on break out of kvm ple handler upon
>  seeing source runqueue length = 1, but it had to export rq length).
>  Peter came up with the elegant idea of return -ESRCH in scheduler core.
> 
> Signed-off-by: Peter Zijlstra <peterz@infradead.org>
> Raghavendra, Checking the rq length of target vcpu condition added.(thanks Avi)
> Reviewed-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
> Signed-off-by: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
> ---
> 
>  kernel/sched/core.c |   25 +++++++++++++++++++------
>  1 file changed, 19 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 2d8927f..fc219a5 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4289,7 +4289,10 @@ EXPORT_SYMBOL(yield);
>   * 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.
> + * Returns:
> + *	true (>0) if we indeed boosted the target task.
> + *	false (0) if we failed to boost the target.
> + *	-ESRCH if there's no task to yield to.
>   */
>  bool __sched yield_to(struct task_struct *p, bool preempt)
>  {
> @@ -4303,6 +4306,15 @@ bool __sched yield_to(struct task_struct *p, bool preempt)
>  
>  again:
>  	p_rq = task_rq(p);
> +	/*
> +	 * If we're the only runnable task on the rq and target rq also
> +	 * has only one task, there's absolutely no point in yielding.
> +	 */
> +	if (rq->nr_running == 1 && p_rq->nr_running == 1) {
> +		yielded = -ESRCH;
> +		goto out_irq;
> +	}
> +
>  	double_rq_lock(rq, p_rq);
>  	while (task_rq(p) != p_rq) {
>  		double_rq_unlock(rq, p_rq);
> @@ -4310,13 +4322,13 @@ again:
>  	}
>  
>  	if (!curr->sched_class->yield_to_task)
> -		goto out;
> +		goto out_unlock;
>  
>  	if (curr->sched_class != p->sched_class)
> -		goto out;
> +		goto out_unlock;
>  
>  	if (task_running(p_rq, p) || p->state)
> -		goto out;
> +		goto out_unlock;
>  
>  	yielded = curr->sched_class->yield_to_task(rq, p, preempt);
>  	if (yielded) {
> @@ -4329,11 +4341,12 @@ again:
>  			resched_task(p_rq->curr);
>  	}
>  
> -out:
> +out_unlock:
>  	double_rq_unlock(rq, p_rq);
> +out_irq:
>  	local_irq_restore(flags);
>  
> -	if (yielded)
> +	if (yielded > 0)
>  		schedule();
>  
>  	return yielded;
>

Acked-by: Andrew Jones <drjones@redhat.com>