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

Ingo Molnar
2013-01-24T10:32:19Z

* Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com> 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>
> Acked-by: Andrew Jones <drjones@redhat.com>
> Tested-by: Chegu Vinod <chegu_vinod@hp.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;
> + }

Looks good to me in principle.

Would be nice to get more consistent benchmark numbers. Once
those are unambiguously showing that this is a win:

Acked-by: Ingo Molnar <mingo@kernel.org>

Thanks,

Ingo