From mboxrd@z Thu Jan  1 00:00:00 1970
From: Ingo Molnar <mingo@kernel.org>
Subject: Re: [PATCH V3 RESEND RFC 1/2] sched: Bail out of yield_to when
 source and target runqueue has one task
Date: Thu, 24 Jan 2013 11:32:13 +0100
Message-ID: <20130124103213.GD27602@gmail.com>
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 <20130122073913.24731.65118.sendpatchset@codeblue.in.ibm.com>
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Cc: Peter Zijlstra <peterz@infradead.org>,
	Avi Kivity <avi.kivity@gmail.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Gleb Natapov <gleb@redhat.com>, Ingo Molnar <mingo@redhat.com>,
	Marcelo Tosatti <mtosatti@redhat.com>,
	Rik van Riel <riel@redhat.com>,
	Srikar <srikar@linux.vnet.ibm.com>,
	"Nikunj A. Dadhania" <nikunj@linux.vnet.ibm.com>,
	KVM <kvm@vger.kernel.org>, Jiannan Ouyang <ouyang@cs.pitt.edu>,
	Chegu Vinod <chegu_vinod@hp.com>,
	"Andrew M. Theurer" <habanero@linux.vnet.ibm.com>,
	LKML <linux-kernel@vger.kernel.org>,
	Srivatsa Vaddagiri <srivatsa.vaddagiri@gmail.com>,
	Andrew Jones <drjones@redhat.com>
To: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
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* 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