linux-kernel - Re: [PATCH RFC 2/2] kvm: Be courteous to other VMs in overcommitted scenario in PLE handler

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Message-Id: <20120921224636.18cccbd6c33b161975f2a8cb@gmail.com>
Date:	Fri, 21 Sep 2012 22:46:36 +0900
From:	Takuya Yoshikawa <takuya.yoshikawa@...il.com>
To:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Ingo Molnar <mingo@...hat.com>, Avi Kivity <avi@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Srikar <srikar@...ux.vnet.ibm.com>,
	"Nikunj A. Dadhania" <nikunj@...ux.vnet.ibm.com>,
	KVM <kvm@...r.kernel.org>, Jiannan Ouyang <ouyang@...pitt.edu>,
	chegu vinod <chegu_vinod@...com>,
	"Andrew M. Theurer" <habanero@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Srivatsa Vaddagiri <srivatsa.vaddagiri@...il.com>,
	Gleb Natapov <gleb@...hat.com>
Subject: Re: [PATCH RFC 2/2] kvm: Be courteous to other VMs in overcommitted
 scenario in PLE handler

On Fri, 21 Sep 2012 17:30:20 +0530
Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com> wrote:

> From: Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
> 
> When PLE handler fails to find a better candidate to yield_to, it
> goes back and does spin again. This is acceptable when we do not
> have overcommit.
> But in overcommitted scenarios (especially when we have large
> number of small guests), it is better to yield.
> 
> Reviewed-by: Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
> Signed-off-by: Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
> ---
>  virt/kvm/kvm_main.c |    4 ++++
>  1 files changed, 4 insertions(+), 0 deletions(-)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 8323685..713b677 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -1660,6 +1660,10 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me)
>  			}
>  		}
>  	}
> +	/* In overcommitted cases, yield instead of spinning */
> +	if (!yielded && rq_nr_running() > 1)
> +		schedule();

How about doing cond_resched() instead?

I'm not sure whether checking more sched stuff in KVM code is a
good thing.

	Takuya
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