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

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Date:	Fri, 21 Sep 2012 23:15:40 +0530
From:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
To:	Rik van Riel <riel@...hat.com>
CC:	Takuya Yoshikawa <takuya.yoshikawa@...il.com>,
	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>,
	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 09/21/2012 07:22 PM, Rik van Riel wrote:
> On 09/21/2012 09:46 AM, Takuya Yoshikawa wrote:
>> 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?
>
> Actually, an actual call to yield() may be better.
>
> That will set scheduler hints to make the scheduler pick
> another task for one round, while preserving this task's
> top position in the runqueue.

I am not a scheduler expert, but I am also inclined towards
Rik's suggestion here since we set skip buddy here. Takuya?

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