linux-kernel - Re: [PATCH RFC 0/2] kvm: Better yield_to candidate using preemption notifiers

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Date:	Tue, 05 Mar 2013 17:54:09 +0530
From:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
To:	Andrew Jones <drjones@...hat.com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Avi Kivity <avi.kivity@...il.com>,
	Gleb Natapov <gleb@...hat.com>, Ingo Molnar <mingo@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Srikar <srikar@...ux.vnet.ibm.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"Nikunj A. Dadhania" <nikunj@...ux.vnet.ibm.com>,
	KVM <kvm@...r.kernel.org>, Thomas Gleixner <tglx@...utronix.de>,
	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>
Subject: Re: [PATCH RFC 0/2] kvm: Better yield_to candidate using preemption
 notifiers

On 03/05/2013 03:23 PM, Andrew Jones wrote:
> On Mon, Mar 04, 2013 at 11:31:46PM +0530, Raghavendra K T wrote:
>>   This patch series further filters better vcpu candidate to yield to
>> in PLE handler. The main idea is to record the preempted vcpus using
>> preempt notifiers and iterate only those preempted vcpus in the
>> handler. Note that the vcpus which were in spinloop during pause loop
>> exit are already filtered.
>
> The %improvement and patch series look good.
>

Thank you for the review.

>>
>> Thanks Jiannan, Avi for bringing the idea and Gleb, PeterZ for
>> precious suggestions during the discussion.
>> Thanks Srikar for suggesting to avoid rcu lock while checking task state
>> that has improved overcommit cases.
>>
>> There are basically two approches for the implementation.
>>
>> Method 1: Uses per vcpu preempt flag (this series).
>>
>> Method 2: We keep a bitmap of preempted vcpus. using this we can easily
>> iterate over preempted vcpus.
>>
>> Note that method 2 needs an extra index variable to identify/map bitmap to
>> vcpu and it also needs static vcpu allocation.
>
> We definitely don't want something that requires static vcpu allocation.
> I think it'd be better to add another counter for the vcpu bit assignment.
>

So do you mean some thing parallel to online_vcpus?

>>
>> I am also posting Method 2 approach for reference in case it interests.
>
> I guess the interest in Method2 would come from perf numbers. Did you try
> comparing Method1 vs. Method2?
>

Yes I did. Performance wise method2 is almost equal to method1. But I
believe if there is any difference it may show when we have large vcpu
guest. (Currently I have only 32 core host).

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