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

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Date:	Tue, 5 Mar 2013 13:40:13 +0100
From:	Andrew Jones <drjones@...hat.com>
To:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.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 Tue, Mar 05, 2013 at 05:54:09PM +0530, Raghavendra K T wrote:
> 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?

Yes, one that only grows. However, then, if a vcpu is unplugged, its bit
would have to be skipped over.

> 
> >>
> >>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).
> 

OK, probably not worth it at this point then.

thanks,
drew
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