linux-kernel - Re: [PATCH v7 10/11] KVM: MMU: collapse TLB flushes when zap all pages

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Date:	Fri, 31 May 2013 01:24:43 +0900
From:	Takuya Yoshikawa <takuya.yoshikawa@...il.com>
To:	Gleb Natapov <gleb@...hat.com>
Cc:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>,
	Marcelo Tosatti <mtosatti@...hat.com>, avi.kivity@...il.com,
	pbonzini@...hat.com, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org
Subject: Re: [PATCH v7 10/11] KVM: MMU: collapse TLB flushes when zap all
 pages

On Thu, 30 May 2013 03:53:38 +0300
Gleb Natapov <gleb@...hat.com> wrote:

> On Wed, May 29, 2013 at 09:19:41PM +0800, Xiao Guangrong wrote:
> > On 05/29/2013 08:39 PM, Marcelo Tosatti wrote:
> > > On Wed, May 29, 2013 at 11:03:19AM +0800, Xiao Guangrong wrote:
> > >>>>> the pages since other vcpus may be doing locklessly shadow
> > >>>>> page walking
> > >>>
> > >>> Ah, yes, i agree with you.
> > >>>
> > >>> We can introduce a list, say kvm->arch.obsolte_pages, to link all of the
> > >>> zapped-page, the page-shrink will free the page on that list first.
> > >>>
> > >>> Marcelo, if you do not have objection on  patch 1 ~ 8 and 11, could you please
> > >>> let them merged first, and do add some comments and tlb optimization later?
> > >>
> > >> Exclude patch 11 please, since it depends on the "collapse" optimization.
> > > 
> > > I'm fine with patch 1 being merged. I think the remaining patches need better
> > > understanding or explanation. The problems i see are:
> > > 
> > > 1) The magic number "10" to zap before considering reschedule is
> > > annoying. It would be good to understand why it is needed at all.
> > 
> > ......
> > 
> > > 
> > > But then again, the testcase is measuring kvm_mmu_zap_all performance
> > > alone which we know is not a common operation, so perhaps there is
> > > no need for that minimum-pages-to-zap-before-reschedule.
> > 
> > Well. Although, this is not the common operation, but this operation
> > can be triggered by VCPU - it one VCPU take long time on zap-all-pages,
> > other vcpus is missing IPI-synce, or missing IO. This is easily cause
> > soft lockups if the vcpu is doing memslot-releated things.
> > 
> +1. If it is trigarable by a guest it may slow down the guest, but we
> should not allow for it to slow down a host.
> 

Well, I don't object to the minimum-pages-to-zap-before-reschedule idea
itself, but if you're going to take patch 4, please at least add a warning
in the changelog that the magic number "10" was selected without good enough
reasoning.

"[ It improves kernel building 0.6% ~ 1% ]" alone will make it hard for
others to change the number later.

I actually once tried to do a similar thing for other code.  So I have a
possible reasoning for this, and 10 should probably be changed later.

	Takuya
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