linux-kernel - Re: [PATCH v4 06/10] KVM: MMU: fast path of handling guest page fault

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Date:	Thu, 3 May 2012 09:15:58 +0900
From:	Takuya Yoshikawa <takuya.yoshikawa@...il.com>
To:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
Cc:	Marcelo Tosatti <mtosatti@...hat.com>, Avi Kivity <avi@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH v4 06/10] KVM: MMU: fast path of handling guest page
 fault

On Wed, 02 May 2012 13:39:51 +0800
Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com> wrote:

> > Was the problem really mmu_lock contention?

> Takuya, i am so tired to argue the advantage of lockless write-protect
> and lockless O(1) dirty-log again and again.

You are missing my point.  Please do not take my comments as an objection
to your whole work: whey do you feel so?

I thought that your new fast-page-fault path was fast and optimized
the guest during dirty logging.

So in this v4, you might get a similar result even before dropping
mmu_lock, without 07/10?, if the problem Marcelo explained was not there.

Of course there is a problem of mmu_lock contention.  What I am suggesting
is to split that problem and do measurement separately so that part of
your work can be merged soon.

Your guest size and workload was small to make get_dirty hold mmu_lock
long time.  If you want to appeal the real value of lock-less, you need to
do another measurment.

But this is your work and it's up to you.  Although I was thinking to help
your measurement, I cannot do that knowing the fact that you would not
welcome my help.

> > Although I am not certain about what will be really needed in the
> > final form, if this kind of maybe-needed-overhead is going to be
> > added little by little, I worry about possible regression.

> Well, will you suggest Linus to reject all patches and stop
> all discussion for the "possible regression" reason?

My concern was for Marcelo's examples, not your current implementation.
If you can show explicitely what will be needed in the final form,
I do not have any concern.

Sorry for disturbing.

Thanks,
	Takuya
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