From mboxrd@z Thu Jan 1 00:00:00 1970 From: Xiao Guangrong Subject: Re: [PATCH v4 06/10] KVM: MMU: fast path of handling guest page fault Date: Wed, 02 May 2012 13:39:51 +0800 Message-ID: <4FA0C8A7.9000001@linux.vnet.ibm.com> References: <4F9776D2.7020506@linux.vnet.ibm.com> <4F9777A4.208@linux.vnet.ibm.com> <20120426234535.GA5057@amt.cnet> <4F9A3445.2060305@linux.vnet.ibm.com> <20120427145213.GB28796@amt.cnet> <20120429175004.b54d8c095a60d98c8cdbc942@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Cc: Marcelo Tosatti , Avi Kivity , LKML , KVM To: Takuya Yoshikawa Return-path: In-Reply-To: <20120429175004.b54d8c095a60d98c8cdbc942@gmail.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On 04/29/2012 04:50 PM, Takuya Yoshikawa wrote: > On Fri, 27 Apr 2012 11:52:13 -0300 > Marcelo Tosatti wrote: > >> Yes but the objective you are aiming for is to read and write sptes >> without mmu_lock. That is, i am not talking about this patch. >> Please read carefully the two examples i gave (separated by "example)"). > > The real objective is not still clear. > > The ~10% improvement reported before was on macro benchmarks during live > migration. At least, that optimization was the initial objective. > > But at some point, the objective suddenly changed to "lock-less" without > understanding what introduced the original improvement. > > 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. > If the path being introduced by this patch is really fast, isn't it > possible to achieve the same improvement still using mmu_lock? > > > Note: During live migration, the fact that the guest gets faulted is > itself a limitation. We could easily see noticeable slowdown of a > program even if it runs only between two GET_DIRTY_LOGs. > Obviously no. It depends on what the guest is doing, from my autotest test, it very easily to see that, the huge improvement is on bench-migration not pure-migration. > >> The rules for code under mmu_lock should be: >> >> 1) Spte updates under mmu lock must always be atomic and >> with locked instructions. >> 2) Spte values must be read once, and appropriate action >> must be taken when writing them back in case their value >> has changed (remote TLB flush might be required). > > 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?