From: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
To: Takuya Yoshikawa <takuya.yoshikawa@gmail.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>,
Avi Kivity <avi@redhat.com>, LKML <linux-kernel@vger.kernel.org>,
KVM <kvm@vger.kernel.org>
Subject: Re: [PATCH v4 06/10] KVM: MMU: fast path of handling guest page fault
Date: Wed, 02 May 2012 13:39:51 +0800 [thread overview]
Message-ID: <4FA0C8A7.9000001@linux.vnet.ibm.com> (raw)
In-Reply-To: <20120429175004.b54d8c095a60d98c8cdbc942@gmail.com>
On 04/29/2012 04:50 PM, Takuya Yoshikawa wrote:
> On Fri, 27 Apr 2012 11:52:13 -0300
> Marcelo Tosatti <mtosatti@redhat.com> 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?
next prev parent reply other threads:[~2012-05-02 5:40 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-04-25 4:00 [PATCH v4 00/10] KVM: MMU: fast page fault Xiao Guangrong
2012-04-25 4:01 ` [PATCH v4 01/10] KVM: MMU: return bool in __rmap_write_protect Xiao Guangrong
2012-04-25 4:01 ` [PATCH v4 02/10] KVM: MMU: abstract spte write-protect Xiao Guangrong
2012-04-25 4:02 ` [PATCH v4 03/10] KVM: VMX: export PFEC.P bit on ept Xiao Guangrong
2012-04-25 4:02 ` [PATCH v4 04/10] KVM: MMU: introduce SPTE_MMU_WRITEABLE bit Xiao Guangrong
2012-04-25 4:03 ` [PATCH v4 05/10] KVM: MMU: introduce SPTE_WRITE_PROTECT bit Xiao Guangrong
2012-04-25 4:03 ` [PATCH v4 06/10] KVM: MMU: fast path of handling guest page fault Xiao Guangrong
2012-04-26 23:45 ` Marcelo Tosatti
2012-04-27 5:53 ` Xiao Guangrong
2012-04-27 14:52 ` Marcelo Tosatti
2012-04-28 6:10 ` Xiao Guangrong
2012-05-01 1:34 ` Marcelo Tosatti
2012-05-02 5:28 ` Xiao Guangrong
2012-05-02 21:07 ` Marcelo Tosatti
2012-05-03 11:26 ` Xiao Guangrong
2012-05-05 14:08 ` Marcelo Tosatti
2012-05-06 9:36 ` Avi Kivity
2012-05-07 6:52 ` Xiao Guangrong
2012-04-29 8:50 ` Takuya Yoshikawa
2012-05-01 2:31 ` Marcelo Tosatti
2012-05-02 5:39 ` Xiao Guangrong [this message]
2012-05-02 21:10 ` Marcelo Tosatti
2012-05-03 12:09 ` Xiao Guangrong
2012-05-03 12:13 ` Avi Kivity
2012-05-03 0:15 ` Takuya Yoshikawa
2012-05-03 12:23 ` Xiao Guangrong
2012-05-03 12:40 ` Takuya Yoshikawa
2012-04-25 4:04 ` [PATCH v4 07/10] KVM: MMU: lockless update spte on fast page fault path Xiao Guangrong
2012-04-25 4:04 ` [PATCH v4 08/10] KVM: MMU: trace fast page fault Xiao Guangrong
2012-04-25 4:05 ` [PATCH v4 09/10] KVM: MMU: fix kvm_mmu_pagetable_walk tracepoint Xiao Guangrong
2012-04-25 4:06 ` [PATCH v4 10/10] KVM: MMU: document mmu-lock and fast page fault Xiao Guangrong
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