linux-kernel - Re: [PATCH v3 00/15] KVM: MMU: fast zap all shadow pages

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Message-ID: <5176363C.10303@linux.vnet.ibm.com>
Date:	Tue, 23 Apr 2013 15:20:28 +0800
From:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
To:	Gleb Natapov <gleb@...hat.com>
CC:	mtosatti@...hat.com, avi.kivity@...il.com,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v3 00/15] KVM: MMU: fast zap all shadow pages

On 04/23/2013 02:28 PM, Gleb Natapov wrote:
> On Tue, Apr 23, 2013 at 08:19:02AM +0800, Xiao Guangrong wrote:
>> On 04/22/2013 05:21 PM, Gleb Natapov wrote:
>>> On Sun, Apr 21, 2013 at 10:09:29PM +0800, Xiao Guangrong wrote:
>>>> On 04/21/2013 09:03 PM, Gleb Natapov wrote:
>>>>> On Tue, Apr 16, 2013 at 02:32:38PM +0800, Xiao Guangrong wrote:
>>>>>> This patchset is based on my previous two patchset:
>>>>>> [PATCH 0/2] KVM: x86: avoid potential soft lockup and unneeded mmu reload
>>>>>> (https://lkml.org/lkml/2013/4/1/2)
>>>>>>
>>>>>> [PATCH v2 0/6] KVM: MMU: fast invalid all mmio sptes
>>>>>> (https://lkml.org/lkml/2013/4/1/134)
>>>>>>
>>>>>> Changlog:
>>>>>> V3:
>>>>>>   completely redesign the algorithm, please see below.
>>>>>>
>>>>> This looks pretty complicated. Is it still needed in order to avoid soft
>>>>> lockups after "avoid potential soft lockup and unneeded mmu reload" patch?
>>>>
>>>> Yes.
>>>>
>>>> I discussed this point with Marcelo:
>>>>
>>>> ======
>>>> BTW, to my honest, i do not think spin_needbreak is a good way - it does
>>>> not fix the hot-lock contention and it just occupies more cpu time to avoid
>>>> possible soft lock-ups.
>>>>
>>>> Especially, zap-all-shadow-pages can let other vcpus fault and vcpus contest
>>>> mmu-lock, then zap-all-shadow-pages release mmu-lock and wait, other vcpus
>>>> create page tables again. zap-all-shadow-page need long time to be finished,
>>>> the worst case is, it can not completed forever on intensive vcpu and memory
>>>> usage.
>>>>
>>> So what about mixed approach: use generation numbers and reload roots to
>>> quickly invalidate all shadow pages and then do kvm_mmu_zap_all_invalid().
>>> kvm_mmu_zap_all_invalid() is a new function that invalidates only shadow
>>> pages with stale generation number (and uses lock break technique). It
>>> may traverse active_mmu_pages from tail to head since new shadow pages
>>> will be added to the head of the list or it may use invalid slot rmap to
>>> find exactly what should be invalidated.
>>
>> I prefer to unmapping the invalid rmap instead of zapping stale shadow pages
>> in kvm_mmu_zap_all_invalid(), the former is faster.
>>
> Not sure what do you mean here. What is "unmapping the invalid rmap"?

it is like you said below:
======
kvm_mmu_zap_all_invalid(slot) will only zap shadow pages that are
reachable from the slot's rmap
======
My suggestion is zapping the spte that are linked in the slot's rmap.

> 
>> This way may help but not good, after reload mmu with the new generation number,
>> all of the vcpu will fault in a long time, try to hold mmu-lock is not good even
>> if use lock break technique.
> If kvm_mmu_zap_all_invalid(slot) will only zap shadow pages that are
> reachable from the slot's rmap, as opposite to zapping all invalid
> shadow pages, it will have much less work to do. The slots that we
> add/remove during hot plug are usually small. To guaranty reasonable
> forward progress we can break the lock only after certain amount of
> shadow pages are invalidated. All other invalid shadow pages will be
> zapped in make_mmu_pages_available() and zapping will be spread between
> page faults.

No interested in hot-remove memory?

BTW, could you please review my previous patchsets and apply them if its
looks ok? ;)

[PATCH 0/2] KVM: x86: avoid potential soft lockup and unneeded mmu reload
(https://lkml.org/lkml/2013/4/1/2)

[PATCH v2 0/6] KVM: MMU: fast invalid all mmio sptes
(https://lkml.org/lkml/2013/4/1/134)

Thanks!


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