linux-kernel - Re: [PATCH v2 0/7] KVM: MMU: fast zap all shadow pages

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Date:	Fri, 22 Mar 2013 20:03:04 +0800
From:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
To:	Gleb Natapov <gleb@...hat.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v2 0/7] KVM: MMU: fast zap all shadow pages

On 03/22/2013 07:47 PM, Gleb Natapov wrote:
> On Fri, Mar 22, 2013 at 07:39:24PM +0800, Xiao Guangrong wrote:
>> On 03/22/2013 07:28 PM, Gleb Natapov wrote:
>>> On Fri, Mar 22, 2013 at 07:10:44PM +0800, Xiao Guangrong wrote:
>>>> On 03/22/2013 06:54 PM, Marcelo Tosatti wrote:
>>>>
>>>>>>
>>>>>>>
>>>>>>> And then have codepaths that nuke shadow pages break from the spinlock,
>>>>>>
>>>>>> I think this is not needed any more. We can let mmu_notify use the generation
>>>>>> number to invalid all shadow pages, then we only need to free them after
>>>>>> all vcpus down and mmu_notify unregistered - at this point, no lock contention,
>>>>>> we can directly free them.
>>>>>>
>>>>>>> such as kvm_mmu_slot_remove_write_access does now (spin_needbreak).
>>>>>>
>>>>>> 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.
>>>>>
>>>>> Yes, but the suggestion is to use spin_needbreak on the VM shutdown
>>>>> cases, where there is no detailed concern about performance. Such as
>>>>> mmu_notifier_release, kvm_destroy_vm, etc. In those cases what matters
>>>>> most is that host remains unaffected (and that it finishes in a
>>>>> reasonable time).
>>>>
>>>> Okay. I agree with you, will give a try.
>>>>
>>>>>
>>>>>> I still think the right way to fix this kind of thing is optimization for
>>>>>> mmu-lock.
>>>>>
>>>>> And then for the cases where performance matters just increase a
>>>>> VM global generetion number, zap the roots and then on kvm_mmu_get_page:
>>>>>
>>>>> kvm_mmu_get_page() {
>>>>> 	sp = lookup_hash(gfn)
>>>>> 	if (sp->role = role) {
>>>>> 		if (sp->mmu_gen_number != kvm->arch.mmu_gen_number) {
>>>>> 			kvm_mmu_commit_zap_page(sp); (no need for TLB flushes as its unreachable)
>>>>> 			kvm_mmu_init_page(sp);
>>>>> 			proceed as if the page was just allocated
>>>>> 		}
>>>>> 	}
>>>>> }
>>>>>
>>>>> It makes the kvm_mmu_zap_all path even faster than you have now.
>>>>> I suppose this was your idea correct with the generation number correct?
>>>>
>>>> Wow, great minds think alike, this is exactly what i am doing. ;)
>>>>
>>> Not that I disagree with above code, but why not make mmu_gen_number to be
>>> part of a role and remove old pages in kvm_mmu_free_some_pages() whenever
>>> limit is reached like we looks to be doing with role.invalid pages now.
>>
>> These pages can be reused after purge its entries and delete it from parents
>> list, it can reduce the pressure of memory allocator. Also, we can move it to
>> the head of active_list so that the pages with invalid_gen can be reclaimed first.
>>
> You mean tail of the active_list, since kvm_mmu_free_some_pages()
> removes pages from tail? Since pages with new mmu_gen_number will be put

I mean purge the invalid-gen page first, then update its valid-gen to current-gen,
then move it to the head of active_list:

kvm_mmu_get_page() {
	sp = lookup_hash(gfn)
	if (sp->role = role) {
		if (sp->mmu_gen_number != kvm->arch.mmu_gen_number) {
			kvm_mmu_purge_page(sp); (no need for TLB flushes as its unreachable)
			sp->mmu_gen_number = kvm->arch.mmu_gen_number;
			@@@@@@ move sp to the head of active list @@@@@@
		}
	}
}


> at the head of the list it is natural that tail will contain pages with
> outdated generation numbers without need to explicitly move them.

Currently, only the new allocated page can be moved to the head of
active_list. The existing pages are not moved by kvm_mmu_get_page.
It seems a bug.

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