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

From: Xiao Guangrong
Date: Fri Mar 22 2013 - 08:37:46 EST

On 03/22/2013 08:12 PM, Gleb Natapov wrote:
> On Fri, Mar 22, 2013 at 08:03:04PM +0800, Xiao Guangrong wrote:
>> 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 @@@@@@
>> }
>> }
>> }
> And I am saying that if you make mmu_gen_number part of the role you do
> not need to change kvm_mmu_get_page() at all. It will just work.

Oh, i got what your said. But i want to reuse these page (without
free and re-allocate). What do you think about this?

>>> 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.
> Ideally it needs to be LRU list based on accessed bit scanning.

Yes, but unfortunately, A bit does not be supported on some intel cpus...

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