Date: Fri, 22 Mar 2013 21:15:24 +0200
From: Gleb Natapov <gleb@redhat.com>
To: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org
Subject: Re: [PATCH v2 0/7] KVM: MMU: fast zap all shadow pages
Message-ID: <20130322191524.GY9382@redhat.com>
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On Fri, Mar 22, 2013 at 08:37:33PM +0800, Xiao Guangrong wrote:
> 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?
> 
We did not do that for sp->role.invalid pages although we could do what
is proposed above for them too (am I right?). If there is measurable
advantage of reusing invalid pages in kvm_mmu_get_page() lets do it like
that, but if not then less code is better.

> > 
> >>> 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...
> 
Yes, but there is not much we can do there.

--
			Gleb.
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