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

[Gleb Natapov <gleb@redhat.com>]

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.
 
--
			Gleb.