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

[Marcelo Tosatti <mtosatti@redhat.com>]

On Fri, Mar 22, 2013 at 09:15:24PM +0200, Gleb Natapov wrote:
> 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.

The number of sp->role.invalid=1 pages is small (only shadow roots). It
can grow but is bounded to a handful. No improvement visible there.

The number of shadow pages with old mmu_gen_number is potentially large.

Returning all shadow pages to the allocator is problematic because it
takes a long time (therefore the suggestion to postpone it).

Spreading the work to free (or reuse) those shadow pages to individual
page fault instances alleviates the mmu_lock hold time issue without
significant reduction to post kvm_mmu_zap_all operation (which has to
rebuilt all pagetables anyway).

You prefer to modify SLAB allocator to aggressively free these stale
shadow pages rather than kvm_mmu_get_page to reuse them?