On 04/22/2013 05:21 PM, Gleb Natapov wrote: > On Sun, Apr 21, 2013 at 10:09:29PM +0800, Xiao Guangrong wrote: >> On 04/21/2013 09:03 PM, Gleb Natapov wrote: >>> On Tue, Apr 16, 2013 at 02:32:38PM +0800, Xiao Guangrong wrote: >>>> This patchset is based on my previous two patchset: >>>> [PATCH 0/2] KVM: x86: avoid potential soft lockup and unneeded mmu reload >>>> (https://lkml.org/lkml/2013/4/1/2) >>>> >>>> [PATCH v2 0/6] KVM: MMU: fast invalid all mmio sptes >>>> (https://lkml.org/lkml/2013/4/1/134) >>>> >>>> Changlog: >>>> V3: >>>> completely redesign the algorithm, please see below. >>>> >>> This looks pretty complicated. Is it still needed in order to avoid soft >>> lockups after "avoid potential soft lockup and unneeded mmu reload" patch? >> >> Yes. >> >> I discussed this point with Marcelo: >> >> ====== >> 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. >> > So what about mixed approach: use generation numbers and reload roots to > quickly invalidate all shadow pages and then do kvm_mmu_zap_all_invalid(). > kvm_mmu_zap_all_invalid() is a new function that invalidates only shadow > pages with stale generation number (and uses lock break technique). It > may traverse active_mmu_pages from tail to head since new shadow pages > will be added to the head of the list or it may use invalid slot rmap to > find exactly what should be invalidated. I prefer to unmapping the invalid rmap instead of zapping stale shadow pages in kvm_mmu_zap_all_invalid(), the former is faster. This way may help but not good, after reload mmu with the new generation number, all of the vcpu will fault in a long time, try to hold mmu-lock is not good even if use lock break technique. I think we can do this step first, then unmapping invalid rmap out of mmu-lock later. > >> I still think the right way to fix this kind of thing is optimization for >> mmu-lock. >> ====== >> >> Which parts scare you? Let's find a way to optimize for it. ;). For example, >> if you do not like unmap_memslot_rmap_nolock(), we can simplify it - We can >> use walk_shadow_page_lockless_begin() and walk_shadow_page_lockless_end() to >> protect spte instead of kvm->being_unmaped_rmap. >> > > kvm->being_unmaped_rmap is particularly tricky, although looks Okay. Will use walk_shadow_page_lockless_begin() and walk_shadow_page_lockless_end instead. > correct. Additional indirection with rmap ops also does not help following > the code. I'd rather have if(slot is invalid) in a couple of places where > things should be done differently. In most places it will be WARN_ON(slot > is invalid). Less change, good to me, will do. ;) Thanks! -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
