Messages in this thread | | | Date | Sat, 5 Mar 2022 20:53:42 +0100 | Subject | Re: [PATCH v4 21/30] KVM: x86/mmu: Zap invalidated roots via asynchronous worker | From | Paolo Bonzini <> |
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On 3/5/22 01:34, Sean Christopherson wrote: > On Fri, Mar 04, 2022, Paolo Bonzini wrote: >> On 3/4/22 17:02, Sean Christopherson wrote: >>> On Fri, Mar 04, 2022, Paolo Bonzini wrote: >>>> On 3/3/22 22:32, Sean Christopherson wrote: >>>> I didn't remove the paragraph from the commit message, but I think it's >>>> unnecessary now. The workqueue is flushed in kvm_mmu_zap_all_fast() and >>>> kvm_mmu_uninit_tdp_mmu(), unlike the buggy patch, so it doesn't need to take >>>> a reference to the VM. >>>> >>>> I think I don't even need to check kvm->users_count in the defunct root >>>> case, as long as kvm_mmu_uninit_tdp_mmu() flushes and destroys the workqueue >>>> before it checks that the lists are empty. >>> >>> Yes, that should work. IIRC, the WARN_ONs will tell us/you quite quickly if >>> we're wrong :-) mmu_notifier_unregister() will call the "slow" kvm_mmu_zap_all() >>> and thus ensure all non-root pages zapped, but "leaking" a worker will trigger >>> the WARN_ON that there are no roots on the list. >> >> Good, for the record these are the commit messages I have:
I'm seeing some hangs in ~50% of installation jobs, both Windows and Linux. I have not yet tried to reproduce outside the automated tests, or to bisect, but I'll try to push at least the first part of the series for 5.18.
Paolo
>> KVM: x86/mmu: Zap invalidated roots via asynchronous worker >> Use the system worker threads to zap the roots invalidated >> by the TDP MMU's "fast zap" mechanism, implemented by >> kvm_tdp_mmu_invalidate_all_roots(). >> At this point, apart from allowing some parallelism in the zapping of >> roots, the workqueue is a glorified linked list: work items are added and >> flushed entirely within a single kvm->slots_lock critical section. However, >> the workqueue fixes a latent issue where kvm_mmu_zap_all_invalidated_roots() >> assumes that it owns a reference to all invalid roots; therefore, no >> one can set the invalid bit outside kvm_mmu_zap_all_fast(). Putting the >> invalidated roots on a linked list... erm, on a workqueue ensures that >> tdp_mmu_zap_root_work() only puts back those extra references that >> kvm_mmu_zap_all_invalidated_roots() had gifted to it. >> >> and >> >> KVM: x86/mmu: Zap defunct roots via asynchronous worker >> Zap defunct roots, a.k.a. roots that have been invalidated after their >> last reference was initially dropped, asynchronously via the existing work >> queue instead of forcing the work upon the unfortunate task that happened >> to drop the last reference. >> If a vCPU task drops the last reference, the vCPU is effectively blocked >> by the host for the entire duration of the zap. If the root being zapped >> happens be fully populated with 4kb leaf SPTEs, e.g. due to dirty logging >> being active, the zap can take several hundred seconds. Unsurprisingly, >> most guests are unhappy if a vCPU disappears for hundreds of seconds. >> E.g. running a synthetic selftest that triggers a vCPU root zap with >> ~64tb of guest memory and 4kb SPTEs blocks the vCPU for 900+ seconds. >> Offloading the zap to a worker drops the block time to <100ms. >> There is an important nuance to this change. If the same work item >> was queued twice before the work function has run, it would only >> execute once and one reference would be leaked. Therefore, now that >> queueing items is not anymore protected by write_lock(&kvm->mmu_lock), >> kvm_tdp_mmu_invalidate_all_roots() has to check root->role.invalid and >> skip already invalid roots. On the other hand, kvm_mmu_zap_all_fast() >> must return only after those skipped roots have been zapped as well. >> These two requirements can be satisfied only if _all_ places that >> change invalid to true now schedule the worker before releasing the >> mmu_lock. There are just two, kvm_tdp_mmu_put_root() and >> kvm_tdp_mmu_invalidate_all_roots(). > > Very nice! >
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