Re: [PATCH v4 21/30] KVM: x86/mmu: Zap invalidated roots via asynchronous worker
From: Paolo Bonzini
Date: Sat Mar 05 2022 - 14:54:00 EST
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!