Re: [PATCH v2 00/28] Allow parallel MMU operations with TDP MMU

[Paolo Bonzini]

On 02/02/21 19:57, Ben Gardon wrote:
> The TDP MMU was implemented to simplify and improve the performance of
> KVM's memory management on modern hardware with TDP (EPT / NPT). To build
> on the existing performance improvements of the TDP MMU, add the ability
> to handle vCPU page faults, enabling and disabling dirty logging, and
> removing mappings, in parallel. In the current implementation,
> vCPU page faults (actually EPT/NPT violations/misconfigurations) are the
> largest source of MMU lock contention on VMs with many vCPUs. This
> contention, and the resulting page fault latency, can soft-lock guests
> and degrade performance. Handling page faults in parallel is especially
> useful when booting VMs, enabling dirty logging, and handling demand
> paging. In all these cases vCPUs are constantly incurring  page faults on
> each new page accessed.
>
> Broadly, the following changes were required to allow parallel page
> faults (and other MMU operations):
> -- Contention detection and yielding added to rwlocks to bring them up to
>     feature parity with spin locks, at least as far as the use of the MMU
>     lock is concerned.
> -- TDP MMU page table memory is protected with RCU and freed in RCU
>     callbacks to allow multiple threads to operate on that memory
>     concurrently.
> -- The MMU lock was changed to an rwlock on x86. This allows the page
>     fault handlers to acquire the MMU lock in read mode and handle page
>     faults in parallel, and other operations to maintain exclusive use of
>     the lock by acquiring it in write mode.
> -- An additional lock is added to protect some data structures needed by
>     the page fault handlers, for relatively infrequent operations.
> -- The page fault handler is modified to use atomic cmpxchgs to set SPTEs
>     and some page fault handler operations are modified slightly to work
>     concurrently with other threads.
>
> This series also contains a few bug fixes and optimizations, related to
> the above, but not strictly part of enabling parallel page fault handling.
>
> Correctness testing:
> The following tests were performed with an SMP kernel and DBX kernel on an
> Intel Skylake machine. The tests were run both with and without the TDP
> MMU enabled.
> -- This series introduces no new failures in kvm-unit-tests
> SMP + no TDP MMU no new failures
> SMP + TDP MMU no new failures
> DBX + no TDP MMU no new failures
> DBX + TDP MMU no new failures

What's DBX?  Lockdep etc.?

> -- All KVM selftests behave as expected
> SMP + no TDP MMU all pass except ./x86_64/vmx_preemption_timer_test
> SMP + TDP MMU all pass except ./x86_64/vmx_preemption_timer_test
> (./x86_64/vmx_preemption_timer_test also fails without this patch set,
> both with the TDP MMU on and off.)

Yes, it's flaky.  It depends on your host.

> DBX + no TDP MMU all pass
> DBX + TDP MMU all pass
> -- A VM can be booted running a Debian 9 and all memory accessed
> SMP + no TDP MMU works
> SMP + TDP MMU works
> DBX + no TDP MMU works
> DBX + TDP MMU works
>
> This series can be viewed in Gerrit at:
> https://linux-review.googlesource.com/c/linux/kernel/git/torvalds/linux/+/7172

Looks good!  I'll wait for a few days of reviews, but I'd like to queue 
this for 5.12 and I plan to make it the default in 5.13 or 5.12-rc 
(depending on when I can ask Red Hat QE to give it a shake).

It also needs more documentation though.  I'll do that myself based on 
your KVM Forum talk so that I can teach myself more of it.

Paolo