Re: [PATCH v6 03/11] KVM: arm64: Relax locking for kvm_test_age_gfn and kvm_age_gfn

[James Houghton <jthoughton@google.com>]

On Tue, Jul 23, 2024 at 6:11 PM James Houghton <jthoughton@google.com> wrote:
>
> Replace the MMU write locks (taken in the memslot iteration loop) for
> read locks.
>
> Grabbing the read lock instead of the write lock is safe because the
> only requirement we have is that the stage-2 page tables do not get
> deallocated while we are walking them. The stage2_age_walker() callback
> is safe to race with itself; update the comment to reflect the
> synchronization change.
>
> Signed-off-by: James Houghton <jthoughton@google.com>
> ---

Here is some data to show that this patch at least *can* be helpful:

# arm64 patched to do aging (i.e., set HAVE_KVM_MMU_NOTIFIER_YOUNG_FAST_ONLY)
# The test is faulting memory in while doing aging as fast as possible.
# taskset -c 0-32 ./access_tracking_perf_test -l -r /dev/cgroup/memory
-p -v 32 -m 3

# Write lock
vcpu wall time                : 3.039207157s
lru_gen avg pass duration     : 1.660541541s, (passes:2, total:3.321083083s)

# Read lock
vcpu wall time                : 3.010848445s
lru_gen avg pass duration     : 0.306623698s, (passes:11, total:3.372860688s)

Aging is able to run significantly faster, but vCPU runtime isn't
affected much (in this test).

It would be really nice to motivate this patch with a test that didn't
require patching the kernel... Oliver and Marc, please let me know if
you'd like to see more data. I'm also happy to simply drop this patch.