On Thu, Jul 25, 2024, James Houghton wrote:

On Tue, Jul 23, 2024 at 6:11 PM James Houghton [off-list ref] wrote:

quoted

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 <redacted>
---

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).

Were you expecting vCPU runtime to improve (more)?  If so, lack of movement could
be due to KVM arm64 taking mmap_lock for read when handling faults:

https://lore.kernel.org/all/Zr0ZbPQHVNzmvwa6@google.com (local)