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

[Sean Christopherson]

On Thu, Jul 25, 2024, James Houghton wrote:
> On Tue, Jul 23, 2024 at 6:11 PM James Houghton <jthoughton at 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 at 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).

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