Re: [Patch v4 03/18] KVM: x86/mmu: Track count of pages in KVM MMU page caches globally

[David Matlack <dmatlack@google.com>]

On Thu, Mar 09, 2023 at 04:28:10PM -0800, Vipin Sharma wrote:
> On Thu, Mar 9, 2023 at 3:53 PM David Matlack <dmatlack@google.com> wrote:
> >
> > On Mon, Mar 06, 2023 at 02:41:12PM -0800, Vipin Sharma wrote:
> > > Create a global counter for total number of pages available
> > > in MMU page caches across all VMs. Add mmu_shadow_page_cache
> > > pages to this counter.
> >
> > I think I prefer counting the objects on-demand in mmu_shrink_count(),
> > instead of keeping track of the count. Keeping track of the count adds
> > complexity to the topup/alloc paths for the sole benefit of the
> > shrinker. I'd rather contain that complexity to the shrinker code unless
> > there is a compelling reason to optimize mmu_shrink_count().
> >
> > IIRC we discussed this at one point. Was there a reason to take this
> > approach that I'm just forgetting?
> 
> To count on demand, we first need to lock on kvm_lock and then for
> each VMs iterate through each vCPU, take a lock, and sum the objects
> count in caches. When the NUMA support will be introduced in this
> series then it means we have to iterate even more caches. We
> can't/shouldn't use mutex_trylock() as it will not give the correct
> picture and when shrink_scan is called count can be totally different.

Yeah good point. Hm, do we need to take the cache mutex to calculate the
count though? mmu_shrink_count() is inherently racy (something could get
freed or allocated in between count() and scan()). I don't think holding
the mutex buys us anything over just reading the count without the
mutex.

e.g.

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index df8dcb7e5de7..c80a5c52f0ea 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -6739,10 +6739,20 @@ static unsigned long mmu_shrink_scan(struct shrinker *shrink,
 static unsigned long mmu_shrink_count(struct shrinker *shrink,
                                      struct shrink_control *sc)
 {
-       s64 count = percpu_counter_sum(&kvm_total_unused_cached_pages);
+       struct kvm *kvm, *next_kvm;
+       unsigned long count = 0;

-       WARN_ON(count < 0);
-       return count <= 0 ? SHRINK_EMPTY : count;
+       mutex_lock(&kvm_lock);
+       list_for_each_entry_safe(kvm, next_kvm, &vm_list, vm_list) {
+               struct kvm_vcpu *vcpu;
+               unsigned long i;
+
+               kvm_for_each_vcpu(i, vcpu, kvm)
+                       count += READ_ONCE(vcpu->arch.mmu_shadow_page_cache.nobjs);
+       }
+       mutex_unlock(&kvm_lock);
+
+       return count == 0 ? SHRINK_EMPTY : count;

 }

Then the only concern is an additional acquire of kvm_lock. But it
should be fairly quick (quicker than mmu_shrink_scan()). If we can
tolerate the kvm_lock overhead of mmu_shrink_scan(), then we should be
able to tolerate some here.

> 
> scan_count() API comment says to not do any deadlock check (I don't
> know what does that mean) and percpu_counter is very fast when we are
> adding/subtracting pages so the effect of using it to keep global
> count is very minimal. Since, there is not much impact to using
> percpu_count compared to previous one, we ended our discussion with
> keeping this per cpu counter.

Yeah it's just the code complexity of maintaing
kvm_total_unused_cached_pages that I'm hoping to avoid. We have to
create the counter, destroy it, and keep it up to date. Some
kvm_mmu_memory_caches have to update the counter, and others don't. It's
just adds a lot of bookkeeping code that I'm not convinced is worth the
it.