[Devel] [PATCH rh7 2/2] kvm: move actual VM memory shrink out of kvm_lock

[Konstantin Khorenko]

We face a situation when a node with many cpu cores (88) and a lot
of RAM (1Tb) and many VMs (300) has almost all cpu cores busy in
mmu_shrink_scan():
all but one just wait for kvm_lock,
the last one is busy with actual memory shrink for a VM.

Let's allow parallel VM shrinking:
- just inc the VM usage count, so it's not destroyed under us
- drop the kvm_lock, so over shrinkers are free to go
- and shrink our VM without holding the kvm_lock
- dec the VM usage count after we finish with shrinking

As we shrink only single VM, we don't need the protect the vm_list
all the way during shrink.

https://jira.sw.ru/browse/PSBM-95077

Signed-off-by: Konstantin Khorenko <khorenko at virtuozzo.com>
---
 arch/x86/kvm/mmu.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 1d576bf305e1..0c3ded90dd38 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -5359,8 +5359,10 @@ mmu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
 		 * to shrink more than one VM and it is very unlikely to see
 		 * !n_used_mmu_pages so many times.
 		 */
-		if (!nr_to_scan--)
+		if (!nr_to_scan--) {
+			spin_unlock(&kvm_lock);
 			break;
+		}
 
 		/* Does not matter if we will shrink current VM or not, let's
 		 * move it to the tail, so next shrink won't hit it again soon.
@@ -5381,6 +5383,9 @@ mmu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
 		      !kvm_has_zapped_obsolete_pages(kvm))
 			continue;
 
+		kvm_get_kvm(kvm);
+		spin_unlock(&kvm_lock);
+
 		idx = srcu_read_lock(&kvm->srcu);
 		spin_lock(&kvm->mmu_lock);
 
@@ -5398,10 +5403,11 @@ mmu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
 		spin_unlock(&kvm->mmu_lock);
 		srcu_read_unlock(&kvm->srcu, idx);
 
+		kvm_put_kvm(kvm);
+
 		break;
 	}
 
-	spin_unlock(&kvm_lock);
 	return freed;
 
 }
-- 
2.15.1