KVM: MMU: zap pages in batch

Zap at lease 10 pages before releasing mmu-lock to reduce the overload
caused by requiring lock

After the patch, kvm_zap_obsolete_pages can forward progress anyway,
so update the comments

[ It improves the case 0.6% ~ 1% that do kernel building meanwhile read
  PCI ROM. ]

Note: i am not sure that "10" is the best speculative value, i just
guessed that '10' can make vcpu do not spend long time on
kvm_zap_obsolete_pages and do not cause mmu-lock too hungry.

Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Reviewed-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>

1 files changed, 11 insertions, 24 deletions

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 0880b9b425d7..fe9d6f10e7a9 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -4197,14 +4197,18 @@ void kvm_mmu_slot_remove_write_access(struct kvm *kvm, int slot)
         spin_unlock(&kvm->mmu_lock);
 }
 
+#define BATCH_ZAP_PAGES 10
 static void kvm_zap_obsolete_pages(struct kvm *kvm)
 {
         struct kvm_mmu_page *sp, *node;
         LIST_HEAD(invalid_list);
+        int batch = 0;
 
 restart:
         list_for_each_entry_safe_reverse(sp, node,
               &kvm->arch.active_mmu_pages, link) {
+                int ret;
+
                 /*
                  * No obsolete page exists before new created page since
                  * active_mmu_pages is the FIFO list.
@@ -4213,28 +4217,6 @@ restart:
                         break;
 
                 /*
-                 * Do not repeatedly zap a root page to avoid unnecessary
-                 * KVM_REQ_MMU_RELOAD, otherwise we may not be able to
-                 * progress:
-                 *    vcpu 0                        vcpu 1
-                 *                         call vcpu_enter_guest():
-                 *                            1): handle KVM_REQ_MMU_RELOAD
-                 *                                and require mmu-lock to
-                 *                                load mmu
-                 * repeat:
-                 *    1): zap root page and
-                 *        send KVM_REQ_MMU_RELOAD
-                 *
-                 *    2): if (cond_resched_lock(mmu-lock))
-                 *
-                 *                            2): hold mmu-lock and load mmu
-                 *
-                 *                            3): see KVM_REQ_MMU_RELOAD bit
-                 *                                on vcpu->requests is set
-                 *                                then return 1 to call
-                 *                                vcpu_enter_guest() again.
-                 *            goto repeat;
-                 *
                  * Since we are reversely walking the list and the invalid
                  * list will be moved to the head, skip the invalid page
                  * can help us to avoid the infinity list walking.
@@ -4242,13 +4224,18 @@ restart:
                 if (sp->role.invalid)
                         continue;
 
-                if (need_resched() || spin_needbreak(&kvm->mmu_lock)) {
+                if (batch >= BATCH_ZAP_PAGES &&
+                      (need_resched() || spin_needbreak(&kvm->mmu_lock))) {
+                        batch = 0;
                         kvm_mmu_commit_zap_page(kvm, &invalid_list);
                         cond_resched_lock(&kvm->mmu_lock);
                         goto restart;
                 }
 
-                if (kvm_mmu_prepare_zap_page(kvm, sp, &invalid_list))
+                ret = kvm_mmu_prepare_zap_page(kvm, sp, &invalid_list);
+                batch += ret;
+
+                if (ret)
                         goto restart;
         }