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Commit e7d11c7a authored by Xiao Guangrong's avatar Xiao Guangrong Committed by Gleb Natapov
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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: default avatarXiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Reviewed-by: default avatarMarcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: default avatarGleb Natapov <gleb@redhat.com>
parent 7f52af74

arch/x86/kvm/mmu.c

+11 −24
Original line number Diff line number Diff line
@@ -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 @@ static void kvm_zap_obsolete_pages(struct kvm *kvm) 
			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 @@ static void kvm_zap_obsolete_pages(struct kvm *kvm) 
		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;

	}