[RFC,8/9] reduce kvm_lock hold times in mmu_skrink()

Commit Message

David Hansen June 15, 2010, 1:55 p.m. UTC

None

Patch

diff -puN arch/x86/kvm/mmu.c~optimize_shrinker-3 arch/x86/kvm/mmu.c
--- linux-2.6.git/arch/x86/kvm/mmu.c~optimize_shrinker-3	2010-06-11 08:39:17.000000000 -0700
+++ linux-2.6.git-dave/arch/x86/kvm/mmu.c	2010-06-11 08:39:17.000000000 -0700
@@ -2930,7 +2930,8 @@  static int kvm_mmu_remove_some_alloc_mmu
 
 static int shrink_kvm_mmu(struct kvm *kvm, int nr_to_scan)
 {
-	int idx, freed_pages;
+	int idx;
+	int freed_pages = 0;
 
 	idx = srcu_read_lock(&kvm->srcu);
 	spin_lock(&kvm->mmu_lock);
@@ -2950,23 +2951,50 @@  static int shrink_kvm_mmu(struct kvm *kv
 
 static int mmu_shrink(int nr_to_scan, gfp_t gfp_mask)
 {
+	int err;
+	int freed;
 	struct kvm *kvm;
 
 	if (nr_to_scan == 0)
 		goto out;
 
+retry:
+	nr_to_scan--;
 	spin_lock(&kvm_lock);
-
-	list_for_each_entry(kvm, &vm_list, vm_list) {
-		int freed = shrink_kvm_mmu(kvm, nr_to_scan);
-		if (!freed)
-			continue;
-
-		list_move_tail(&kvm->vm_list, &vm_list);
-		break;
+	if (list_empty(&vm_list)) {
+		spin_unlock(&kvm_lock);
+		goto out;
 	}
-
+	kvm = list_first_entry(&vm_list, struct kvm, vm_list);
+	/*
+	 * With a reference to the kvm object, it can not go away
+	 * nor get removed from the vm_list.
+	 */
+	err = kvm_get_kvm(kvm);
+	/* Did someone race and start destroying the kvm object? */
+	if (err) {
+		spin_unlock(&kvm_lock);
+		goto retry;
+	}
+	/*
+	 * Stick this kvm on the end of the list so the next
+	 * interation will shrink a different one.  Do this here
+	 * so that we normally don't have to reacquire the lock.
+	 */
+	list_move_tail(&kvm->vm_list, &vm_list);
+	/*
+	 * Which lets us release the global lock, holding it for
+	 * the minimal amount of time possible, and ensuring that
+	 * we don't hold it during the (presumably slow) shrink
+	 * operation itself.
+	 */
 	spin_unlock(&kvm_lock);
+	freed = shrink_kvm_mmu(kvm, nr_to_scan);
+
+	kvm_put_kvm(kvm);
+
+	if (!freed && nr_to_scan > 0)
+		goto retry;
 
 out:
 	return kvm_total_used_mmu_pages;
diff -puN virt/kvm/kvm_main.c~optimize_shrinker-3 virt/kvm/kvm_main.c
diff -puN kernel/profile.c~optimize_shrinker-3 kernel/profile.c
--- linux-2.6.git/kernel/profile.c~optimize_shrinker-3	2010-06-11 09:09:43.000000000 -0700
+++ linux-2.6.git-dave/kernel/profile.c	2010-06-11 09:12:24.000000000 -0700
@@ -314,6 +314,8 @@  void profile_hits(int type, void *__pc, 
 	if (prof_on != type || !prof_buffer)
 		return;
 	pc = min((pc - (unsigned long)_stext) >> prof_shift, prof_len - 1);
+	if ((pc == prof_len - 1) && printk_ratelimit())
+		printk("profile_hits(%d, %p, %d)\n", type, __pc, nr_hits);
 	i = primary = (pc & (NR_PROFILE_GRP - 1)) << PROFILE_GRPSHIFT;
 	secondary = (~(pc << 1) & (NR_PROFILE_GRP - 1)) << PROFILE_GRPSHIFT;
 	cpu = get_cpu();