[PATCH v2 1/6] vhost: use binary search instead of linear in find_region()

[Igor Mammedov]

For default region layouts performance stays the same
as linear search i.e. it takes around 210ns average for
translate_desc() that inlines find_region().

But it scales better with larger amount of regions,
235ns BS vs 300ns LS with 55 memory regions
and it will be about the same values when allowed number
of slots is increased to 509 like it has been done in KVM.

Signed-off-by: Igor Mammedov <imammedo@xxxxxxxxxx>
---
v2:
  move kvfree() to 2/2 where it belongs
---
 drivers/vhost/vhost.c | 36 +++++++++++++++++++++++++++---------
 1 file changed, 27 insertions(+), 9 deletions(-)

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 2ee2826..f1e07b8 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -25,6 +25,7 @@
 #include <linux/kthread.h>
 #include <linux/cgroup.h>
 #include <linux/module.h>
+#include <linux/sort.h>
 
 #include "vhost.h"
 
@@ -590,6 +591,16 @@ int vhost_vq_access_ok(struct vhost_virtqueue *vq)
 }
 EXPORT_SYMBOL_GPL(vhost_vq_access_ok);
 
+static int vhost_memory_reg_sort_cmp(const void *p1, const void *p2)
+{
+	const struct vhost_memory_region *r1 = p1, *r2 = p2;
+	if (r1->guest_phys_addr < r2->guest_phys_addr)
+		return 1;
+	if (r1->guest_phys_addr > r2->guest_phys_addr)
+		return -1;
+	return 0;
+}
+
 static long vhost_set_memory(struct vhost_dev *d, struct vhost_memory __user *m)
 {
 	struct vhost_memory mem, *newmem, *oldmem;
@@ -612,6 +623,8 @@ static long vhost_set_memory(struct vhost_dev *d, struct vhost_memory __user *m)
 		kfree(newmem);
 		return -EFAULT;
 	}
+	sort(newmem->regions, newmem->nregions, sizeof(*newmem->regions),
+		vhost_memory_reg_sort_cmp, NULL);
 
 	if (!memory_access_ok(d, newmem, 0)) {
 		kfree(newmem);
@@ -913,17 +926,22 @@ EXPORT_SYMBOL_GPL(vhost_dev_ioctl);
 static const struct vhost_memory_region *find_region(struct vhost_memory *mem,
 						     __u64 addr, __u32 len)
 {
-	struct vhost_memory_region *reg;
-	int i;
+	const struct vhost_memory_region *reg;
+	int start = 0, end = mem->nregions;
 
-	/* linear search is not brilliant, but we really have on the order of 6
-	 * regions in practice */
-	for (i = 0; i < mem->nregions; ++i) {
-		reg = mem->regions + i;
-		if (reg->guest_phys_addr <= addr &&
-		    reg->guest_phys_addr + reg->memory_size - 1 >= addr)
-			return reg;
+	while (start < end) {
+		int slot = start + (end - start) / 2;
+		reg = mem->regions + slot;
+		if (addr >= reg->guest_phys_addr)
+			end = slot;
+		else
+			start = slot + 1;
 	}
+
+	reg = mem->regions + start;
+	if (addr >= reg->guest_phys_addr &&
+		reg->guest_phys_addr + reg->memory_size > addr)
+		return reg;
 	return NULL;
 }
 
-- 
1.8.3.1

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