On Wed, 27 Oct 2021 04:55:16 -0400, Michael S. Tsirkin [off-list ref] wrote:

On Wed, Oct 27, 2021 at 02:19:11PM +0800, Xuan Zhuo wrote:

quoted

In the case of using indirect, indirect desc must be allocated and
released each time, which increases a lot of cpu overhead.

Here, a cache is added for indirect. If the number of indirect desc to be
applied for is less than VIRT_QUEUE_CACHE_DESC_NUM, the desc array with
the size of VIRT_QUEUE_CACHE_DESC_NUM is fixed and cached for reuse.

Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
---
 drivers/virtio/virtio.c      |  6 ++++
 drivers/virtio/virtio_ring.c | 63 ++++++++++++++++++++++++++++++------
 include/linux/virtio.h       | 10 ++++++
 3 files changed, 70 insertions(+), 9 deletions(-)

diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
index 0a5b54034d4b..04bcb74e5b9a 100644
--- a/drivers/virtio/virtio.c
+++ b/drivers/virtio/virtio.c

@@ -431,6 +431,12 @@ bool is_virtio_device(struct device *dev)
 }
 EXPORT_SYMBOL_GPL(is_virtio_device);

+void virtio_use_desc_cache(struct virtio_device *dev, bool val)
+{
+	dev->desc_cache = val;
+}
+EXPORT_SYMBOL_GPL(virtio_use_desc_cache);
+
 void unregister_virtio_device(struct virtio_device *dev)
 {
 	int index = dev->index; /* save for after device release */

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index dd95dfd85e98..0b9a8544b0e8 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c

@@ -117,6 +117,10 @@ struct vring_virtqueue {
 	/* Hint for event idx: already triggered no need to disable. */
 	bool event_triggered;

+	/* Is indirect cache used? */
+	bool use_desc_cache;
+	void *desc_cache_chain;
+
 	union {
 		/* Available for split ring */
 		struct {

Let's use llist_head and friends please (I am guessing we want
single-linked to avoid writing into indirect buffer after use,
invalidating the cache, but please document why in a comment).  Do not
open-code it.

Also hide all casts in inline wrappers.

quoted

@@ -423,12 +427,47 @@ static unsigned int vring_unmap_one_split(const struct vring_virtqueue *vq,
 	return extra[i].next;
 }

-static struct vring_desc *alloc_indirect_split(struct virtqueue *_vq,
+#define VIRT_QUEUE_CACHE_DESC_NUM 4

Add an inline wrapper for checking sg versus VIRT_QUEUE_CACHE_DESC_NUM.

quoted

+
+static void desc_cache_chain_free_split(void *chain)
+{
+	struct vring_desc *desc;
+
+	while (chain) {
+		desc = chain;
+		chain = (void *)desc->addr;
+		kfree(desc);
+	}
+}
+
+static void desc_cache_put_split(struct vring_virtqueue *vq,
+				 struct vring_desc *desc, int n)
+{
+	if (vq->use_desc_cache && n <= VIRT_QUEUE_CACHE_DESC_NUM) {
+		desc->addr = (u64)vq->desc_cache_chain;
+		vq->desc_cache_chain = desc;
+	} else {
+		kfree(desc);
+	}
+}

quoted

+
+static struct vring_desc *alloc_indirect_split(struct vring_virtqueue *vq,
 					       unsigned int total_sg,
 					       gfp_t gfp)
 {
 	struct vring_desc *desc;
-	unsigned int i;
+	unsigned int i, n;
+
+	if (vq->use_desc_cache && total_sg <= VIRT_QUEUE_CACHE_DESC_NUM) {
+		if (vq->desc_cache_chain) {
+			desc = vq->desc_cache_chain;
+			vq->desc_cache_chain = (void *)desc->addr;
+			goto got;
+		}
+		n = VIRT_QUEUE_CACHE_DESC_NUM;

Hmm. This will allocate more entries than actually used. Why do it?

If total_sg here is 2, we only allocate two desc here, but if total_sg is 3
later, it will be difficult to use the desc allocated this time.

So if total_sg is less than or equal to 4, a desc array of size 4 is allocated,
so that subsequent desc arrays can be reused when total_sg is less than or
equal to 4. If total_sg is greater than 4, use kmalloc_array to allocate
directly without using the cache.

Thanks.

quoted

+	} else {
+		n = total_sg;
+	}

 	/*
 	 * We require lowmem mappings for the descriptors because

@@ -437,12 +476,13 @@ static struct vring_desc *alloc_indirect_split(struct virtqueue *_vq,
 	 */
 	gfp &= ~__GFP_HIGHMEM;

-	desc = kmalloc_array(total_sg, sizeof(struct vring_desc), gfp);
+	desc = kmalloc_array(n, sizeof(struct vring_desc), gfp);
 	if (!desc)
 		return NULL;

+got:
 	for (i = 0; i < total_sg; i++)
-		desc[i].next = cpu_to_virtio16(_vq->vdev, i + 1);
+		desc[i].next = cpu_to_virtio16(vq->vq.vdev, i + 1);
 	return desc;
 }

@@ -508,7 +548,7 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
 	head = vq->free_head;

 	if (virtqueue_use_indirect(_vq, total_sg))
-		desc = alloc_indirect_split(_vq, total_sg, gfp);
+		desc = alloc_indirect_split(vq, total_sg, gfp);
 	else {
 		desc = NULL;
 		WARN_ON_ONCE(total_sg > vq->split.vring.num && !vq->indirect);

@@ -652,7 +692,7 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
 	}

 	if (indirect)
-		kfree(desc);
+		desc_cache_put_split(vq, desc, total_sg);

 	END_USE(vq);
 	return -ENOMEM;

@@ -717,7 +757,7 @@ static void detach_buf_split(struct vring_virtqueue *vq, unsigned int head,
 	if (vq->indirect) {
 		struct vring_desc *indir_desc =
 				vq->split.desc_state[head].indir_desc;
-		u32 len;
+		u32 len, n;

 		/* Free the indirect table, if any, now that it's unmapped. */
 		if (!indir_desc)

@@ -729,10 +769,12 @@ static void detach_buf_split(struct vring_virtqueue *vq, unsigned int head,
 				VRING_DESC_F_INDIRECT));
 		BUG_ON(len == 0 || len % sizeof(struct vring_desc));

-		for (j = 0; j < len / sizeof(struct vring_desc); j++)
+		n = len / sizeof(struct vring_desc);
+
+		for (j = 0; j < n; j++)
 			vring_unmap_one_split_indirect(vq, &indir_desc[j]);

-		kfree(indir_desc);
+		desc_cache_put_split(vq, indir_desc, n);
 		vq->split.desc_state[head].indir_desc = NULL;
 	} else if (ctx) {
 		*ctx = vq->split.desc_state[head].indir_desc;

@@ -2199,6 +2241,8 @@ struct virtqueue *__vring_new_virtqueue(unsigned int index,
 	vq->indirect = virtio_has_feature(vdev, VIRTIO_RING_F_INDIRECT_DESC) &&
 		!context;
 	vq->event = virtio_has_feature(vdev, VIRTIO_RING_F_EVENT_IDX);
+	vq->desc_cache_chain = NULL;
+	vq->use_desc_cache = vdev->desc_cache;

 	if (virtio_has_feature(vdev, VIRTIO_F_ORDER_PLATFORM))
 		vq->weak_barriers = false;

@@ -2329,6 +2373,7 @@ void vring_del_virtqueue(struct virtqueue *_vq)
 	if (!vq->packed_ring) {
 		kfree(vq->split.desc_state);
 		kfree(vq->split.desc_extra);
+		desc_cache_chain_free_split(vq->desc_cache_chain);
 	}
 	kfree(vq);
 }

diff --git a/include/linux/virtio.h b/include/linux/virtio.h
index 41edbc01ffa4..d84b7b8f4070 100644
--- a/include/linux/virtio.h
+++ b/include/linux/virtio.h

@@ -109,6 +109,7 @@ struct virtio_device {
 	bool failed;
 	bool config_enabled;
 	bool config_change_pending;
+	bool desc_cache;
 	spinlock_t config_lock;
 	spinlock_t vqs_list_lock; /* Protects VQs list access */
 	struct device dev;

@@ -130,6 +131,15 @@ int register_virtio_device(struct virtio_device *dev);
 void unregister_virtio_device(struct virtio_device *dev);
 bool is_virtio_device(struct device *dev);

+/**
+ * virtio_use_desc_cache - virtio ring use desc cache
+ *
+ * virtio will cache the allocated indirect desc.
+ *
+ * This function must be called before find_vqs.
+ */
+void virtio_use_desc_cache(struct virtio_device *dev, bool val);
+
 void virtio_break_device(struct virtio_device *dev);

 void virtio_config_changed(struct virtio_device *dev);
--

2.31.0

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