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Rusty Russell

2014-Sep-03 04:29 UTC

[PATCH 0/3] virtio: simplify virtio_ring.

I resurrected these patches after prompting from Andy Lutomirski's
recent patches.  I put them on the back-burner because vring_bench
had a 15% slowdown on my laptop: pktgen testing revealed a speedup,
if anything, so I've cleaned them up.

Rusty Russell (3):
  virtio_net: pass well-formed sgs to virtqueue_add_*()
  virtio_ring: assume sgs are always well-formed.
  virtio_ring: unify direct/indirect code paths.

 drivers/net/virtio_net.c     |   5 +-
 drivers/virtio/virtio_ring.c | 185 +++++++++++++++----------------------------
 2 files changed, 69 insertions(+), 121 deletions(-)

-- 
1.9.1

Rusty Russell

2014-Sep-03 04:29 UTC

head link

[PATCH 1/3] virtio_net: pass well-formed sgs to virtqueue_add_*()

This is the only driver which doesn't hand virtqueue_add_inbuf and
virtqueue_add_outbuf a well-formed, well-terminated sg.  Fix it,
so we can make virtio_add_* simpler.

pktgen results:
	modprobe pktgen
	echo 'add_device eth0' > /proc/net/pktgen/kpktgend_0
	echo nowait 1 > /proc/net/pktgen/eth0
	echo count 1000000 > /proc/net/pktgen/eth0
	echo clone_skb 100000 > /proc/net/pktgen/eth0
	echo dst_mac 4e:14:25:a9:30:ac > /proc/net/pktgen/eth0
	echo dst 192.168.1.2 > /proc/net/pktgen/eth0
	for i in `seq 20`; do echo start > /proc/net/pktgen/pgctrl; tail -n1
/proc/net/pktgen/eth0; done

Before:
  746547-793084(786421+/-9.6e+03)pps 346-367(364.4+/-4.4)Mb/sec
(346397808-367990976(3.649e+08+/-4.5e+06)bps) errors: 0

After:
  767390-792966(785159+/-6.5e+03)pps 356-367(363.75+/-2.9)Mb/sec
(356068960-367936224(3.64314e+08+/-3e+06)bps) errors: 0

Signed-off-by: Rusty Russell <rusty at rustcorp.com.au>
---
 drivers/net/virtio_net.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 59caa06f34a6..31cac511b3c3 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -546,8 +546,8 @@ static int add_recvbuf_small(struct receive_queue *rq, gfp_t
gfp)
 	skb_put(skb, GOOD_PACKET_LEN);
 
 	hdr = skb_vnet_hdr(skb);
+	sg_init_table(rq->sg, MAX_SKB_FRAGS + 2);
 	sg_set_buf(rq->sg, &hdr->hdr, sizeof hdr->hdr);
-
 	skb_to_sgvec(skb, rq->sg + 1, 0, skb->len);
 
 	err = virtqueue_add_inbuf(rq->vq, rq->sg, 2, skb, gfp);
@@ -563,6 +563,8 @@ static int add_recvbuf_big(struct receive_queue *rq, gfp_t
gfp)
 	char *p;
 	int i, err, offset;
 
+	sg_init_table(rq->sg, MAX_SKB_FRAGS + 2);
+
 	/* page in rq->sg[MAX_SKB_FRAGS + 1] is list tail */
 	for (i = MAX_SKB_FRAGS + 1; i > 1; --i) {
 		first = get_a_page(rq, gfp);
@@ -899,6 +901,7 @@ static int xmit_skb(struct send_queue *sq, struct sk_buff
*skb)
 	if (vi->mergeable_rx_bufs)
 		hdr->mhdr.num_buffers = 0;
 
+	sg_init_table(sq->sg, MAX_SKB_FRAGS + 2);
 	if (can_push) {
 		__skb_push(skb, hdr_len);
 		num_sg = skb_to_sgvec(skb, sq->sg, 0, skb->len);
-- 
1.9.1

Rusty Russell

2014-Sep-03 04:29 UTC

head link

[PATCH 2/3] virtio_ring: assume sgs are always well-formed.

We used to have several callers which just used arrays.  They're
gone, so we can use sg_next() everywhere, simplifying the code.

On my laptop, this slowed down vring_bench by 15%:

vring_bench before:
	936153354-967745359(9.44739e+08+/-6.1e+06)ns
vring_bench after:
	1061485790-1104800648(1.08254e+09+/-6.6e+06)ns

However, a more realistic test using pktgen on a AMD FX(tm)-8320 saw
a few percent improvement:

pktgen before:
  767390-792966(785159+/-6.5e+03)pps 356-367(363.75+/-2.9)Mb/sec
(356068960-367936224(3.64314e+08+/-3e+06)bps) errors: 0

pktgen after:
   787781-796334(793165+/-2.4e+03)pps 365-369(367.5+/-1.2)Mb/sec
(365530384-369498976(3.68028e+08+/-1.1e+06)bps) errors: 0

Signed-off-by: Rusty Russell <rusty at rustcorp.com.au>
---
 drivers/virtio/virtio_ring.c | 68 +++++++++++++-------------------------------
 1 file changed, 19 insertions(+), 49 deletions(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 4d08f45a9c29..374399c62080 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -99,28 +99,10 @@ struct vring_virtqueue
 
 #define to_vvq(_vq) container_of(_vq, struct vring_virtqueue, vq)
 
-static inline struct scatterlist *sg_next_chained(struct scatterlist *sg,
-						  unsigned int *count)
-{
-	return sg_next(sg);
-}
-
-static inline struct scatterlist *sg_next_arr(struct scatterlist *sg,
-					      unsigned int *count)
-{
-	if (--(*count) == 0)
-		return NULL;
-	return sg + 1;
-}
-
 /* Set up an indirect table of descriptors and add it to the queue. */
 static inline int vring_add_indirect(struct vring_virtqueue *vq,
 				     struct scatterlist *sgs[],
-				     struct scatterlist *(*next)
-				       (struct scatterlist *, unsigned int *),
 				     unsigned int total_sg,
-				     unsigned int total_out,
-				     unsigned int total_in,
 				     unsigned int out_sgs,
 				     unsigned int in_sgs,
 				     gfp_t gfp)
@@ -144,7 +126,7 @@ static inline int vring_add_indirect(struct vring_virtqueue
*vq,
 	/* Transfer entries from the sg lists into the indirect page */
 	i = 0;
 	for (n = 0; n < out_sgs; n++) {
-		for (sg = sgs[n]; sg; sg = next(sg, &total_out)) {
+		for (sg = sgs[n]; sg; sg = sg_next(sg)) {
 			desc[i].flags = VRING_DESC_F_NEXT;
 			desc[i].addr = sg_phys(sg);
 			desc[i].len = sg->length;
@@ -153,7 +135,7 @@ static inline int vring_add_indirect(struct vring_virtqueue
*vq,
 		}
 	}
 	for (; n < (out_sgs + in_sgs); n++) {
-		for (sg = sgs[n]; sg; sg = next(sg, &total_in)) {
+		for (sg = sgs[n]; sg; sg = sg_next(sg)) {
 			desc[i].flags = VRING_DESC_F_NEXT|VRING_DESC_F_WRITE;
 			desc[i].addr = sg_phys(sg);
 			desc[i].len = sg->length;
@@ -186,10 +168,7 @@ static inline int vring_add_indirect(struct vring_virtqueue
*vq,
 
 static inline int virtqueue_add(struct virtqueue *_vq,
 				struct scatterlist *sgs[],
-				struct scatterlist *(*next)
-				  (struct scatterlist *, unsigned int *),
-				unsigned int total_out,
-				unsigned int total_in,
+				unsigned int total_sg,
 				unsigned int out_sgs,
 				unsigned int in_sgs,
 				void *data,
@@ -197,7 +176,7 @@ static inline int virtqueue_add(struct virtqueue *_vq,
 {
 	struct vring_virtqueue *vq = to_vvq(_vq);
 	struct scatterlist *sg;
-	unsigned int i, n, avail, uninitialized_var(prev), total_sg;
+	unsigned int i, n, avail, uninitialized_var(prev);
 	int head;
 
 	START_USE(vq);
@@ -222,13 +201,10 @@ static inline int virtqueue_add(struct virtqueue *_vq,
 	}
 #endif
 
-	total_sg = total_in + total_out;
-
 	/* If the host supports indirect descriptor tables, and we have multiple
 	 * buffers, then go indirect. FIXME: tune this threshold */
 	if (vq->indirect && total_sg > 1 && vq->vq.num_free)
{
-		head = vring_add_indirect(vq, sgs, next, total_sg, total_out,
-					  total_in,
+		head = vring_add_indirect(vq, sgs, total_sg, 
 					  out_sgs, in_sgs, gfp);
 		if (likely(head >= 0))
 			goto add_head;
@@ -254,7 +230,7 @@ static inline int virtqueue_add(struct virtqueue *_vq,
 
 	head = i = vq->free_head;
 	for (n = 0; n < out_sgs; n++) {
-		for (sg = sgs[n]; sg; sg = next(sg, &total_out)) {
+		for (sg = sgs[n]; sg; sg = sg_next(sg)) {
 			vq->vring.desc[i].flags = VRING_DESC_F_NEXT;
 			vq->vring.desc[i].addr = sg_phys(sg);
 			vq->vring.desc[i].len = sg->length;
@@ -263,7 +239,7 @@ static inline int virtqueue_add(struct virtqueue *_vq,
 		}
 	}
 	for (; n < (out_sgs + in_sgs); n++) {
-		for (sg = sgs[n]; sg; sg = next(sg, &total_in)) {
+		for (sg = sgs[n]; sg; sg = sg_next(sg)) {
 			vq->vring.desc[i].flags = VRING_DESC_F_NEXT|VRING_DESC_F_WRITE;
 			vq->vring.desc[i].addr = sg_phys(sg);
 			vq->vring.desc[i].len = sg->length;
@@ -324,29 +300,23 @@ int virtqueue_add_sgs(struct virtqueue *_vq,
 		      void *data,
 		      gfp_t gfp)
 {
-	unsigned int i, total_out, total_in;
+	unsigned int i, total_sg = 0;
 
 	/* Count them first. */
-	for (i = total_out = total_in = 0; i < out_sgs; i++) {
-		struct scatterlist *sg;
-		for (sg = sgs[i]; sg; sg = sg_next(sg))
-			total_out++;
-	}
-	for (; i < out_sgs + in_sgs; i++) {
+	for (i = 0; i < out_sgs + in_sgs; i++) {
 		struct scatterlist *sg;
 		for (sg = sgs[i]; sg; sg = sg_next(sg))
-			total_in++;
+			total_sg++;
 	}
-	return virtqueue_add(_vq, sgs, sg_next_chained,
-			     total_out, total_in, out_sgs, in_sgs, data, gfp);
+	return virtqueue_add(_vq, sgs, total_sg, out_sgs, in_sgs, data, gfp);
 }
 EXPORT_SYMBOL_GPL(virtqueue_add_sgs);
 
 /**
  * virtqueue_add_outbuf - expose output buffers to other end
  * @vq: the struct virtqueue we're talking about.
- * @sgs: array of scatterlists (need not be terminated!)
- * @num: the number of scatterlists readable by other side
+ * @sg: scatterlist (must be well-formed and terminated!)
+ * @num: the number of entries in @sg readable by other side
  * @data: the token identifying the buffer.
  * @gfp: how to do memory allocations (if necessary).
  *
@@ -356,19 +326,19 @@ EXPORT_SYMBOL_GPL(virtqueue_add_sgs);
  * Returns zero or a negative error (ie. ENOSPC, ENOMEM, EIO).
  */
 int virtqueue_add_outbuf(struct virtqueue *vq,
-			 struct scatterlist sg[], unsigned int num,
+			 struct scatterlist *sg, unsigned int num,
 			 void *data,
 			 gfp_t gfp)
 {
-	return virtqueue_add(vq, &sg, sg_next_arr, num, 0, 1, 0, data, gfp);
+	return virtqueue_add(vq, &sg, num, 1, 0, data, gfp);
 }
 EXPORT_SYMBOL_GPL(virtqueue_add_outbuf);
 
 /**
  * virtqueue_add_inbuf - expose input buffers to other end
  * @vq: the struct virtqueue we're talking about.
- * @sgs: array of scatterlists (need not be terminated!)
- * @num: the number of scatterlists writable by other side
+ * @sg: scatterlist (must be well-formed and terminated!)
+ * @num: the number of entries in @sg writable by other side
  * @data: the token identifying the buffer.
  * @gfp: how to do memory allocations (if necessary).
  *
@@ -378,11 +348,11 @@ EXPORT_SYMBOL_GPL(virtqueue_add_outbuf);
  * Returns zero or a negative error (ie. ENOSPC, ENOMEM, EIO).
  */
 int virtqueue_add_inbuf(struct virtqueue *vq,
-			struct scatterlist sg[], unsigned int num,
+			struct scatterlist *sg, unsigned int num,
 			void *data,
 			gfp_t gfp)
 {
-	return virtqueue_add(vq, &sg, sg_next_arr, 0, num, 0, 1, data, gfp);
+	return virtqueue_add(vq, &sg, num, 0, 1, data, gfp);
 }
 EXPORT_SYMBOL_GPL(virtqueue_add_inbuf);
 
-- 
1.9.1

Rusty Russell

2014-Sep-03 04:29 UTC

head link

[PATCH 3/3] virtio_ring: unify direct/indirect code paths.

virtqueue_add() populates the virtqueue descriptor table from the sgs
given.  If it uses an indirect descriptor table, then it puts a single
descriptor in the descriptor table pointing to the kmalloc'ed indirect
table where the sg is populated.

Previously vring_add_indirect() did the allocation and the simple
linear layout.  We replace that with alloc_indirect() which allocates
the indirect table then chains it like the normal descriptor table so
we can reuse the core logic.

This slows down pktgen by less than 1/2 a percent (which uses direct
descriptors), as well as vring_bench, but it's far neater.

vring_bench before:
	1061485790-1104800648(1.08254e+09+/-6.6e+06)ns
vring_bench after:
	1125610268-1183528965(1.14172e+09+/-8e+06)ns

pktgen before:
   787781-796334(793165+/-2.4e+03)pps 365-369(367.5+/-1.2)Mb/sec
(365530384-369498976(3.68028e+08+/-1.1e+06)bps) errors: 0

pktgen after:
   779988-790404(786391+/-2.5e+03)pps 361-366(364.35+/-1.3)Mb/sec
(361914432-366747456(3.64885e+08+/-1.2e+06)bps) errors: 0

Now, if we make force indirect descriptors by turning off any_header_sg
in virtio_net.c:

pktgen before:
  713773-721062(718374+/-2.1e+03)pps 331-334(332.95+/-0.92)Mb/sec
(331190672-334572768(3.33325e+08+/-9.6e+05)bps) errors: 0
pktgen after:
  710542-719195(714898+/-2.4e+03)pps 329-333(331.15+/-1.1)Mb/sec
(329691488-333706480(3.31713e+08+/-1.1e+06)bps) errors: 0

Signed-off-by: Rusty Russell <rusty at rustcorp.com.au>
---
 drivers/virtio/virtio_ring.c | 125 +++++++++++++++++--------------------------
 1 file changed, 50 insertions(+), 75 deletions(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 374399c62080..a4ebbffac141 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -99,18 +99,10 @@ struct vring_virtqueue
 
 #define to_vvq(_vq) container_of(_vq, struct vring_virtqueue, vq)
 
-/* Set up an indirect table of descriptors and add it to the queue. */
-static inline int vring_add_indirect(struct vring_virtqueue *vq,
-				     struct scatterlist *sgs[],
-				     unsigned int total_sg,
-				     unsigned int out_sgs,
-				     unsigned int in_sgs,
-				     gfp_t gfp)
+static struct vring_desc *alloc_indirect(unsigned int total_sg, gfp_t gfp)
 {
-	struct vring_desc *desc;
-	unsigned head;
-	struct scatterlist *sg;
-	int i, n;
+ 	struct vring_desc *desc;
+	unsigned int i;
 
 	/*
 	 * We require lowmem mappings for the descriptors because
@@ -119,51 +111,13 @@ static inline int vring_add_indirect(struct
vring_virtqueue *vq,
 	 */
 	gfp &= ~(__GFP_HIGHMEM | __GFP_HIGH);
 
-	desc = kmalloc(total_sg * sizeof(struct vring_desc), gfp);
-	if (!desc)
-		return -ENOMEM;
-
-	/* Transfer entries from the sg lists into the indirect page */
-	i = 0;
-	for (n = 0; n < out_sgs; n++) {
-		for (sg = sgs[n]; sg; sg = sg_next(sg)) {
-			desc[i].flags = VRING_DESC_F_NEXT;
-			desc[i].addr = sg_phys(sg);
-			desc[i].len = sg->length;
-			desc[i].next = i+1;
-			i++;
-		}
-	}
-	for (; n < (out_sgs + in_sgs); n++) {
-		for (sg = sgs[n]; sg; sg = sg_next(sg)) {
-			desc[i].flags = VRING_DESC_F_NEXT|VRING_DESC_F_WRITE;
-			desc[i].addr = sg_phys(sg);
-			desc[i].len = sg->length;
-			desc[i].next = i+1;
-			i++;
-		}
-	}
-	BUG_ON(i != total_sg);
-
-	/* Last one doesn't continue. */
-	desc[i-1].flags &= ~VRING_DESC_F_NEXT;
-	desc[i-1].next = 0;
-
-	/* We're about to use a buffer */
-	vq->vq.num_free--;
-
-	/* Use a single buffer which doesn't continue */
-	head = vq->free_head;
-	vq->vring.desc[head].flags = VRING_DESC_F_INDIRECT;
-	vq->vring.desc[head].addr = virt_to_phys(desc);
-	/* kmemleak gives a false positive, as it's hidden by virt_to_phys */
-	kmemleak_ignore(desc);
-	vq->vring.desc[head].len = i * sizeof(struct vring_desc);
-
-	/* Update free pointer */
-	vq->free_head = vq->vring.desc[head].next;
+ 	desc = kmalloc(total_sg * sizeof(struct vring_desc), gfp);
+ 	if (!desc)
+		return NULL;
 
-	return head;
+	for (i = 0; i < total_sg; i++)
+		desc[i].next = i+1;
+	return desc;
 }
 
 static inline int virtqueue_add(struct virtqueue *_vq,
@@ -176,8 +130,10 @@ static inline int virtqueue_add(struct virtqueue *_vq,
 {
 	struct vring_virtqueue *vq = to_vvq(_vq);
 	struct scatterlist *sg;
+	struct vring_desc *desc;
 	unsigned int i, n, avail, uninitialized_var(prev);
 	int head;
+	bool indirect;
 
 	START_USE(vq);
 
@@ -201,18 +157,36 @@ static inline int virtqueue_add(struct virtqueue *_vq,
 	}
 #endif
 
+	BUG_ON(total_sg > vq->vring.num);
+	BUG_ON(total_sg == 0);
+
+	head = vq->free_head;
+
 	/* If the host supports indirect descriptor tables, and we have multiple
 	 * buffers, then go indirect. FIXME: tune this threshold */
-	if (vq->indirect && total_sg > 1 && vq->vq.num_free)
{
-		head = vring_add_indirect(vq, sgs, total_sg, 
-					  out_sgs, in_sgs, gfp);
-		if (likely(head >= 0))
-			goto add_head;
+	if (vq->indirect && total_sg > 1 && vq->vq.num_free)
+		desc = alloc_indirect(total_sg, gfp);
+	else
+		desc = NULL;
+
+	if (desc) {
+		/* Use a single buffer which doesn't continue */
+		vq->vring.desc[head].flags = VRING_DESC_F_INDIRECT;
+		vq->vring.desc[head].addr = virt_to_phys(desc);
+		/* avoid kmemleak false positive (hidden by virt_to_phys) */
+		kmemleak_ignore(desc);
+		vq->vring.desc[head].len = total_sg * sizeof(struct vring_desc);
+
+		/* Set up rest to use this indirect table. */
+		i = 0;
+		total_sg = 1;
+		indirect = true;
+	} else {
+		desc = vq->vring.desc;
+		i = head;
+		indirect = false;
 	}
 
-	BUG_ON(total_sg > vq->vring.num);
-	BUG_ON(total_sg == 0);
-
 	if (vq->vq.num_free < total_sg) {
 		pr_debug("Can't add buf len %i - avail = %i\n",
 			 total_sg, vq->vq.num_free);
@@ -228,32 +202,33 @@ static inline int virtqueue_add(struct virtqueue *_vq,
 	/* We're about to use some buffers from the free list. */
 	vq->vq.num_free -= total_sg;
 
-	head = i = vq->free_head;
 	for (n = 0; n < out_sgs; n++) {
 		for (sg = sgs[n]; sg; sg = sg_next(sg)) {
-			vq->vring.desc[i].flags = VRING_DESC_F_NEXT;
-			vq->vring.desc[i].addr = sg_phys(sg);
-			vq->vring.desc[i].len = sg->length;
+			desc[i].flags = VRING_DESC_F_NEXT;
+			desc[i].addr = sg_phys(sg);
+			desc[i].len = sg->length;
 			prev = i;
-			i = vq->vring.desc[i].next;
+			i = desc[i].next;
 		}
 	}
 	for (; n < (out_sgs + in_sgs); n++) {
 		for (sg = sgs[n]; sg; sg = sg_next(sg)) {
-			vq->vring.desc[i].flags = VRING_DESC_F_NEXT|VRING_DESC_F_WRITE;
-			vq->vring.desc[i].addr = sg_phys(sg);
-			vq->vring.desc[i].len = sg->length;
+			desc[i].flags = VRING_DESC_F_NEXT|VRING_DESC_F_WRITE;
+			desc[i].addr = sg_phys(sg);
+			desc[i].len = sg->length;
 			prev = i;
-			i = vq->vring.desc[i].next;
+			i = desc[i].next;
 		}
 	}
 	/* Last one doesn't continue. */
-	vq->vring.desc[prev].flags &= ~VRING_DESC_F_NEXT;
+	desc[prev].flags &= ~VRING_DESC_F_NEXT;
 
 	/* Update free pointer */
-	vq->free_head = i;
+	if (indirect)
+		vq->free_head = vq->vring.desc[head].next;
+	else
+		vq->free_head = i;
 
-add_head:
 	/* Set token. */
 	vq->data[head] = data;
 
-- 
1.9.1

Andy Lutomirski

2014-Sep-03 04:54 UTC

head link

[PATCH 1/3] virtio_net: pass well-formed sgs to virtqueue_add_*()

On Tue, Sep 2, 2014 at 9:29 PM, Rusty Russell <rusty at rustcorp.com.au>
wrote:> This is the only driver which doesn't hand virtqueue_add_inbuf and
> virtqueue_add_outbuf a well-formed, well-terminated sg.  Fix it,
> so we can make virtio_add_* simpler.
OK, I get it now: you're reinitializing the table every time, clearing
old end marks.  There's room to microoptimize this, but it's probably
not worth it.

IOW, looks good to me.

--Andy

Jesper Dangaard Brouer

2014-Sep-03 10:23 UTC

head link

[PATCH 1/3] virtio_net: pass well-formed sgs to virtqueue_add_*()

On Wed,  3 Sep 2014 13:59:14 +0930
Rusty Russell <rusty at rustcorp.com.au> wrote:
> pktgen results:
> 	modprobe pktgen
> 	echo 'add_device eth0' > /proc/net/pktgen/kpktgend_0
> 	echo nowait 1 > /proc/net/pktgen/eth0
Maybe your pktgen "nowait" (patch) should be a flag instead?

 echo "flag EXIT_NO_WAIT" > /proc/net/pktgen/eth0
> 	echo count 1000000 > /proc/net/pktgen/eth0
> 	echo clone_skb 100000 > /proc/net/pktgen/eth0
> 	echo dst_mac 4e:14:25:a9:30:ac > /proc/net/pktgen/eth0
> 	echo dst 192.168.1.2 > /proc/net/pktgen/eth0
> 	for i in `seq 20`; do echo start > /proc/net/pktgen/pgctrl; tail -n1
/proc/net/pktgen/eth0; done
> 

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Sr. Network Kernel Developer at Red Hat
  Author of http://www.iptv-analyzer.org
  LinkedIn: http://www.linkedin.com/in/brouer

Andy Lutomirski

2014-Sep-04 01:59 UTC

head link

[PATCH 3/3] virtio_ring: unify direct/indirect code paths.

On Tue, Sep 2, 2014 at 9:29 PM, Rusty Russell <rusty at rustcorp.com.au>
wrote:> virtqueue_add() populates the virtqueue descriptor table from the sgs
> given.  If it uses an indirect descriptor table, then it puts a single
> descriptor in the descriptor table pointing to the kmalloc'ed indirect
> table where the sg is populated.
>
> Previously vring_add_indirect() did the allocation and the simple
> linear layout.  We replace that with alloc_indirect() which allocates
> the indirect table then chains it like the normal descriptor table so
> we can reuse the core logic.
>
> +       if (vq->indirect && total_sg > 1 &&
vq->vq.num_free)
> +               desc = alloc_indirect(total_sg, gfp);
> +       else
> +               desc = NULL;
> +
> +       if (desc) {
> +               /* Use a single buffer which doesn't continue */
> +               vq->vring.desc[head].flags = VRING_DESC_F_INDIRECT;
> +               vq->vring.desc[head].addr = virt_to_phys(desc);
> +               /* avoid kmemleak false positive (hidden by virt_to_phys)
*/
> +               kmemleak_ignore(desc);
> +               vq->vring.desc[head].len = total_sg * sizeof(struct
vring_desc);
> +
> +               /* Set up rest to use this indirect table. */
> +               i = 0;
> +               total_sg = 1;
This is a little too magical for me.  Would it make sense to add a new
variable for this (total_root_descs or something)?

--Andy

Paolo Bonzini

2014-Sep-05 10:55 UTC

head link

[PATCH 3/3] virtio_ring: unify direct/indirect code paths.

Il 03/09/2014 06:29, Rusty Russell ha scritto:> + 	desc = kmalloc(total_sg * sizeof(struct vring_desc), gfp);
> + 	if (!desc)
> +		return NULL;
>  
> -	return head;
> +	for (i = 0; i < total_sg; i++)
> +		desc[i].next = i+1;
> +	return desc;
>  }
Would it make sense to keep a cache of a few (say) 8- or 16-element
indirect descriptors?  You'd only have to do this ugly (and slowish) for
loop on the first allocation.

Also, since this is mostly an aesthetic patch,
> +	if (indirect)
> +		vq->free_head = vq->vring.desc[head].next;
> +	else
> +		vq->free_head = i;
I'd move the indirect case above, where the vring.desc[head] is actually
allocated.

Paolo

David Miller

2014-Sep-05 21:27 UTC

head link

[PATCH 0/3] virtio: simplify virtio_ring.

From: Rusty Russell <rusty at rustcorp.com.au>
Date: Wed,  3 Sep 2014 13:59:13 +0930
> I resurrected these patches after prompting from Andy Lutomirski's
> recent patches.  I put them on the back-burner because vring_bench
> had a 15% slowdown on my laptop: pktgen testing revealed a speedup,
> if anything, so I've cleaned them up.
If you want to merge this via your virtio tree that's fine with me.

For virtio_net:

Acked-by: David S. Miller <davem at davemloft.net>

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