Subject: [RFC PATCH net-next v3 1/2] net: veth: Improving page pool recycling

From: Liang Chen <[email-masked]>
Date: Wed, 16 Aug 2023 20:30:28 +0800

Page pool is supported for veth. But for XDP_TX and XDP_REDIRECT cases,
the pages are not effectively recycled. "ethtool -S" statistics for the
page pool are as follows:

NIC statistics:
rx_pp_alloc_fast: 18041186
rx_pp_alloc_slow: 286369
rx_pp_recycle_ring: 0
rx_pp_recycle_released_ref: 18327555

This failure to recycle page pool pages is a result of the code snippet
below, which converts page pool pages into regular pages and releases
the skb data structure:

veth_xdp_get(xdp);
consume_skb(skb);

The reason behind is some skbs received from the veth peer are not page
pool pages, and remain so after conversion to xdp frame. In order to not
confusing __xdp_return with mixed regular pages and page pool pages, they
are all converted to regular pages. So registering xdp memory model as
MEM_TYPE_PAGE_SHARED is sufficient.

If we replace the above code with kfree_skb_partial, directly releasing
the skb data structure, we can retain the original page pool page behavior.
However, directly changing the xdp memory model to MEM_TYPE_PAGE_POOL is
not a solution as explained above. Therefore, we introduced an additionally
MEM_TYPE_PAGE_POOL model for each rq.

The following tests were conducted using pktgen to generate traffic and
evaluate the performance improvement after page pool pages get successfully
recycled in scenarios involving XDP_TX, XDP_REDIRECT, and AF_XDP.

Test environment setup:
ns1                 ns2
veth0   <-peer->    veth1
veth2   <-peer->    veth3

Test Results:
pktgen -> veth1 -> veth0(XDP_TX) -> veth1(XDP_DROP)
    without PP recycle: 1,855,069
    with PP recycle: 2,033,439
    improvement: ~10%

pktgen -> veth1 -> veth0(XDP_TX) -> veth1(XDP_PASS)
    without PP recycle: 1,494,890
    with PP recycle: 1,585,462
    improvement: 5%

pktgen -> veth1 -> veth0(XDP_REDIRECT) -> veth2 -> veth3(XDP_DROP)
    without PP recycle: 1,631,582
    with PP recycle: 1,787,342
    improvement: ~10%

pktgen -> veth1 -> veth0(XDP_REDIRECT) -> veth2 -> veth3(XDP_PASS)
    without PP recycle: 1,309,265
    with PP recycle: 1,391,587
    improvement: ~6%

pktgen -> veth1 -> veth0(AF_XDP) -> user space(DROP)
    without PP recycle: 1,674,021
    with PP recycle: 1,811,844
    improvement: ~8%

Additionally, the performance improvement were measured when converting to
xdp_buff doesn't require buffer copy and original skb uses regular pages,
i.e. page pool recycle not involved. This still gives around 2% improvement
attributed to the changes from consume_skb to kfree_skb_partial.

Signed-off-by: Liang Chen <[email-masked]>
---
 drivers/net/veth.c | 64 ++++++++++++++++++++++++++++++++++++++++------
 1 file changed, 56 insertions(+), 8 deletions(-)

diff --git a/drivers/net/veth.c b/drivers/net/veth.c
index 509e901da41d..7234eb0297dd 100644
--- a/drivers/net/veth.c
+++ b/drivers/net/veth.c
@@ -62,6 +62,7 @@ struct veth_rq {
 	struct net_device	*dev;
 	struct bpf_prog __rcu	*xdp_prog;
 	struct xdp_mem_info	xdp_mem;
+	struct xdp_mem_info	xdp_mem_pp;
 	struct veth_rq_stats	stats;
 	bool			rx_notify_masked;
 	struct ptr_ring		xdp_ring;
@@ -728,7 +729,8 @@ static void veth_xdp_get(struct xdp_buff *xdp)
 
 static int veth_convert_skb_to_xdp_buff(struct veth_rq *rq,
 					struct xdp_buff *xdp,
-					struct sk_buff **pskb)
+					struct sk_buff **pskb,
+					bool *local_pp_alloc)
 {
 	struct sk_buff *skb = *pskb;
 	u32 frame_sz;
@@ -802,6 +804,7 @@ static int veth_convert_skb_to_xdp_buff(struct veth_rq *rq,
 
 		consume_skb(skb);
 		skb = nskb;
+		*local_pp_alloc = true;
 	}
 
 	/* SKB "head" area always have tailroom for skb_shared_info */
@@ -827,6 +830,37 @@ static int veth_convert_skb_to_xdp_buff(struct veth_rq *rq,
 	return -ENOMEM;
 }
 
+static void __skb2xdp_steal_data(struct sk_buff *skb,
+				 struct xdp_buff *xdp,
+				 struct veth_rq *rq,
+				 bool local_pp_alloc)
+{
+	if (local_pp_alloc) {
+		/* This is the most common case where the skb was reallocated locally in
+		 * veth_convert_skb_to_xdp_buff, and it's safe to use the xdp_mem_pp model.
+		 */
+		xdp->rxq->mem = rq->xdp_mem_pp;
+		kfree_skb_partial(skb, true);
+	} else if (!skb->pp_recycle) {
+		/* We can safely use kfree_skb_partial here because this cannot be an fclone
+		 * skb. Fclone skbs are allocated via __alloc_skb, with their head buffer
+		 * allocated by kmalloc_reserve (i.e. skb->head_frag = 0), satisfying the
+		 * skb_head_is_locked condition in veth_convert_skb_to_xdp_buff, and are
+		 * thus reallocated.
+		 */
+		xdp->rxq->mem = rq->xdp_mem;
+		kfree_skb_partial(skb, true);
+	} else {
+		/* skbs in this case may include page_pool pages from peer. We cannot use
+		 * rq->xdp_mem_pp as for the local_pp_alloc case, because they might already
+		 * be associated with different xdp_mem_info.
+		 */
+		veth_xdp_get(xdp);
+		consume_skb(skb);
+		xdp->rxq->mem = rq->xdp_mem;
+	}
+}
+
 static struct sk_buff *veth_xdp_rcv_skb(struct veth_rq *rq,
 					struct sk_buff *skb,
 					struct veth_xdp_tx_bq *bq,
@@ -836,6 +870,7 @@ static struct sk_buff *veth_xdp_rcv_skb(struct veth_rq *rq,
 	struct bpf_prog *xdp_prog;
 	struct veth_xdp_buff vxbuf;
 	struct xdp_buff *xdp = &vxbuf.xdp;
+	bool local_pp_alloc = false;
 	u32 act, metalen;
 	int off;
 
@@ -849,7 +884,7 @@ static struct sk_buff *veth_xdp_rcv_skb(struct veth_rq *rq,
 	}
 
 	__skb_push(skb, skb->data - skb_mac_header(skb));
-	if (veth_convert_skb_to_xdp_buff(rq, xdp, &skb))
+	if (veth_convert_skb_to_xdp_buff(rq, xdp, &skb, &local_pp_alloc))
 		goto drop;
 	vxbuf.skb = skb;
 
@@ -862,9 +897,8 @@ static struct sk_buff *veth_xdp_rcv_skb(struct veth_rq *rq,
 	case XDP_PASS:
 		break;
 	case XDP_TX:
-		veth_xdp_get(xdp);
-		consume_skb(skb);
-		xdp->rxq->mem = rq->xdp_mem;
+		__skb2xdp_steal_data(skb, xdp, rq, local_pp_alloc);
+
 		if (unlikely(veth_xdp_tx(rq, xdp, bq) < 0)) {
 			trace_xdp_exception(rq->dev, xdp_prog, act);
 			stats->rx_drops++;
@@ -874,9 +908,8 @@ static struct sk_buff *veth_xdp_rcv_skb(struct veth_rq *rq,
 		rcu_read_unlock();
 		goto xdp_xmit;
 	case XDP_REDIRECT:
-		veth_xdp_get(xdp);
-		consume_skb(skb);
-		xdp->rxq->mem = rq->xdp_mem;
+		__skb2xdp_steal_data(skb, xdp, rq, local_pp_alloc);
+
 		if (xdp_do_redirect(rq->dev, xdp, xdp_prog)) {
 			stats->rx_drops++;
 			goto err_xdp;
@@ -1061,6 +1094,14 @@ static int __veth_napi_enable_range(struct net_device *dev, int start, int end)
 			goto err_page_pool;
 	}
 
+	for (i = start; i < end; i++) {
+		err = xdp_reg_mem_model(&priv->rq[i].xdp_mem_pp,
+					MEM_TYPE_PAGE_POOL,
+					priv->rq[i].page_pool);
+		if (err)
+			goto err_reg_mem;
+	}
+
 	for (i = start; i < end; i++) {
 		struct veth_rq *rq = &priv->rq[i];
 
@@ -1082,6 +1123,10 @@ static int __veth_napi_enable_range(struct net_device *dev, int start, int end)
 	for (i--; i >= start; i--)
 		ptr_ring_cleanup(&priv->rq[i].xdp_ring, veth_ptr_free);
 	i = end;
+err_reg_mem:
+	for (i--; i >= start; i--)
+		xdp_unreg_mem_model(&priv->rq[i].xdp_mem_pp);
+	i = end;
 err_page_pool:
 	for (i--; i >= start; i--) {
 		page_pool_destroy(priv->rq[i].page_pool);
@@ -1117,6 +1162,9 @@ static void veth_napi_del_range(struct net_device *dev, int start, int end)
 		ptr_ring_cleanup(&rq->xdp_ring, veth_ptr_free);
 	}
 
+	for (i = start; i < end; i++)
+		xdp_unreg_mem_model(&priv->rq[i].xdp_mem_pp);
+
 	for (i = start; i < end; i++) {
 		page_pool_destroy(priv->rq[i].page_pool);
 		priv->rq[i].page_pool = NULL;
-- 
2.40.1