Subject: [PATCH 1/4] net: skb_orphan on dev_hard_start_xmit

From: Rusty Russell <[email-masked]>
Date: Fri, 29 May 2009 23:44:55 +0930

Various drivers do skb_orphan() because they do not free transmitted
skbs in a timely manner (causing apps which hit their socket limits to
stall, socket close to hang, etc.).

DaveM points out that there are advantages to doing it generally (it's
more likely to be on same CPU than after xmit), and I couldn't find
any new starvation issues in simple benchmarking here.

This patch adds skb_orphan to the start of dev_hard_start_xmit(): it
can be premature in the NETDEV_TX_BUSY case, but that's uncommon.

I removed the drivers' skb_orphan(), though it's harmless.

Signed-off-by: Rusty Russell <[email-masked]>
Cc: Divy Le Ray <[email-masked]>
Cc: Roland Dreier <[email-masked]>
Cc: Pavel Emelianov <[email-masked]>
Cc: Dan Williams <[email-masked]>
Cc: [email-masked]
---
 drivers/net/cxgb3/sge.c            |   27 ---------------------------
 drivers/net/loopback.c             |    2 --
 drivers/net/mlx4/en_tx.c           |    4 ----
 drivers/net/niu.c                  |    3 +--
 drivers/net/veth.c                 |    2 --
 drivers/net/wireless/libertas/tx.c |    3 ---
 net/core/dev.c                     |   21 +++++----------------
 7 files changed, 6 insertions(+), 56 deletions(-)

diff --git a/drivers/net/cxgb3/sge.c b/drivers/net/cxgb3/sge.c
--- a/drivers/net/cxgb3/sge.c
+++ b/drivers/net/cxgb3/sge.c
@@ -1288,33 +1288,6 @@ int t3_eth_xmit(struct sk_buff *skb, str
 	dev->trans_start = jiffies;
 	spin_unlock(&q->lock);
 
-	/*
-	 * We do not use Tx completion interrupts to free DMAd Tx packets.
-	 * This is good for performamce but means that we rely on new Tx
-	 * packets arriving to run the destructors of completed packets,
-	 * which open up space in their sockets' send queues.  Sometimes
-	 * we do not get such new packets causing Tx to stall.  A single
-	 * UDP transmitter is a good example of this situation.  We have
-	 * a clean up timer that periodically reclaims completed packets
-	 * but it doesn't run often enough (nor do we want it to) to prevent
-	 * lengthy stalls.  A solution to this problem is to run the
-	 * destructor early, after the packet is queued but before it's DMAd.
-	 * A cons is that we lie to socket memory accounting, but the amount
-	 * of extra memory is reasonable (limited by the number of Tx
-	 * descriptors), the packets do actually get freed quickly by new
-	 * packets almost always, and for protocols like TCP that wait for
-	 * acks to really free up the data the extra memory is even less.
-	 * On the positive side we run the destructors on the sending CPU
-	 * rather than on a potentially different completing CPU, usually a
-	 * good thing.  We also run them without holding our Tx queue lock,
-	 * unlike what reclaim_completed_tx() would otherwise do.
-	 *
-	 * Run the destructor before telling the DMA engine about the packet
-	 * to make sure it doesn't complete and get freed prematurely.
-	 */
-	if (likely(!skb_shared(skb)))
-		skb_orphan(skb);
-
 	write_tx_pkt_wr(adap, skb, pi, pidx, gen, q, ndesc, compl);
 	check_ring_tx_db(adap, q);
 	return NETDEV_TX_OK;
diff --git a/drivers/net/loopback.c b/drivers/net/loopback.c
--- a/drivers/net/loopback.c
+++ b/drivers/net/loopback.c
@@ -72,8 +72,6 @@ static int loopback_xmit(struct sk_buff 
 {
 	struct pcpu_lstats *pcpu_lstats, *lb_stats;
 
-	skb_orphan(skb);
-
 	skb->protocol = eth_type_trans(skb,dev);
 
 	/* it's OK to use per_cpu_ptr() because BHs are off */
diff --git a/drivers/net/mlx4/en_tx.c b/drivers/net/mlx4/en_tx.c
--- a/drivers/net/mlx4/en_tx.c
+++ b/drivers/net/mlx4/en_tx.c
@@ -807,10 +807,6 @@ int mlx4_en_xmit(struct sk_buff *skb, st
 	if (tx_desc == (struct mlx4_en_tx_desc *) ring->bounce_buf)
 		tx_desc = mlx4_en_bounce_to_desc(priv, ring, index, desc_size);
 
-	/* Run destructor before passing skb to HW */
-	if (likely(!skb_shared(skb)))
-		skb_orphan(skb);
-
 	/* Ensure new descirptor hits memory
 	 * before setting ownership of this descriptor to HW */
 	wmb();
diff --git a/drivers/net/niu.c b/drivers/net/niu.c
--- a/drivers/net/niu.c
+++ b/drivers/net/niu.c
@@ -6667,8 +6667,7 @@ static int niu_start_xmit(struct sk_buff
 		}
 		kfree_skb(skb);
 		skb = skb_new;
-	} else
-		skb_orphan(skb);
+	}
 
 	align = ((unsigned long) skb->data & (16 - 1));
 	headroom = align + sizeof(struct tx_pkt_hdr);
diff --git a/drivers/net/veth.c b/drivers/net/veth.c
--- a/drivers/net/veth.c
+++ b/drivers/net/veth.c
@@ -155,8 +155,6 @@ static int veth_xmit(struct sk_buff *skb
 	struct veth_net_stats *stats, *rcv_stats;
 	int length, cpu;
 
-	skb_orphan(skb);
-
 	priv = netdev_priv(dev);
 	rcv = priv->peer;
 	rcv_priv = netdev_priv(rcv);
diff --git a/drivers/net/wireless/libertas/tx.c 
b/drivers/net/wireless/libertas/tx.c
--- a/drivers/net/wireless/libertas/tx.c
+++ b/drivers/net/wireless/libertas/tx.c
@@ -154,9 +154,6 @@ int lbs_hard_start_xmit(struct sk_buff *
 	if (priv->monitormode) {
 		/* Keep the skb to echo it back once Tx feedback is
 		   received from FW */
-		skb_orphan(skb);
-
-		/* Keep the skb around for when we get feedback */
 		priv->currenttxskb = skb;
 	} else {
  free:
diff --git a/net/core/dev.c b/net/core/dev.c
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -1677,6 +1677,10 @@ int dev_hard_start_xmit(struct sk_buff *
 	const struct net_device_ops *ops = dev->netdev_ops;
 	int rc;
 
+	/* Call destructor here.  It's SMP-cache-friendly and avoids issues
+	 * with drivers which can hold transmitted skbs for long times */
+	skb_orphan(skb);
+
 	if (likely(!skb->next)) {
 		if (!list_empty(&ptype_all))
 			dev_queue_xmit_nit(skb, dev);
@@ -1688,22 +1692,7 @@ int dev_hard_start_xmit(struct sk_buff *
 				goto gso;
 		}
 
-		rc = ops->ndo_start_xmit(skb, dev);
-		/*
-		 * TODO: if skb_orphan() was called by
-		 * dev->hard_start_xmit() (for example, the unmodified
-		 * igb driver does that; bnx2 doesn't), then
-		 * skb_tx_software_timestamp() will be unable to send
-		 * back the time stamp.
-		 *
-		 * How can this be prevented? Always create another
-		 * reference to the socket before calling
-		 * dev->hard_start_xmit()? Prevent that skb_orphan()
-		 * does anything in dev->hard_start_xmit() by clearing
-		 * the skb destructor before the call and restoring it
-		 * afterwards, then doing the skb_orphan() ourselves?
-		 */
-		return rc;
+		return ops->ndo_start_xmit(skb, dev);
 	}
 
 gso: