Subject: [PATCH, v2] net: release dst entry in dev_hard_start_xmit()

From: Eric Dumazet <[email-masked]>
Date: Tue, 12 May 2009 21:26:35 +0200

Eric Dumazet a écrit :
> Eric Dumazet a écrit :
>> One point of contention in high network loads is the dst_release() performed
>> when a transmited skb is freed. This is because NIC tx completion calls
>> dev_kree_skb() long after original call to dev_queue_xmit(skb).
>>
>> CPU cache is cold and the atomic op in dst_release() stalls. On SMP, this is
>> quite visible if one CPU is 100% handling softirqs for a network device,
>> since dst_clone() is done by other cpus, involving cache line ping pongs.
>>
>> It seems right place to release dst is in dev_hard_start_xmit(), for most
>> devices but ones that are virtual, and some exceptions.
>>
>> David Miller suggested to define a new device flag, set in alloc_netdev_mq()
>> (so that most devices set it at init time), and carefuly unset in devices
>> which dont want a NULL skb->dst in their ndo_start_xmit().
>>
>> List of devices that must clear this flag is :
>>
>> - loopback device, because it calls netif_rx() and quoting Patrick :
>>     "ip_route_input() doesn't accept loopback addresses, so loopback packets
>>      already need to have a dst_entry attached."
>> - appletalk/ipddp.c : needs skb->dst in its xmit function
>>
>> - And all devices that call again dev_queue_xmit() from their xmit function
>> (as some classifiers need skb->dst) : bonding, macvlan, eql, ifb, hdlc_fr
>>
>> Signed-off-by: Eric Dumazet <[email-masked]>
>> ---
>>  drivers/net/appletalk/ipddp.c   |    1 +
>>  drivers/net/bonding/bond_main.c |    1 +
>>  drivers/net/eql.c               |    1 +
>>  drivers/net/ifb.c               |    1 +
>>  drivers/net/loopback.c          |    1 +
>>  drivers/net/macvlan.c           |    1 +
>>  drivers/net/wan/hdlc_fr.c       |    1 +
>>  include/linux/if.h              |    3 +++
>>  net/core/dev.c                  |    9 +++++++++
>>  9 files changed, 19 insertions(+)
>>
>> diff --git a/drivers/net/appletalk/ipddp.c b/drivers/net/appletalk/ipddp.c
>> index da64ba8..0268561 100644
>> --- a/drivers/net/appletalk/ipddp.c
>> +++ b/drivers/net/appletalk/ipddp.c
>> @@ -71,6 +71,7 @@ static struct net_device * __init ipddp_init(void)
>>  	if (!dev)
>>  		return ERR_PTR(-ENOMEM);
>>  
>> +	dev->priv_flags &= IFF_XMIT_DST_RELEASE;
> 
> 
> Oops, I forgot the  ~ here, sorry

Here is a second version, including change to vlan code as well.

Thank you

[PATCH] net: release dst entry in dev_hard_start_xmit()

One point of contention in high network loads is the dst_release() performed
when a transmited skb is freed. This is because NIC tx completion calls
dev_kree_skb() long after original call to dev_queue_xmit(skb).

CPU cache is cold and the atomic op in dst_release() stalls. On SMP, this is
quite visible if one CPU is 100% handling softirqs for a network device,
since dst_clone() is done by other cpus, involving cache line ping pongs.

It seems right place to release dst is in dev_hard_start_xmit(), for most
devices but ones that are virtual, and some exceptions.

David Miller suggested to define a new device flag, set in alloc_netdev_mq()
(so that most devices set it at init time), and carefuly unset in devices
which dont want a NULL skb->dst in their ndo_start_xmit().

List of devices that must clear this flag is :

- loopback device, because it calls netif_rx() and quoting Patrick :
    "ip_route_input() doesn't accept loopback addresses, so loopback packets
     already need to have a dst_entry attached."
- appletalk/ipddp.c : needs skb->dst in its xmit function

- And all devices that call again dev_queue_xmit() from their xmit function
(as some classifiers need skb->dst) : bonding, vlan, macvlan, eql, ifb, hdlc_fr

Signed-off-by: Eric Dumazet <[email-masked]>
---
 drivers/net/appletalk/ipddp.c   |    1 +
 drivers/net/bonding/bond_main.c |    1 +
 drivers/net/eql.c               |    1 +
 drivers/net/ifb.c               |    1 +
 drivers/net/loopback.c          |    1 +
 drivers/net/macvlan.c           |    1 +
 drivers/net/wan/hdlc_fr.c       |    1 +
 include/linux/if.h              |    3 +++
 net/8021q/vlan_dev.c            |    1 +
 net/core/dev.c                  |    9 +++++++++
 10 files changed, 20 insertions(+)

diff --git a/drivers/net/appletalk/ipddp.c b/drivers/net/appletalk/ipddp.c
index da64ba8..f939e92 100644
--- a/drivers/net/appletalk/ipddp.c
+++ b/drivers/net/appletalk/ipddp.c
@@ -71,6 +71,7 @@ static struct net_device * __init ipddp_init(void)
 	if (!dev)
 		return ERR_PTR(-ENOMEM);
 
+	dev->priv_flags &= ~IFF_XMIT_DST_RELEASE;
 	strcpy(dev->name, "ipddp%d");
 
 	if (version_printed++ == 0)
diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
index 815191d..a29f421 100644
--- a/drivers/net/bonding/bond_main.c
+++ b/drivers/net/bonding/bond_main.c
@@ -5148,6 +5148,7 @@ int bond_create(char *name, struct bond_params *params)
 		goto out_rtnl;
 	}
 
+	bond_dev->priv_flags &= ~IFF_XMIT_DST_RELEASE;
 	if (!name) {
 		res = dev_alloc_name(bond_dev, "bond%d");
 		if (res < 0)
diff --git a/drivers/net/eql.c b/drivers/net/eql.c
index 5210bb1..19b7dd9 100644
--- a/drivers/net/eql.c
+++ b/drivers/net/eql.c
@@ -194,6 +194,7 @@ static void __init eql_setup(struct net_device *dev)
 
 	dev->type       	= ARPHRD_SLIP;
 	dev->tx_queue_len 	= 5;		/* Hands them off fast */
+	dev->priv_flags	       &= ~IFF_XMIT_DST_RELEASE;
 }
 
 static int eql_open(struct net_device *dev)
diff --git a/drivers/net/ifb.c b/drivers/net/ifb.c
index 60a2630..96713ef 100644
--- a/drivers/net/ifb.c
+++ b/drivers/net/ifb.c
@@ -156,6 +156,7 @@ static void ifb_setup(struct net_device *dev)
 
 	dev->flags |= IFF_NOARP;
 	dev->flags &= ~IFF_MULTICAST;
+	dev->priv_flags &= ~IFF_XMIT_DST_RELEASE;
 	random_ether_addr(dev->dev_addr);
 }
 
diff --git a/drivers/net/loopback.c b/drivers/net/loopback.c
index 6f71157..da472c6 100644
--- a/drivers/net/loopback.c
+++ b/drivers/net/loopback.c
@@ -170,6 +170,7 @@ static void loopback_setup(struct net_device *dev)
 	dev->tx_queue_len	= 0;
 	dev->type		= ARPHRD_LOOPBACK;	/* 0x0001*/
 	dev->flags		= IFF_LOOPBACK;
+	dev->priv_flags	       &= ~IFF_XMIT_DST_RELEASE;
 	dev->features 		= NETIF_F_SG | NETIF_F_FRAGLIST
 		| NETIF_F_TSO
 		| NETIF_F_NO_CSUM
diff --git a/drivers/net/macvlan.c b/drivers/net/macvlan.c
index 329cd50..d5334b4 100644
--- a/drivers/net/macvlan.c
+++ b/drivers/net/macvlan.c
@@ -414,6 +414,7 @@ static void macvlan_setup(struct net_device *dev)
 {
 	ether_setup(dev);
 
+	dev->priv_flags	       &= ~IFF_XMIT_DST_RELEASE;
 	dev->netdev_ops		= &macvlan_netdev_ops;
 	dev->destructor		= free_netdev;
 	dev->header_ops		= &macvlan_hard_header_ops,
diff --git a/drivers/net/wan/hdlc_fr.c b/drivers/net/wan/hdlc_fr.c
index 8005301..bfa0161 100644
--- a/drivers/net/wan/hdlc_fr.c
+++ b/drivers/net/wan/hdlc_fr.c
@@ -1054,6 +1054,7 @@ static void pvc_setup(struct net_device *dev)
 	dev->flags = IFF_POINTOPOINT;
 	dev->hard_header_len = 10;
 	dev->addr_len = 2;
+	dev->priv_flags &= ~IFF_XMIT_DST_RELEASE;
 }
 
 static const struct net_device_ops pvc_ops = {
diff --git a/include/linux/if.h b/include/linux/if.h
index 1108f3e..b9a6229 100644
--- a/include/linux/if.h
+++ b/include/linux/if.h
@@ -67,6 +67,9 @@
 #define IFF_ISATAP	0x80		/* ISATAP interface (RFC4214)	*/
 #define IFF_MASTER_ARPMON 0x100		/* bonding master, ARP mon in use */
 #define IFF_WAN_HDLC	0x200		/* WAN HDLC device		*/
+#define IFF_XMIT_DST_RELEASE 0x400	/* dev_hard_start_xmit() is allowed to
+					 * release skb->dst
+					 */
 
 #define IF_GET_IFACE	0x0001		/* for querying only */
 #define IF_GET_PROTO	0x0002
diff --git a/net/8021q/vlan_dev.c b/net/8021q/vlan_dev.c
index 25ba41e..faa535f 100644
--- a/net/8021q/vlan_dev.c
+++ b/net/8021q/vlan_dev.c
@@ -739,6 +739,7 @@ void vlan_setup(struct net_device *dev)
 	ether_setup(dev);
 
 	dev->priv_flags		|= IFF_802_1Q_VLAN;
+	dev->priv_flags		&= ~IFF_XMIT_DST_RELEASE;
 	dev->tx_queue_len	= 0;
 
 	dev->netdev_ops		= &vlan_netdev_ops;
diff --git a/net/core/dev.c b/net/core/dev.c
index 14dd725..b9aef53 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -1688,6 +1688,14 @@ int dev_hard_start_xmit(struct sk_buff *skb, struct net_device *dev,
 				goto gso;
 		}
 
+		/*
+		 * If device doesnt need skb->dst, release it right now while
+		 * its hot in this cpu cache
+		 */
+		if ((dev->priv_flags & IFF_XMIT_DST_RELEASE) && skb->dst) {
+			dst_release(skb->dst);
+			skb->dst = NULL;
+		}
 		rc = ops->ndo_start_xmit(skb, dev);
 		/*
 		 * TODO: if skb_orphan() was called by
@@ -5028,6 +5036,7 @@ struct net_device *alloc_netdev_mq(int sizeof_priv, const char *name,
 	netdev_init_queues(dev);
 
 	INIT_LIST_HEAD(&dev->napi_list);
+	dev->priv_flags = IFF_XMIT_DST_RELEASE;
 	setup(dev);
 	strcpy(dev->name, name);
 	return dev;