Eric Dumazet a écrit :
quoted text
> 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 <dada1@cosmosbay.com>
> ---
>  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

quoted text
>  	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/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;
> 

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