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Date: Tue, 12 May 2009 10:21:12 +0200
From: Eric Dumazet <dada1@...mosbay.com>
To: "David S. Miller" <davem@...emloft.net>
CC: Linux Netdev List <netdev@...r.kernel.org>,
Jarek Poplawski <jarkao2@...il.com>,
Patrick McHardy <kaber@...sh.net>
Subject: Re: [PATCH] net: release dst entry in dev_hard_start_xmit()
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 <dada1@...mosbay.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
> 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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