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Date: Mon, 25 Aug 2014 20:42:59 -0700
From: Tom Herbert <therbert@...gle.com>
To: David Miller <davem@...emloft.net>
Cc: Linux Netdev List <netdev@...r.kernel.org>,
Jamal Hadi Salim <jhs@...atatu.com>,
Hannes Frederic Sowa <hannes@...essinduktion.org>,
Eric Dumazet <edumazet@...gle.com>,
Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
Rusty Russell <rusty@...tcorp.com.au>,
Daniel Borkmann <dborkman@...hat.com>, brouer@...hat.com
Subject: Re: [PATCH 1/2] net: Remove ndo_xmit_flush netdev operation, use
signalling instead.
On Mon, Aug 25, 2014 at 4:35 PM, David Miller <davem@...emloft.net> wrote:
>
> As reported by Jesper Dangaard Brouer, for high packet rates the
> overhead of having another indirect call in the TX path is
> non-trivial.
>
> There is the indirect call itself, and then there is all of the
> reloading of the state to refetch the tail pointer value and
> then write the device register.
>
> Move to a more passive scheme, which requires very light modifications
> to the device drivers.
>
> The signal is a new skb->xmit_more value, if it is non-zero it means
> that more SKBs are pending to be transmitted on the same queue as the
> current SKB. And therefore, the driver may elide the tail pointer
> update.
>
> Right now skb->xmit_more is always zero.
>
> Signed-off-by: David S. Miller <davem@...emloft.net>
> ---
> drivers/net/ethernet/intel/igb/igb_main.c | 36 +++++++++++--------------------
> drivers/net/virtio_net.c | 12 +++--------
> include/linux/netdevice.h | 25 ++-------------------
> include/linux/skbuff.h | 2 ++
> 4 files changed, 19 insertions(+), 56 deletions(-)
>
> diff --git a/drivers/net/ethernet/intel/igb/igb_main.c b/drivers/net/ethernet/intel/igb/igb_main.c
> index b9c020a..89c29b4 100644
> --- a/drivers/net/ethernet/intel/igb/igb_main.c
> +++ b/drivers/net/ethernet/intel/igb/igb_main.c
> @@ -136,7 +136,6 @@ static void igb_update_phy_info(unsigned long);
> static void igb_watchdog(unsigned long);
> static void igb_watchdog_task(struct work_struct *);
> static netdev_tx_t igb_xmit_frame(struct sk_buff *skb, struct net_device *);
> -static void igb_xmit_flush(struct net_device *netdev, u16 queue);
> static struct rtnl_link_stats64 *igb_get_stats64(struct net_device *dev,
> struct rtnl_link_stats64 *stats);
> static int igb_change_mtu(struct net_device *, int);
> @@ -2076,7 +2075,6 @@ static const struct net_device_ops igb_netdev_ops = {
> .ndo_open = igb_open,
> .ndo_stop = igb_close,
> .ndo_start_xmit = igb_xmit_frame,
> - .ndo_xmit_flush = igb_xmit_flush,
> .ndo_get_stats64 = igb_get_stats64,
> .ndo_set_rx_mode = igb_set_rx_mode,
> .ndo_set_mac_address = igb_set_mac,
> @@ -4917,6 +4915,14 @@ static void igb_tx_map(struct igb_ring *tx_ring,
>
> tx_ring->next_to_use = i;
>
> + if (!skb->xmit_more) {
> + writel(i, tx_ring->tail);
> +
> + /* we need this if more than one processor can write to our tail
> + * at a time, it synchronizes IO on IA64/Altix systems
> + */
> + mmiowb();
> + }
> return;
>
I would suggest the flush should be done if !skb->xmit_more or queue
is being stopped. So maybe pull this code into it's own function e.g.
igb_flush(tx_ring). Then do:
if (igb_maybe_stop_tx(tx_ring, DESC_NEEDED) || !skb->more)
igb_flush(tx_ring);
> dma_error:
> @@ -5052,20 +5058,17 @@ out_drop:
> return NETDEV_TX_OK;
> }
>
> -static struct igb_ring *__igb_tx_queue_mapping(struct igb_adapter *adapter, unsigned int r_idx)
> +static inline struct igb_ring *igb_tx_queue_mapping(struct igb_adapter *adapter,
> + struct sk_buff *skb)
> {
> + unsigned int r_idx = skb->queue_mapping;
> +
> if (r_idx >= adapter->num_tx_queues)
> r_idx = r_idx % adapter->num_tx_queues;
>
> return adapter->tx_ring[r_idx];
> }
>
> -static inline struct igb_ring *igb_tx_queue_mapping(struct igb_adapter *adapter,
> - struct sk_buff *skb)
> -{
> - return __igb_tx_queue_mapping(adapter, skb->queue_mapping);
> -}
> -
> static netdev_tx_t igb_xmit_frame(struct sk_buff *skb,
> struct net_device *netdev)
> {
> @@ -5094,21 +5097,6 @@ static netdev_tx_t igb_xmit_frame(struct sk_buff *skb,
> return igb_xmit_frame_ring(skb, igb_tx_queue_mapping(adapter, skb));
> }
>
> -static void igb_xmit_flush(struct net_device *netdev, u16 queue)
> -{
> - struct igb_adapter *adapter = netdev_priv(netdev);
> - struct igb_ring *tx_ring;
> -
> - tx_ring = __igb_tx_queue_mapping(adapter, queue);
> -
> - writel(tx_ring->next_to_use, tx_ring->tail);
> -
> - /* we need this if more than one processor can write to our tail
> - * at a time, it synchronizes IO on IA64/Altix systems
> - */
> - mmiowb();
> -}
> -
> /**
> * igb_tx_timeout - Respond to a Tx Hang
> * @netdev: network interface device structure
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 6242108..f0c2824 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -953,15 +953,10 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
> }
> }
>
> - return NETDEV_TX_OK;
> -}
> + if (!skb->xmit_more)
> + virtqueue_kick(sq->vq);
>
> -static void xmit_flush(struct net_device *dev, u16 qnum)
> -{
> - struct virtnet_info *vi = netdev_priv(dev);
> - struct send_queue *sq = &vi->sq[qnum];
> -
> - virtqueue_kick(sq->vq);
> + return NETDEV_TX_OK;
> }
>
> /*
> @@ -1393,7 +1388,6 @@ static const struct net_device_ops virtnet_netdev = {
> .ndo_open = virtnet_open,
> .ndo_stop = virtnet_close,
> .ndo_start_xmit = start_xmit,
> - .ndo_xmit_flush = xmit_flush,
> .ndo_validate_addr = eth_validate_addr,
> .ndo_set_mac_address = virtnet_set_mac_address,
> .ndo_set_rx_mode = virtnet_set_rx_mode,
> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index 220c509..039b237 100644
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -782,19 +782,6 @@ typedef u16 (*select_queue_fallback_t)(struct net_device *dev,
> * (can also return NETDEV_TX_LOCKED iff NETIF_F_LLTX)
> * Required can not be NULL.
> *
> - * void (*ndo_xmit_flush)(struct net_device *dev, u16 queue);
> - * A driver implements this function when it wishes to support
> - * deferred TX queue flushing. The idea is that the expensive
> - * operation to trigger TX queue processing can be done after
> - * N calls to ndo_start_xmit rather than being done every single
> - * time. In this regime ndo_start_xmit will be called one or more
> - * times, and then a final ndo_xmit_flush call will be made to
> - * have the driver tell the device about the new pending TX queue
> - * entries. The kernel keeps track of which queues need flushing
> - * by monitoring skb->queue_mapping of the packets it submits to
> - * ndo_start_xmit. This is the queue value that will be passed
> - * to ndo_xmit_flush.
> - *
> * u16 (*ndo_select_queue)(struct net_device *dev, struct sk_buff *skb,
> * void *accel_priv, select_queue_fallback_t fallback);
> * Called to decide which queue to when device supports multiple
> @@ -1018,7 +1005,6 @@ struct net_device_ops {
> int (*ndo_stop)(struct net_device *dev);
> netdev_tx_t (*ndo_start_xmit) (struct sk_buff *skb,
> struct net_device *dev);
> - void (*ndo_xmit_flush)(struct net_device *dev, u16 queue);
> u16 (*ndo_select_queue)(struct net_device *dev,
> struct sk_buff *skb,
> void *accel_priv,
> @@ -3447,15 +3433,8 @@ int __init dev_proc_init(void);
> static inline netdev_tx_t __netdev_start_xmit(const struct net_device_ops *ops,
> struct sk_buff *skb, struct net_device *dev)
> {
> - netdev_tx_t ret;
> - u16 q;
> -
> - q = skb->queue_mapping;
> - ret = ops->ndo_start_xmit(skb, dev);
> - if (dev_xmit_complete(ret) && ops->ndo_xmit_flush)
> - ops->ndo_xmit_flush(dev, q);
> -
> - return ret;
> + skb->xmit_more = 0;
> + return ops->ndo_start_xmit(skb, dev);
> }
>
> static inline netdev_tx_t netdev_start_xmit(struct sk_buff *skb, struct net_device *dev)
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index 18ddf96..9b3802a 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -452,6 +452,7 @@ static inline u32 skb_mstamp_us_delta(const struct skb_mstamp *t1,
> * @tc_verd: traffic control verdict
> * @hash: the packet hash
> * @queue_mapping: Queue mapping for multiqueue devices
> + * @xmit_more: More SKBs are pending for this queue
> * @ndisc_nodetype: router type (from link layer)
> * @ooo_okay: allow the mapping of a socket to a queue to be changed
> * @l4_hash: indicate hash is a canonical 4-tuple hash over transport
> @@ -558,6 +559,7 @@ struct sk_buff {
>
> __u16 queue_mapping;
> kmemcheck_bitfield_begin(flags2);
> + __u8 xmit_more:1;
> #ifdef CONFIG_IPV6_NDISC_NODETYPE
> __u8 ndisc_nodetype:2;
> #endif
> --
> 1.7.11.7
>
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