netdev - Re: [PATCH net-next] net: introduce netdevice gso_min

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Date:	Sun, 5 Oct 2014 11:45:46 -0700
From:	Tom Herbert <therbert@...gle.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Amir Vadai <amirv@...lanox.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Linux Netdev List <netdev@...r.kernel.org>,
	Yevgeny Petrilin <yevgenyp@...lanox.com>,
	Or Gerlitz <ogerlitz@...lanox.com>,
	Ido Shamay <idos@...lanox.com>
Subject: Re: [PATCH net-next] net: introduce netdevice gso_min_segs attribute

On Sun, Oct 5, 2014 at 10:11 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> From: Eric Dumazet <edumazet@...gle.com>
>
> Some TSO engines might have a too heavy setup cost, that impacts
> performance on hosts sending small bursts (2 MSS per packet).
>
> This patch adds a device gso_min_segs, allowing drivers to set
> a minimum segment size for TSO packets, according to the NIC
> performance.
>
Eric, this seems like another device specific limitation in TSO we are
exposing to the stack. Can't we just put things like this in
ndo_gso_check so driver can apply whatever criteria it wants in
deciding if it worth it or even to rather do TSO?

> Tested on a mlx4 NIC, this allows to get a ~110% increase of
> throughput when sending 2 MSS per packet.
>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> ---
> mlx4 patch will be sent later, its a one liner.
>
>  include/linux/netdevice.h |    4 +++-
>  net/core/dev.c            |    9 ++++++---
>  2 files changed, 9 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index 22d54b9b700d..2df86f50261c 100644
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -1416,6 +1416,8 @@ enum netdev_priv_flags {
>   *     @gso_max_size:  Maximum size of generic segmentation offload
>   *     @gso_max_segs:  Maximum number of segments that can be passed to the
>   *                     NIC for GSO
> + *     @gso_min_segs:  Minimum number of segments that can be passed to the
> + *                     NIC for GSO
>   *
>   *     @dcbnl_ops:     Data Center Bridging netlink ops
>   *     @num_tc:        Number of traffic classes in the net device
> @@ -1666,7 +1668,7 @@ struct net_device {
>         unsigned int            gso_max_size;
>  #define GSO_MAX_SEGS           65535
>         u16                     gso_max_segs;
> -
> +       u16                     gso_min_segs;
>  #ifdef CONFIG_DCB
>         const struct dcbnl_rtnl_ops *dcbnl_ops;
>  #endif
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 1a90530f83ff..16e8ebbd3316 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -2567,10 +2567,12 @@ static netdev_features_t harmonize_features(struct sk_buff *skb,
>
>  netdev_features_t netif_skb_features(struct sk_buff *skb)
>  {
> +       const struct net_device *dev = skb->dev;
> +       netdev_features_t features = dev->features;
> +       u16 gso_segs = skb_shinfo(skb)->gso_segs;
>         __be16 protocol = skb->protocol;
> -       netdev_features_t features = skb->dev->features;
>
> -       if (skb_shinfo(skb)->gso_segs > skb->dev->gso_max_segs)
> +       if (gso_segs > dev->gso_max_segs || gso_segs < dev->gso_min_segs)
>                 features &= ~NETIF_F_GSO_MASK;
>
>         if (protocol == htons(ETH_P_8021Q) || protocol == htons(ETH_P_8021AD)) {
> @@ -2581,7 +2583,7 @@ netdev_features_t netif_skb_features(struct sk_buff *skb)
>         }
>
>         features = netdev_intersect_features(features,
> -                                            skb->dev->vlan_features |
> +                                            dev->vlan_features |
>                                              NETIF_F_HW_VLAN_CTAG_TX |
>                                              NETIF_F_HW_VLAN_STAG_TX);
>
> @@ -6658,6 +6660,7 @@ struct net_device *alloc_netdev_mqs(int sizeof_priv, const char *name,
>
>         dev->gso_max_size = GSO_MAX_SIZE;
>         dev->gso_max_segs = GSO_MAX_SEGS;
> +       dev->gso_min_segs = 0;
>
>         INIT_LIST_HEAD(&dev->napi_list);
>         INIT_LIST_HEAD(&dev->unreg_list);
>
>
> --
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