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Date: Fri, 21 Jun 2024 08:15:15 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Yan Zhai <yan@...udflare.com>,
netdev@...r.kernel.org
Cc: "David S. Miller" <davem@...emloft.net>,
Eric Dumazet <edumazet@...gle.com>,
Jakub Kicinski <kuba@...nel.org>,
Paolo Abeni <pabeni@...hat.com>,
Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Jesper Dangaard Brouer <hawk@...nel.org>,
John Fastabend <john.fastabend@...il.com>,
Willem de Bruijn <willemb@...gle.com>,
Simon Horman <horms@...nel.org>,
Florian Westphal <fw@...len.de>,
Mina Almasry <almasrymina@...gle.com>,
Abhishek Chauhan <quic_abchauha@...cinc.com>,
David Howells <dhowells@...hat.com>,
Alexander Lobakin <aleksander.lobakin@...el.com>,
David Ahern <dsahern@...nel.org>,
Richard Gobert <richardbgobert@...il.com>,
Antoine Tenart <atenart@...nel.org>,
Yan Zhai <yan@...udflare.com>,
Felix Fietkau <nbd@....name>,
Soheil Hassas Yeganeh <soheil@...gle.com>,
Pavel Begunkov <asml.silence@...il.com>,
Lorenzo Bianconi <lorenzo@...nel.org>,
Thomas Weißschuh <linux@...ssschuh.net>,
netdev@...r.kernel.org,
linux-kernel@...r.kernel.org,
bpf@...r.kernel.org
Subject: Re: [RFC net-next 1/9] skb: introduce gro_disabled bit
Yan Zhai wrote:
> Software GRO is currently controlled by a single switch, i.e.
>
> ethtool -K dev gro on|off
>
> However, this is not always desired. When GRO is enabled, even if the
> kernel cannot GRO certain traffic, it has to run through the GRO receive
> handlers with no benefit.
>
> There are also scenarios that turning off GRO is a requirement. For
> example, our production environment has a scenario that a TC egress hook
> may add multiple encapsulation headers to forwarded skbs for load
> balancing and isolation purpose. The encapsulation is implemented via
> BPF. But the problem arises then: there is no way to properly offload a
> double-encapsulated packet, since skb only has network_header and
> inner_network_header to track one layer of encapsulation, but not two.
> On the other hand, not all the traffic through this device needs double
> encapsulation. But we have to turn off GRO completely for any ingress
> device as a result.
>
> Introduce a bit on skb so that GRO engine can be notified to skip GRO on
> this skb, rather than having to be 0-or-1 for all traffic.
>
> Signed-off-by: Yan Zhai <yan@...udflare.com>
> ---
> include/linux/netdevice.h | 9 +++++++--
> include/linux/skbuff.h | 10 ++++++++++
> net/Kconfig | 10 ++++++++++
> net/core/gro.c | 2 +-
> net/core/gro_cells.c | 2 +-
> net/core/skbuff.c | 4 ++++
> 6 files changed, 33 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index c83b390191d4..2ca0870b1221 100644
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -2415,11 +2415,16 @@ struct net_device {
> ((dev)->devlink_port = (port)); \
> })
>
> -static inline bool netif_elide_gro(const struct net_device *dev)
> +static inline bool netif_elide_gro(const struct sk_buff *skb)
> {
> - if (!(dev->features & NETIF_F_GRO) || dev->xdp_prog)
> + if (!(skb->dev->features & NETIF_F_GRO) || skb->dev->xdp_prog)
> return true;
> +
> +#ifdef CONFIG_SKB_GRO_CONTROL
> + return skb->gro_disabled;
> +#else
> return false;
> +#endif
Yet more branches in the hot path.
Compile time configurability does not help, as that will be
enabled by distros.
For a fairly niche use case. Where functionality of GRO already
works. So just a performance for a very rare case at the cost of a
regression in the common case. A small regression perhaps, but death
by a thousand cuts.
> }
>
> #define NETDEV_ALIGN 32
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index f4cda3fbdb75..48b10ece95b5 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -1008,6 +1008,9 @@ struct sk_buff {
> #if IS_ENABLED(CONFIG_IP_SCTP)
> __u8 csum_not_inet:1;
> #endif
> +#ifdef CONFIG_SKB_GRO_CONTROL
> + __u8 gro_disabled:1;
> +#endif
>
> #if defined(CONFIG_NET_SCHED) || defined(CONFIG_NET_XGRESS)
> __u16 tc_index; /* traffic control index */
> @@ -1215,6 +1218,13 @@ static inline bool skb_wifi_acked_valid(const struct sk_buff *skb)
> #endif
> }
>
> +static inline void skb_disable_gro(struct sk_buff *skb)
> +{
> +#ifdef CONFIG_SKB_GRO_CONTROL
> + skb->gro_disabled = 1;
> +#endif
> +}
> +
> /**
> * skb_unref - decrement the skb's reference count
> * @skb: buffer
> diff --git a/net/Kconfig b/net/Kconfig
> index 9fe65fa26e48..47d1ee92df15 100644
> --- a/net/Kconfig
> +++ b/net/Kconfig
> @@ -289,6 +289,16 @@ config MAX_SKB_FRAGS
> and in drivers using build_skb().
> If unsure, say 17.
>
> +config SKB_GRO_CONTROL
> + bool "allow disable GRO on per-packet basis"
> + default y
> + help
> + By default GRO can only be enabled or disabled per network device.
> + This can be cumbersome for certain scenarios.
> + Toggling this option will allow disabling GRO for selected packets,
> + e.g. by XDP programs, so that it is more flexibile.
> + Extra overhead should be minimal.
> +
> config RPS
> bool "Receive packet steering"
> depends on SMP && SYSFS
> diff --git a/net/core/gro.c b/net/core/gro.c
> index b3b43de1a650..46232a0d1983 100644
> --- a/net/core/gro.c
> +++ b/net/core/gro.c
> @@ -476,7 +476,7 @@ static enum gro_result dev_gro_receive(struct napi_struct *napi, struct sk_buff
> enum gro_result ret;
> int same_flow;
>
> - if (netif_elide_gro(skb->dev))
> + if (netif_elide_gro(skb))
> goto normal;
>
> gro_list_prepare(&gro_list->list, skb);
> diff --git a/net/core/gro_cells.c b/net/core/gro_cells.c
> index ff8e5b64bf6b..1bf15783300f 100644
> --- a/net/core/gro_cells.c
> +++ b/net/core/gro_cells.c
> @@ -20,7 +20,7 @@ int gro_cells_receive(struct gro_cells *gcells, struct sk_buff *skb)
> if (unlikely(!(dev->flags & IFF_UP)))
> goto drop;
>
> - if (!gcells->cells || skb_cloned(skb) || netif_elide_gro(dev)) {
> + if (!gcells->cells || skb_cloned(skb) || netif_elide_gro(skb)) {
> res = netif_rx(skb);
> goto unlock;
> }
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 2315c088e91d..82bd297921c1 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -6030,6 +6030,10 @@ void skb_scrub_packet(struct sk_buff *skb, bool xnet)
> ipvs_reset(skb);
> skb->mark = 0;
> skb_clear_tstamp(skb);
> +#ifdef CONFIG_SKB_GRO_CONTROL
> + /* hand back GRO control to next netns */
> + skb->gro_disabled = 0;
> +#endif
> }
> EXPORT_SYMBOL_GPL(skb_scrub_packet);
>
> --
> 2.30.2
>
>
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