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SubjectRe: [PATCH net] net: dsa: fix flow dissection on Tx path
That is a really great improvement!

Am Do., 5. Dez. 2019 um 11:04 Uhr schrieb Alexander Lobakin <alobakin@dlink.ru>:
>
> Commit 43e665287f93 ("net-next: dsa: fix flow dissection") added an
> ability to override protocol and network offset during flow dissection
> for DSA-enabled devices (i.e. controllers shipped as switch CPU ports)
> in order to fix skb hashing for RPS on Rx path.
>
> However, skb_hash() and added part of code can be invoked not only on
> Rx, but also on Tx path if we have a multi-queued device and:
> - kernel is running on UP system or
> - XPS is not configured.
>
> The call stack in this two cases will be like: dev_queue_xmit() ->
> __dev_queue_xmit() -> netdev_core_pick_tx() -> netdev_pick_tx() ->
> skb_tx_hash() -> skb_get_hash().
>
> The problem is that skbs queued for Tx have both network offset and
> correct protocol already set up even after inserting a CPU tag by DSA
> tagger, so calling tag_ops->flow_dissect() on this path actually only
> breaks flow dissection and hashing.
>
> This can be observed by adding debug prints just before and right after
> tag_ops->flow_dissect() call to the related block of code:
>
> Before the patch:
>
> Rx path (RPS):
>
> [ 19.240001] Rx: proto: 0x00f8, nhoff: 0 /* ETH_P_XDSA */
> [ 19.244271] tag_ops->flow_dissect()
> [ 19.247811] Rx: proto: 0x0800, nhoff: 8 /* ETH_P_IP */
>
> [ 19.215435] Rx: proto: 0x00f8, nhoff: 0 /* ETH_P_XDSA */
> [ 19.219746] tag_ops->flow_dissect()
> [ 19.223241] Rx: proto: 0x0806, nhoff: 8 /* ETH_P_ARP */
>
> [ 18.654057] Rx: proto: 0x00f8, nhoff: 0 /* ETH_P_XDSA */
> [ 18.658332] tag_ops->flow_dissect()
> [ 18.661826] Rx: proto: 0x8100, nhoff: 8 /* ETH_P_8021Q */
>
> Tx path (UP system):
>
> [ 18.759560] Tx: proto: 0x0800, nhoff: 26 /* ETH_P_IP */
> [ 18.763933] tag_ops->flow_dissect()
> [ 18.767485] Tx: proto: 0x920b, nhoff: 34 /* junk */
>
> [ 22.800020] Tx: proto: 0x0806, nhoff: 26 /* ETH_P_ARP */
> [ 22.804392] tag_ops->flow_dissect()
> [ 22.807921] Tx: proto: 0x920b, nhoff: 34 /* junk */
>
> [ 16.898342] Tx: proto: 0x86dd, nhoff: 26 /* ETH_P_IPV6 */
> [ 16.902705] tag_ops->flow_dissect()
> [ 16.906227] Tx: proto: 0x920b, nhoff: 34 /* junk */
>
> After:
>
> Rx path (RPS):
>
> [ 16.520993] Rx: proto: 0x00f8, nhoff: 0 /* ETH_P_XDSA */
> [ 16.525260] tag_ops->flow_dissect()
> [ 16.528808] Rx: proto: 0x0800, nhoff: 8 /* ETH_P_IP */
>
> [ 15.484807] Rx: proto: 0x00f8, nhoff: 0 /* ETH_P_XDSA */
> [ 15.490417] tag_ops->flow_dissect()
> [ 15.495223] Rx: proto: 0x0806, nhoff: 8 /* ETH_P_ARP */
>
> [ 17.134621] Rx: proto: 0x00f8, nhoff: 0 /* ETH_P_XDSA */
> [ 17.138895] tag_ops->flow_dissect()
> [ 17.142388] Rx: proto: 0x8100, nhoff: 8 /* ETH_P_8021Q */
>
> Tx path (UP system):
>
> [ 15.499558] Tx: proto: 0x0800, nhoff: 26 /* ETH_P_IP */
>
> [ 20.664689] Tx: proto: 0x0806, nhoff: 26 /* ETH_P_ARP */
>
> [ 18.565782] Tx: proto: 0x86dd, nhoff: 26 /* ETH_P_IPV6 */
>
> In order to fix that we can add the check 'proto == htons(ETH_P_XDSA)'
> to prevent code from calling tag_ops->flow_dissect() on Tx.
> I also decided to initialize 'offset' variable so tagger callbacks can
> now safely leave it untouched without provoking a chaos.
>
> Fixes: 43e665287f93 ("net-next: dsa: fix flow dissection")
> Signed-off-by: Alexander Lobakin <alobakin@dlink.ru>
> ---
> net/core/flow_dissector.c | 5 +++--
> 1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
> index 69395b804709..d524a693e00f 100644
> --- a/net/core/flow_dissector.c
> +++ b/net/core/flow_dissector.c
> @@ -969,9 +969,10 @@ bool __skb_flow_dissect(const struct net *net,
> nhoff = skb_network_offset(skb);
> hlen = skb_headlen(skb);
> #if IS_ENABLED(CONFIG_NET_DSA)
> - if (unlikely(skb->dev && netdev_uses_dsa(skb->dev))) {
> + if (unlikely(skb->dev && netdev_uses_dsa(skb->dev) &&
> + proto == htons(ETH_P_XDSA))) {
> const struct dsa_device_ops *ops;
> - int offset;
> + int offset = 0;
>
> ops = skb->dev->dsa_ptr->tag_ops;
> if (ops->flow_dissect &&
> --
> 2.24.0
>

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