Re: [PATCH] flow_dissector: introduce skb_get_hash_symmetric()

[Eric Dumazet]

On Wed, May 31, 2023 at 7:22 PM Jon Kohler <jon@xxxxxxxxxxx> wrote:
>
> tun.c changed from skb_get_hash() to __skb_get_hash_symmetric() on
> commit feec084a7cf4 ("tun: use symmetric hash"), which exposes an
> overhead for OVS datapath, where ovs_dp_process_packet() has to
> calculate the hash again because __skb_get_hash_symmetric() does not
> retain the hash that it calculates.
>
> Introduce skb_get_hash_symmetric(), which will get and save the hash
> in one go, so that calcuation work does not go to waste, and plumb it
> into tun.c.
>
> Fixes: feec084a7cf4 ("tun: use symmetric hash")


> Signed-off-by: Jon Kohler <jon@xxxxxxxxxxx>
> CC: Jason Wang <jasowang@xxxxxxxxxx>
> CC: David S. Miller <davem@xxxxxxxxxxxxx>
> ---
>

> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index 0b40417457cd..8112b1ab5735 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -1474,6 +1474,7 @@ __skb_set_sw_hash(struct sk_buff *skb, __u32 hash, bool is_l4)
>
>  void __skb_get_hash(struct sk_buff *skb);
>  u32 __skb_get_hash_symmetric(const struct sk_buff *skb);
> +u32 skb_get_hash_symmetric(struct sk_buff *skb);
>  u32 skb_get_poff(const struct sk_buff *skb);
>  u32 __skb_get_poff(const struct sk_buff *skb, const void *data,
>                    const struct flow_keys_basic *keys, int hlen);
> diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
> index 25fb0bbc310f..d8c0e804bbfe 100644
> --- a/net/core/flow_dissector.c
> +++ b/net/core/flow_dissector.c
> @@ -1747,6 +1747,35 @@ u32 __skb_get_hash_symmetric(const struct sk_buff *skb)
>  }
>  EXPORT_SYMBOL_GPL(__skb_get_hash_symmetric);
>
> +/**
> + * skb_get_hash_symmetric: calculate and set a flow hash in @skb, using
> + * flow_keys_dissector_symmetric.
> + * @skb: sk_buff to calculate flow hash from
> + *
> + * This function is similar to __skb_get_hash_symmetric except that it
> + * retains the hash within the skb, such that it can be reused without
> + * being recalculated later.
> + */
> +u32 skb_get_hash_symmetric(struct sk_buff *skb)
> +{
> +       struct flow_keys keys;
> +       u32 hash;
> +
> +       __flow_hash_secret_init();
> +
> +       memset(&keys, 0, sizeof(keys));
> +       __skb_flow_dissect(NULL, skb, &flow_keys_dissector_symmetric,
> +                          &keys, NULL, 0, 0, 0,
> +                          FLOW_DISSECTOR_F_STOP_AT_FLOW_LABEL);
> +
> +       hash = __flow_hash_from_keys(&keys, &hashrnd);
> +
> +       __skb_set_sw_hash(skb, hash, flow_keys_have_l4(&keys));
> +
> +       return hash;
> +}
> +EXPORT_SYMBOL_GPL(skb_get_hash_symmetric);
> +

Why copy/pasting __skb_get_hash_symmetric() ?

Can you reuse it ?