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From:   Eric Dumazet <edumazet@google.com>
Date:   Wed, 31 May 2023 19:33:45 +0200
Message-ID: <CANn89iLE1d=MuSMTisEbLh6Q_c4rjagd8xuRj0PC-4ua0pDRPA@mail.gmail.com>
Subject: Re: [PATCH] flow_dissector: introduce skb_get_hash_symmetric()
To:     Jon Kohler <jon@nutanix.com>
Cc:     "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Pavel Begunkov <asml.silence@gmail.com>,
        Richard Gobert <richardbgobert@gmail.com>,
        Menglong Dong <imagedong@tencent.com>,
        Wojciech Drewek <wojciech.drewek@intel.com>,
        Guillaume Nault <gnault@redhat.com>,
        John Fastabend <john.fastabend@gmail.com>,
        Stanislav Fomichev <sdf@google.com>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Shmulik Ladkani <shmulik.ladkani@gmail.com>,
        Qingqing Yang <qingqing.yang@broadcom.com>,
        Daniel Xu <dxu@dxuuu.xyz>, Felix Fietkau <nbd@nbd.name>,
        Ludovic Cintrat <ludovic.cintrat@gatewatcher.com>,
        Jason Wang <jasowang@redhat.com>, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
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On Wed, May 31, 2023 at 7:22=E2=80=AFPM Jon Kohler <jon@nutanix.com> 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@nutanix.com>
> CC: Jason Wang <jasowang@redhat.com>
> CC: David S. Miller <davem@davemloft.net>
> ---
>

> 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 =3D __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 ?