netdev - Re: [PATCH net-next v2] bonding: add a vlan+mac tx hashing option

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Date:   Thu, 14 Jan 2021 13:23:14 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Jarod Wilson <jarod@...hat.com>
Cc:     linux-kernel@...r.kernel.org, Jay Vosburgh <j.vosburgh@...il.com>,
        Veaceslav Falico <vfalico@...il.com>,
        Andy Gospodarek <andy@...yhouse.net>,
        "David S. Miller" <davem@...emloft.net>,
        Thomas Davis <tadavis@....gov>, netdev@...r.kernel.org
Subject: Re: [PATCH net-next v2] bonding: add a vlan+mac tx hashing option

On Thu, 14 Jan 2021 16:11:41 -0500 Jarod Wilson wrote:
> In truth, this code started out as a copy of bond_eth_hash(), which also
> only uses the last byte, though of both source and destination macs. In
> the typical use case for the requesting user, the bond is formed from two
> onboard NICs, which typically have adjacent mac addresses, i.e.,
> AA:BB:CC:DD:EE:01 and AA:BB:CC:DD:EE:02, so only the last byte is really
> relevant to hash differently, but in thinking about it, a replacement NIC
> because an onboard one died could have the same last byte, and maybe we
> ought to just go full source mac right off the go here.
> 
> Something like this instead maybe:
> 
> static u32 bond_vlan_srcmac_hash(struct sk_buff *skb)
> {
>         struct ethhdr *mac_hdr = (struct ethhdr *)skb_mac_header(skb);
>         u32 srcmac = 0;
>         u16 vlan;
>         int i;
> 
>         for (i = 0; i < ETH_ALEN; i++)
>                 srcmac = (srcmac << 8) | mac_hdr->h_source[i];
> 
>         if (!skb_vlan_tag_present(skb))
>                 return srcmac;
> 
>         vlan = skb_vlan_tag_get(skb);
> 
>         return vlan ^ srcmac;
> }
> 
> Then the documentation is spot-on, and we're future-proof, though
> marginally less performant in calculating the hash, which may have been a
> consideration when the original function was written, but is probably
> basically irrelevant w/modern systems...

No preference, especially if bond_eth_hash() already uses the last byte.
Just make sure the choice is explained in the commit message.