netdev - Re: [PATCH net-next v2 4/4] bonding: balance ICMP echoes in layer3+4 mode

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Date:   Tue, 29 Oct 2019 23:50:33 +0200
From:   Nikolay Aleksandrov <nikolay@...ulusnetworks.com>
To:     Eric Dumazet <eric.dumazet@...il.com>,
        Matteo Croce <mcroce@...hat.com>, netdev@...r.kernel.org
Cc:     Jay Vosburgh <j.vosburgh@...il.com>,
        Veaceslav Falico <vfalico@...il.com>,
        Andy Gospodarek <andy@...yhouse.net>,
        "David S . Miller" <davem@...emloft.net>,
        Stanislav Fomichev <sdf@...gle.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Song Liu <songliubraving@...com>,
        Alexei Starovoitov <ast@...nel.org>,
        Paul Blakey <paulb@...lanox.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next v2 4/4] bonding: balance ICMP echoes in layer3+4
 mode

On 29/10/2019 23:03, Eric Dumazet wrote:
> 
> 
> On 10/29/19 11:35 AM, Nikolay Aleksandrov wrote:
> 
>> Hi Matteo,
>> Wouldn't it be more useful and simpler to use some field to choose the slave (override the hash
>> completely) in a deterministic way from user-space ?
>> For example the mark can be interpreted as a slave id in the bonding (should be
>> optional, to avoid breaking existing setups). ping already supports -m and
>> anything else can set it, this way it can be used to do monitoring for a specific
>> slave with any protocol and would be a much simpler change.
>> User-space can then implement any logic for the monitoring case and as a minor bonus
>> can monitor the slaves in parallel. And the opposite as well - if people don't want
>> these balanced for some reason, they wouldn't enable it.
>>
> 
> I kind of agree giving user more control. But I do not believe we need to use the mark
> (this might be already used by other layers)
> 
> TCP uses sk->sk_hash to feed skb->hash.
> 
> Anything using skb_set_owner_w() is also using sk->sk_hash if set.
> 
> So presumably we could add a generic SO_TXHASH socket option to let user space
> read/set this field.
> 

Right, I was just giving it as an example. Your suggestion sounds much better and
wouldn't interfere with other layers, plus we already use skb->hash in bond_xmit_hash()
and skb_set_owner_w() sets l4_hash if txhash is present which is perfect.

One thing - how do we deal with sk_rethink_txhash() ? I guess we'll need some way to
signal that the user specified the txhash and it is not to be recomputed ?
That can also be used to avoid the connect txhash set as well if SO_TXHASH was set prior
to the connect. It's quite late here, I'll look into it more tomorrow. :)

Thanks,
 Nik