netdev - Re: [net-next PATCH v4 5/7] net: Enable Tx queue selection based on Rx queues

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Date:   Wed, 27 Jun 2018 17:48:55 -0700
From:   "Nambiar, Amritha" <amritha.nambiar@...el.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Alexander Duyck <alexander.h.duyck@...el.com>,
        "Samudrala, Sridhar" <sridhar.samudrala@...el.com>,
        Alexander Duyck <alexander.duyck@...il.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        Tom Herbert <tom@...bertland.com>
Subject: Re: [net-next PATCH v4 5/7] net: Enable Tx queue selection based on
 Rx queues

On 6/27/2018 3:47 AM, Willem de Bruijn wrote:
>>>> +static int get_xps_queue(struct net_device *dev, struct sk_buff *skb)
>>>>  {
>>>>  #ifdef CONFIG_XPS
>>>>         struct xps_dev_maps *dev_maps;
>>>> -       struct xps_map *map;
>>>> +       struct sock *sk = skb->sk;
>>>>         int queue_index = -1;
>>>>
>>>>         if (!static_key_false(&xps_needed))
>>>>                 return -1;
>>>>
>>>>         rcu_read_lock();
>>>> -       dev_maps = rcu_dereference(dev->xps_cpus_map);
>>>> +       if (!static_key_false(&xps_rxqs_needed))
>>>> +               goto get_cpus_map;
>>>> +
>>>> +       dev_maps = rcu_dereference(dev->xps_rxqs_map);
>>>>         if (dev_maps) {
>>>> -               unsigned int tci = skb->sender_cpu - 1;
>>>> +               int tci = sk_rx_queue_get(sk);
>>>
>>> What if the rx device differs from the tx device?
>>>
>> I think I have 3 options here:
>> 1. Cache the ifindex in sock_common which will introduce a new
>> additional field in sock_common.
>> 2. Use dev_get_by_napi_id to get the device id. This could be expensive,
>> if the rxqs_map is set, this will be done on every packet and involves
>> walking through the hashlist for napi_id lookup.
> 
> The tx queue mapping is cached in the sk for connected sockets, but
> indeed this would be expensive for many workloads.
> 
>> 3. Remove validating device id, similar to how it is in skb_tx_hash
>> where rx_queue recorded is used and if not, fall through to flow hash
>> calculation.
>> What do you think is suitable here?
> 
> Alternatively, just accept the misprediction in this rare case. But do
> make the caveat explicit in the documentation.
> 
Okay, I will add this in the documentation.