netdev - Re: [net-next PATCH v2 2/4] net: Enable Tx queue selection based on Rx queues

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Message-ID: <CAF=yD-JghZY5NN6cHGdHeOTs8xb9KF=mQ=J2P49ojrvp+MsD8w@mail.gmail.com>
Date:   Sat, 19 May 2018 16:27:42 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Tom Herbert <tom@...bertland.com>
Cc:     Amritha Nambiar <amritha.nambiar@...el.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Alexander Duyck <alexander.h.duyck@...el.com>,
        Sridhar Samudrala <sridhar.samudrala@...el.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>
Subject: Re: [net-next PATCH v2 2/4] net: Enable Tx queue selection based on
 Rx queues

On Sat, May 19, 2018 at 4:13 PM, Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
> On Fri, May 18, 2018 at 12:03 AM, Tom Herbert <tom@...bertland.com> wrote:
>> On Tue, May 15, 2018 at 6:26 PM, Amritha Nambiar
>> <amritha.nambiar@...el.com> wrote:
>>> This patch adds support to pick Tx queue based on the Rx queue map
>>> configuration set by the admin through the sysfs attribute
>>> for each Tx queue. If the user configuration for receive
>>> queue map does not apply, then the Tx queue selection falls back
>>> to CPU map based selection and finally to hashing.
>>>
>>> Signed-off-by: Amritha Nambiar <amritha.nambiar@...el.com>
>>> Signed-off-by: Sridhar Samudrala <sridhar.samudrala@...el.com>
>>> ---

>>> +static int get_xps_queue(struct net_device *dev, struct sk_buff *skb)
>>> +{
>>> +#ifdef CONFIG_XPS
>>> +       enum xps_map_type i = XPS_MAP_RXQS;
>>> +       struct xps_dev_maps *dev_maps;
>>> +       struct sock *sk = skb->sk;
>>> +       int queue_index = -1;
>>> +       unsigned int tci = 0;
>>> +
>>> +       if (sk && sk->sk_rx_queue_mapping <= dev->real_num_rx_queues &&
>>> +           dev->ifindex == sk->sk_rx_ifindex)
>>> +               tci = sk->sk_rx_queue_mapping;
>>> +
>>> +       rcu_read_lock();
>>> +       while (queue_index < 0 && i < __XPS_MAP_MAX) {
>>> +               if (i == XPS_MAP_CPUS)
>>
>> This while loop typifies exactly why I don't think the XPS maps should
>> be an array.
>
> +1

as a matter of fact, as enabling both cpu and rxqueue map at the same
time makes no sense, only one map is needed at any one time. The
only difference is in how it is indexed. It should probably not be possible
to configure both at the same time. Keeping a single map probably also
significantly simplifies patch 1/4.