netdev - Re: [RFC PATCH] packet: rework packet_pick_tx_queue() to use common code selection

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Date:   Thu, 28 Feb 2019 11:30:20 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        Saeed Mahameed <saeedm@...lanox.com>,
        Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Willem de Bruijn <willemb@...gle.com>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [RFC PATCH] packet: rework packet_pick_tx_queue() to use common
 code selection

On Thu, Feb 28, 2019 at 10:04 AM Paolo Abeni <pabeni@...hat.com> wrote:
>
> Currently packet_pick_tx_queue() is the only caller of
> ndo_select_queue() using a fallback argument other than
> __netdev_pick_tx.
>
> Leveraging rx queue, we can obtain a similar queue selection
> behavior using core helpers. After this change, ndo_select_queue()
> is always invoked with __netdev_pick_tx() as fallback.
> We can change ndo_select_queue() signature in a followup patch,
> dropping an indirect call per transmitted packet in some scenarios
> (e.g. TCP syn and XDP generic xmit)
>
> This changes slightly how af packet queue selection happens when
> PACKET_QDISC_BYPASS is set. It's now more similar to plan dev_queue_xmit()
> tacking in account both XPS and TC mapping.
>
> Signed-off-by: Paolo Abeni <pabeni@...hat.com>
> ---
> Note: the main goal here is getting rid of the fallback() indirect
> call in the device drivers implementing ndo_select_queue(). We can
> obtain the same result with the INDIRECT_CALL() harness. Both ways
> we need to export __netdev_pick_tx() but here we avoid the need
> for additional branching.
> ---
>  include/linux/netdevice.h | 2 ++
>  net/core/dev.c            | 5 +++--
>  net/packet/af_packet.c    | 9 +++++++--
>  3 files changed, 12 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index c10b60297d28..29a558bded82 100644
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -2152,6 +2152,8 @@ static inline void netdev_for_each_tx_queue(struct net_device *dev,
>                                   &qdisc_xmit_lock_key);        \
>  }
>
> +u16 __netdev_pick_tx(struct net_device *dev, struct sk_buff *skb,
> +                    struct net_device *sb_dev);
>  struct netdev_queue *netdev_pick_tx(struct net_device *dev,
>                                     struct sk_buff *skb,
>                                     struct net_device *sb_dev);
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 2b67f2aa59dd..004d1180671a 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -3704,8 +3704,8 @@ u16 dev_pick_tx_cpu_id(struct net_device *dev, struct sk_buff *skb,
>  }
>  EXPORT_SYMBOL(dev_pick_tx_cpu_id);
>
> -static u16 __netdev_pick_tx(struct net_device *dev, struct sk_buff *skb,
> -                           struct net_device *sb_dev)
> +u16 __netdev_pick_tx(struct net_device *dev, struct sk_buff *skb,
> +                    struct net_device *sb_dev)
>  {
>         struct sock *sk = skb->sk;
>         int queue_index = sk_tx_queue_get(sk);
> @@ -3729,6 +3729,7 @@ static u16 __netdev_pick_tx(struct net_device *dev, struct sk_buff *skb,
>
>         return queue_index;
>  }
> +EXPORT_SYMBOL(__netdev_pick_tx);
>
>  struct netdev_queue *netdev_pick_tx(struct net_device *dev,
>                                     struct sk_buff *skb,
> diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> index 8376bc1c1508..1f7ae4a34d27 100644
> --- a/net/packet/af_packet.c
> +++ b/net/packet/af_packet.c
> @@ -285,14 +285,19 @@ static u16 packet_pick_tx_queue(struct sk_buff *skb)
>  {
>         struct net_device *dev = skb->dev;
>         const struct net_device_ops *ops = dev->netdev_ops;
> +       int hint = __packet_pick_tx_queue(dev, skb, NULL);
>
>         u16 queue_index;
>
> +#ifdef CONFIG_XPS
> +       skb->sender_cpu = hint + 1;

should this not be raw_smp_processor_id() without the modulo by tx queue count?

also, __packet_pick_tx_queue is a one-line wrapper around
dev_pick_tx_cpu_id. If simplifying the entire wrapper can be removed.

>
>
> +#endif
> +       skb_record_rx_queue(skb, hint);

how is this used in the transmit path?

>         if (ops->ndo_select_queue) {
>                 queue_index = ops->ndo_select_queue(dev, skb, NULL,
> -                                                   __packet_pick_tx_queue);
> +                                                   __netdev_pick_tx);
>                 queue_index = netdev_cap_txqueue(dev, queue_index);
>         } else {
> -               queue_index = __packet_pick_tx_queue(dev, skb, NULL);
> +               queue_index = __netdev_pick_tx(dev, skb, NULL);
>         }

this now duplicates the core of netdev_pick_tx. I wonder if we can
just call that instead of open coding.

either by changing dev_direct_xmit to take a struct netdev_queue * (it
only has two callers), or by moving the core into a separate
netdev_pick_tx_core (as __netdev_pick_tx is already used).