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

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Message-ID: <CAF=yD-+1-_t415TZz8qczgPN8j8bGFM4=vrYtcE=LChBXX=U4A@mail.gmail.com>
Date:   Thu, 28 Feb 2019 13:29:09 -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

> > > 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?
>
> yes, here sender_cpu is intentionally 'misused' to keep a behavior more
> similar to the unpatched kernel. I'll drop the modulo in the next
> iteration.
>
> > also, __packet_pick_tx_queue is a one-line wrapper around
> > dev_pick_tx_cpu_id. If simplifying the entire wrapper can be removed.
>
> Yes, ideally that will go with a follow-up patch. dev_pick_tx_cpu_id()
> is going to loose an argument too, and I'll have to touch this chunk
> again. I thought that touching it once would reduce noise.
> >
> > >
> > > +#endif
> > > +       skb_record_rx_queue(skb, hint);
> >
> > how is this used in the transmit path?
>
> __netdev_pick_tx() [no XPS conf in place] -> skb_tx_hash() ->
> skb_rx_queue_recorded() -> the packet is mapped to (), module tc
> mappings.

Ah right.

> > >         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  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).
>
> I tried to minimize the changes in this RFC. I'll go for
> netdev_pick_tx_core() in the next iteration, if there are no problem
> with an additional EXPORT_SYMBOL_GPL().

I think the original patch may have consciously added a trivial
packet_pick_tx_queue to keep the transmit path as short as possible
for this PACKET_QDISC_BYPASS mode. The more we deduplicate between
that and normal path, the less the feature makes sense.

That said, this particular change seems fine. Even more so if it
allows a follow-up optimization that benefits the common case. Thanks
for answering my questions.

Acked-by: Willem de Bruijn <willemb@...gle.com>