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

[Paolo Abeni <pabeni@redhat.com>]

Hi,

Thank you for the feedback.

On Thu, 2019-02-28 at 11:30 -0500, Willem de Bruijn wrote:
> On Thu, Feb 28, 2019 at 10:04 AM Paolo Abeni <pabeni@redhat.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@redhat.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?

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.

> >         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().

Thanks,

Paolo