netdev - Re: [PATCH v4 1/2] core: Factor out flow calculation from get_rps

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Date:	Wed, 4 Aug 2010 17:32:44 +0530
From:	Krishna Kumar2 <krkumar2@...ibm.com>
To:	Changli Gao <xiaosuo@...il.com>
Cc:	arnd@...db.de, bhutchings@...arflare.com, davem@...emloft.net,
	mst@...hat.com, netdev@...r.kernel.org, therbert@...gle.com
Subject: Re: [PATCH v4 1/2] core: Factor out flow calculation from get_rps_cpu

Changli Gao <xiaosuo@...il.com> wrote on 08/04/2010 04:52:58 PM:

> > +#ifdef CONFIG_RPS
> > +
> > +/* One global table that all flow-based protocols share. */
> > +struct rps_sock_flow_table *rps_sock_flow_table __read_mostly;
> > +EXPORT_SYMBOL(rps_sock_flow_table);
> > +
> > +/*
> > + * get_rps_cpu is called from netif_receive_skb and returns the target
> > + * CPU from the RPS map of the receiving queue for a given skb.
> > + * rcu_read_lock must be held on entry.
> > + */
> > +static int get_rps_cpu(struct net_device *dev, struct sk_buff *skb,
> > +                      struct rps_dev_flow **rflowp)
> > +{
> > +       struct netdev_rx_queue *rxqueue;
> > +       struct rps_map *map;
> > +       struct rps_dev_flow_table *flow_table;
> > +       struct rps_sock_flow_table *sock_flow_table;
> > +       int cpu = -1;
> > +       u16 tcpu;
> > +
> > +       if (skb_rx_queue_recorded(skb)) {
> > +               u16 index = skb_get_rx_queue(skb);
> > +               if (unlikely(index >= dev->num_rx_queues)) {
> > +                       WARN_ONCE(dev->num_rx_queues > 1, "%s
> received packet "
> > +                               "on queue %u, but number of RX
> queues is %u\n",
> > +                               dev->name, index, dev->num_rx_queues);
> > +                       goto done;
> > +               }
> > +               rxqueue = dev->_rx + index;
> > +       } else
> > +               rxqueue = dev->_rx;
> > +
> > +       if (!rxqueue->rps_map && !rxqueue->rps_flow_table)
> > +               goto done;
> > +
> > +       skb->rxhash = skb_get_rxhash(skb);
>
> the return value of skb_get_rxhash() maybe skb->rxhash, so we don't
> need to assign it to skb->rxhash again. Use a local variable to save
> the value and __skb_get_rxhash() should cache the rxhash into
> skb->rxhash for future use.

I wanted the function to return a rxhash instead of save+return,
since other users, eg macvtap, doesn't need the rxhash beyond
calculating a rxq with that hash. So for those users, we can
avoid updating both skb->rxhash and a local variable (since with
your suggestion, rps will update either one or two variables
depending on whether rxhash is cached or not). I am not sure
which is better.

Thanks,

- KK

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