netdev - Re: [PATCH net-next 4/4] xps_flows: XPS for packets that don't have a socket

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Date:   Thu, 1 Sep 2016 08:36:31 -0700
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     Tom Herbert <tom@...bertland.com>
Cc:     David Miller <davem@...emloft.net>,
        Netdev <netdev@...r.kernel.org>,
        Kernel Team <kernel-team@...com>,
        Rick Jones <rick.jones2@....com>
Subject: Re: [PATCH net-next 4/4] xps_flows: XPS for packets that don't have a socket

On Wed, Aug 31, 2016 at 5:10 PM, Tom Herbert <tom@...bertland.com> wrote:
> xps_flows maintains a per device flow table that is indexed by the
> skbuff hash. The table is only consulted when there is no queue saved in
> a transmit socket for an skbuff.
>
> Each entry in the flow table contains a queue index and a queue
> pointer. The queue pointer is set when a queue is chosen using a
> flow table entry. This pointer is set to the head pointer in the
> transmit queue (which is maintained by BQL).
>
> The new function get_xps_flows_index that looks up flows in the
> xps_flows table. The entry returned gives the last queue a matching flow
> used. The returned queue is compared against the normal XPS queue. If
> they are different, then we only switch if the tail pointer in the TX
> queue has advanced past the pointer saved in the entry. In this
> way OOO should be avoided when XPS wants to use a different queue.
>
> Signed-off-by: Tom Herbert <tom@...bertland.com>
> ---
>  net/Kconfig    |  6 +++++
>  net/core/dev.c | 85 +++++++++++++++++++++++++++++++++++++++++++++++-----------
>  2 files changed, 76 insertions(+), 15 deletions(-)
>

So it looks like you didn't address the two issues I called out with
this patch last time.  I have called them out again below.

> diff --git a/net/core/dev.c b/net/core/dev.c
> index 34b5322..fc68d19 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c

<snip>

> @@ -3240,26 +3239,82 @@ static inline int get_xps_queue(struct net_device *dev, struct sk_buff *skb)
>  #endif
>  }
>
> -static u16 __netdev_pick_tx(struct net_device *dev, struct sk_buff *skb)
> +/* Must be called with RCU read_lock */
> +static int get_xps_flows_index(struct net_device *dev, struct sk_buff *skb)
>  {
> -       struct sock *sk = skb->sk;
> -       int queue_index = sk_tx_queue_get(sk);
> +#ifdef CONFIG_XPS_FLOWS
> +       struct xps_dev_flow_table *flow_table;
> +       struct xps_dev_flow ent;
> +       int queue_index;
> +       struct netdev_queue *txq;
> +       u32 hash;
>
> -       if (queue_index < 0 || skb->ooo_okay ||
> -           queue_index >= dev->real_num_tx_queues) {
> -               int new_index = get_xps_queue(dev, skb);
> -               if (new_index < 0)
> -                       new_index = skb_tx_hash(dev, skb);
> +       flow_table = rcu_dereference(dev->xps_flow_table);
> +       if (!flow_table)
> +               return -1;
>
> -               if (queue_index != new_index && sk &&
> -                   sk_fullsock(sk) &&
> -                   rcu_access_pointer(sk->sk_dst_cache))
> -                       sk_tx_queue_set(sk, new_index);
> +       queue_index = get_xps_queue(dev, skb);
> +       if (queue_index < 0)
> +               return -1;

I really think what would make more sense here is to just call
skb_tx_hash to acquire the queue_index instead of just exiting.  That
way we don't have the flows toggling back and forth between XPS and
non-XPS cpus.

> -               queue_index = new_index;
> +       hash = skb_get_hash(skb);
> +       if (!hash)
> +               return -1;

So a hash of 0 is perfectly valid.  So this line doesn't make any
sense.  You could just drop these two lines and work with the hash you
generated.

The rest of this looks good.

- Alex