net: Fix skb_tx_hash() for forwarding workloads.

When skb_rx_queue_recorded() is true, we dont want to use jash distribution
as the device driver exactly told us which queue was selected at RX time.
jhash makes a statistical shuffle, but this wont work with 8 static inputs.

Later improvements would be to compute reciprocal value of real_num_tx_queues
to avoid a divide here. But this computation should be done once,
when real_num_tx_queues is set. This needs a separate patch, and a new
field in struct net_device.

Reported-by: Andrew Dickinson <andrew@whydna.net>
Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 5 insertions, 4 deletions

diff --git a/net/core/dev.c b/net/core/dev.c
index 308a7d0c277..e2e9e4af3ac 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -1735,11 +1735,12 @@ u16 skb_tx_hash(const struct net_device *dev, const struct sk_buff *skb)
 {
         u32 hash;
 
-        if (skb_rx_queue_recorded(skb)) {
-                hash = skb_get_rx_queue(skb);
-        } else if (skb->sk && skb->sk->sk_hash) {
+        if (skb_rx_queue_recorded(skb))
+                return skb_get_rx_queue(skb) % dev->real_num_tx_queues;
+
+        if (skb->sk && skb->sk->sk_hash)
                 hash = skb->sk->sk_hash;
-        } else
+        else
                 hash = skb->protocol;
 
         hash = jhash_1word(hash, skb_tx_hashrnd);