netdev - [PATCH net-next] tun: compute the RFS hash only if needed.

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Date:   Wed,  7 Nov 2018 10:34:36 +0100
From:   Paolo Abeni <pabeni@...hat.com>
To:     netdev@...r.kernel.org
Cc:     Jason Wang <jasowang@...hat.com>,
        "David S. Miller" <davem@...emloft.net>
Subject: [PATCH net-next] tun: compute the RFS hash only if needed.

The tun XDP sendmsg code path, unconditionally computes the symmetric
hash of each packet for RFS's sake, even when we could skip it. e.g.
when the device has a single queue.

This change adds the check already in-place for the skb sendmsg path
to avoid unneeded hashing.

The above gives small, but measurable, performance gain for VM xmit
path when zerocopy is not enabled.

Signed-off-by: Paolo Abeni <pabeni@...hat.com>
---
 drivers/net/tun.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 060135ceaf0e..a65779c6d72f 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -2448,7 +2448,8 @@ static int tun_xdp_one(struct tun_struct *tun,
 			goto out;
 	}

-	if (!rcu_dereference(tun->steering_prog))
+	if (!rcu_dereference(tun->steering_prog) && tun->numqueues > 1 &&
+	    !tfile->detached)
 		rxhash = __skb_get_hash_symmetric(skb);

 	netif_receive_skb(skb);
-- 
2.17.2