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

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

[Paolo Abeni]

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@redhat.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

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

[Jason Wang]

On 2018/11/7 下午5:34, Paolo Abeni wrote:
> 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@redhat.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);


Acked-by: Jason Wang <jasowang@redhat.com>

Thanks

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

[David Miller]

From: Paolo Abeni <pabeni@redhat.com>
Date: Wed,  7 Nov 2018 10:34:36 +0100

> 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@redhat.com>

Applied.