[net-next] tuntap: calculate rps hash only when needed

There's no need to calculate rps hash if it was not enabled. So this
patch export rps_needed and check it before trying to get rps
hash. Tests (using pktgen to inject packets to guest) shows this can
improve pps about 13% (when rps is disabled).

Before:
~1150000 pps
After:
~1300000 pps

Cc: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
---
 drivers/net/tun.c | 2 +-
 net/core/dev.c    | 1 +
 2 files changed, 2 insertions(+), 1 deletion(-)

Hi,

[auto build test ERROR on net-next/master]

url:    https://github.com/0day-ci/linux/commits/Jason-Wang/tuntap-calculate-rps-hash-only-when-needed/20160426-095825
config: xtensa-allyesconfig (attached as .config)
compiler: 
reproduce:
        wget https://git.kernel.org/cgit/linux/kernel/git/wfg/lkp-tests.git/plain/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # save the attached .config to linux build tree
        make.cross ARCH=xtensa 

All errors (new ones prefixed by >>):

   drivers/net/tun.c: In function 'tun_net_xmit':
>> drivers/net/tun.c:836:42: error: 'rps_needed' undeclared (first use in this function)
     if (numqueues == 1 && static_key_false(&rps_needed)) {
                                             ^
   drivers/net/tun.c:836:42: note: each undeclared identifier is reported only once for each function it appears in

vim +/rps_needed +836 drivers/net/tun.c

   830		numqueues = ACCESS_ONCE(tun->numqueues);
   831	
   832		/* Drop packet if interface is not attached */
   833		if (txq >= numqueues)
   834			goto drop;
   835	
 > 836		if (numqueues == 1 && static_key_false(&rps_needed)) {
   837			/* Select queue was not called for the skbuff, so we extract the
   838			 * RPS hash and save it into the flow_table here.
   839			 */

---
0-DAY kernel test infrastructure                Open Source Technology Center
https://lists.01.org/pipermail/kbuild-all                   Intel Corporation

On 04/26/2016 09:55 AM, Jason Wang wrote:
> There's no need to calculate rps hash if it was not enabled. So this
> patch export rps_needed and check it before trying to get rps
> hash. Tests (using pktgen to inject packets to guest) shows this can
> improve pps about 13% (when rps is disabled).
>
> Before:
> ~1150000 pps
> After:
> ~1300000 pps
>
> Cc: Michael S. Tsirkin <mst@redhat.com>
> Signed-off-by: Jason Wang <jasowang@redhat.com>
> ---
>  drivers/net/tun.c | 2 +-
>  net/core/dev.c    | 1 +
>  2 files changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index afdf950..746877f 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -819,7 +819,7 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
>  	if (txq >= numqueues)
>  		goto drop;
>  
> -	if (numqueues == 1) {
> +	if (numqueues == 1 && static_key_false(&rps_needed)) {
>  		/* Select queue was not called for the skbuff, so we extract the
>  		 * RPS hash and save it into the flow_table here.
>  		 */
> diff --git a/net/core/dev.c b/net/core/dev.c
> index b9bcbe7..d4ba936 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -3428,6 +3428,7 @@ u32 rps_cpu_mask __read_mostly;
>  EXPORT_SYMBOL(rps_cpu_mask);
>  
>  struct static_key rps_needed __read_mostly;
> +EXPORT_SYMBOL(rps_needed);
>  
>  static struct rps_dev_flow *
>  set_rps_cpu(struct net_device *dev, struct sk_buff *skb,

Kbuild bot reports an error when !CONFIG_RPS. Will send V2 to fix this.