On 2014/1/28 22:49, Eric Dumazet wrote:
On Tue, 2014-01-28 at 16:14 +0800, Qin Chuanyu wrote:
according perf test result，I found that there are 5%-8% cpu cost on
softirq by use netif_rx_ni called in tun_get_user.
so I changed the function which cause skb transmitted more quickly.
from
tun_get_user ->
netif_rx_ni(skb);
to
tun_get_user ->
rcu_read_lock_bh();
netif_receive_skb(skb);
rcu_read_unlock_bh();
No idea why you use rcu here ?
In my first version, I forgot to add lock when called netif_receive_skb
then I met a dad spinlock when using tcpdump.
tcpdump receive skb in netif_receive_skb but also in dev_queue_xmit.
and I have notice dev_queue_xmit add rcu_read_lock_bh before
transmitting skb, and this lock avoid race between softirq and transmit
thread.
/* Disable soft irqs for various locks below. Also
* stops preemption for RCU.
*/
rcu_read_lock_bh();
Now I try to xmit skb in vhost thread, so I did the same thing.
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