tun: do not put self in waitq if doing a nonblock read
Perf shows a relatively high rate (about 8%) race in
spin_lock_irqsave() when doing netperf between external host and
guest. It's mainly becuase the lock contention between the
tun_do_read() and tun_xmit_skb(), so this patch do not put self into
waitqueue to reduce this kind of race. After this patch, it drops to
4%.
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Amos Kong <akong@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 2829bad..ef68e13 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -817,7 +817,8 @@
tun_debug(KERN_INFO, tun, "tun_chr_read\n");
- add_wait_queue(&tun->wq.wait, &wait);
+ if (unlikely(!noblock))
+ add_wait_queue(&tun->wq.wait, &wait);
while (len) {
current->state = TASK_INTERRUPTIBLE;
@@ -848,7 +849,8 @@
}
current->state = TASK_RUNNING;
- remove_wait_queue(&tun->wq.wait, &wait);
+ if (unlikely(!noblock))
+ remove_wait_queue(&tun->wq.wait, &wait);
return ret;
}