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tun: do not put self in waitq if doing a nonblock read

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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>
This commit is contained in:
Amos Kong
2011-06-09 00:27:10 -07:00
committed by David S. Miller
parent 6f7c156c08
commit 61a5ff15eb
1 changed files with 4 additions and 2 deletions
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drivers/net/tun.c
+4 -2
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@@ -817,7 +817,8 @@ static ssize_t tun_do_read(struct tun_struct *tun,
tun_debug(KERN_INFO, tun, "tun_chr_read\n"); 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) { while (len) {
current->state = TASK_INTERRUPTIBLE; current->state = TASK_INTERRUPTIBLE;
@@ -848,7 +849,8 @@ static ssize_t tun_do_read(struct tun_struct *tun,
} }
current->state = TASK_RUNNING; current->state = TASK_RUNNING;
remove_wait_queue(&tun->wq.wait, &wait); if (unlikely(!noblock))
remove_wait_queue(&tun->wq.wait, &wait);
return ret; return ret;
} }