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

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Submitter Amos Kong
Date June 8, 2011, 11:46 p.m.
Message ID <20110608234606.8681.19932.stgit@localhost6.localdomain6>
Download mbox | patch
Permalink /patch/99587/
State Accepted
Delegated to: David Miller
Headers show

Comments

Amos Kong - June 8, 2011, 11:46 p.m.
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>
---
 drivers/net/tun.c |    6 ++++--
 1 files changed, 4 insertions(+), 2 deletions(-)


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David Miller - June 9, 2011, 7:27 a.m.
From: Amos Kong <akong@redhat.com>
Date: Thu, 09 Jun 2011 07:46:06 +0800

> 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>

Applied, thanks.
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Patch

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 74e9405..95dbff4 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -817,7 +817,8 @@  static ssize_t tun_do_read(struct tun_struct *tun,
 
 	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 @@  static ssize_t tun_do_read(struct tun_struct *tun,
 	}
 
 	current->state = TASK_RUNNING;
-	remove_wait_queue(&tun->wq.wait, &wait);
+	if (unlikely(!noblock))
+		remove_wait_queue(&tun->wq.wait, &wait);
 
 	return ret;
 }