Subject: Re: [PATCH] net: allow netdev_wait_allrefs() to run faster

From: Eric Dumazet <[email-masked]>
Date: Sat, 24 Oct 2009 06:35:53 +0200

Paul E. McKenney a écrit :
> On Wed, Oct 21, 2009 at 05:40:07PM +0200, Eric Dumazet wrote:
>> [PATCH] net: allow netdev_wait_allrefs() to run faster
>>
>> netdev_wait_allrefs() waits that all references to a device vanishes.
>>
>> It currently uses a _very_ pessimistic 250 ms delay between each probe.
>> Some users report that no more than 4 devices can be dismantled per second,
>> this is a pretty serious problem for extreme setups.
>>
>> Most likely, references only wait for a rcu grace period that should come
>> fast, so use a schedule_timeout_uninterruptible(1) to allow faster recovery.
> 
> Is this a place where synchronize_rcu_expedited() is appropriate?
> (It went in to 2.6.32-rc1.)
> 

Thanks for the tip Paul

I believe netdev_wait_allrefs() is not a perfect candidate, because 
synchronize_sched_expedited() seems really expensive.

Maybe we could call it once only, if we had to call 1 times
the jiffie delay ?

diff --git a/net/core/dev.c b/net/core/dev.c
index fa88dcd..9b04b9a 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -4970,6 +4970,7 @@ EXPORT_SYMBOL(register_netdev);
 static void netdev_wait_allrefs(struct net_device *dev)
 {
 	unsigned long rebroadcast_time, warning_time;
+	unsigned int count = 0;
 
 	rebroadcast_time = warning_time = jiffies;
 	while (atomic_read(&dev->refcnt) != 0) {
@@ -4995,7 +4996,10 @@ static void netdev_wait_allrefs(struct net_device *dev)
 			rebroadcast_time = jiffies;
 		}
 
-		msleep(250);
+		if (count++ == 1)
+			synchronize_rcu_expedited();
+		else
+			schedule_timeout_uninterruptible(1);
 
 		if (time_after(jiffies, warning_time + 10 * HZ)) {
 			printk(KERN_EMERG "unregister_netdevice: "