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

From: "Paul E. McKenney" <[email-masked]>
Date: Fri, 23 Oct 2009 22:49:43 -0700

On Sat, Oct 24, 2009 at 06:35:53AM +0200, Eric Dumazet wrote:
> 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.

It does indeed keep the CPUs quite busy for a bit.  ;-)

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

This could be a very useful approach!

However, please keep in mind that although synchronize_rcu_expedited()
forces a grace period, it does nothing to speed the invocation of other
RCU callbacks.  In short, synchronize_rcu_expedited() is a faster version
of synchronize_rcu(), but doesn't necessarily help other synchronize_rcu()
or call_rcu() invocations.

The reason I point this out is that it looks to me that the code below is
waiting for some other task which is in turn waiting on a grace period.
But I don't know this code, so could easily be confused.

						Thanx, paul

> 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: "
>