Subject: Re: [PATCH] net: use synchronize_rcu_expedited()

From: "Paul E. McKenney" <[email-masked]>
Date: Tue, 24 May 2011 08:44:45 -0700

On Tue, May 24, 2011 at 11:07:32AM +0200, Eric Dumazet wrote:
> synchronize_rcu() is very slow in various situations (HZ=100,
> CONFIG_NO_HZ=y, CONFIG_PREEMPT=n)
> 
> Extract from my (mostly idle) 8 core machine :
> 
>  synchronize_rcu() in 99985 us
>  synchronize_rcu() in 79982 us
>  synchronize_rcu() in 87612 us
>  synchronize_rcu() in 79827 us
>  synchronize_rcu() in 109860 us
>  synchronize_rcu() in 98039 us
>  synchronize_rcu() in 89841 us
>  synchronize_rcu() in 79842 us
>  synchronize_rcu() in 80151 us
>  synchronize_rcu() in 119833 us
>  synchronize_rcu() in 99858 us
>  synchronize_rcu() in 73999 us
>  synchronize_rcu() in 79855 us
>  synchronize_rcu() in 79853 us
> 
> 
> When we hold RTNL mutex, we would like to spend some cpu cycles but not
> block too long other processes waiting for this mutex.
> 
> We also want to setup/dismantle network features as fast as possible at
> boot/shutdown time.
> 
> This patch makes synchronize_net() call the expedited version if RTNL is
> locked.
> 
> synchronize_rcu_expedited() typical delay is about 20 us on my machine.
> 
>  synchronize_rcu_expedited() in 18 us
>  synchronize_rcu_expedited() in 18 us
>  synchronize_rcu_expedited() in 18 us
>  synchronize_rcu_expedited() in 18 us
>  synchronize_rcu_expedited() in 20 us
>  synchronize_rcu_expedited() in 16 us
>  synchronize_rcu_expedited() in 20 us
>  synchronize_rcu_expedited() in 18 us
>  synchronize_rcu_expedited() in 18 us

Cool!!!

Just out of curiosity, how many CPUs does your system have?

Reviewed-by: Paul E. McKenney <[email-masked]>

> Signed-off-by: Eric Dumazet <[email-masked]>
> CC: Paul E. McKenney <[email-masked]>
> CC: Ben Greear <[email-masked]>
> ---
>  net/core/dev.c |    5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/net/core/dev.c b/net/core/dev.c
> index bcb05cb..ec11d75 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -5954,7 +5954,10 @@ EXPORT_SYMBOL(free_netdev);
>  void synchronize_net(void)
>  {
>  	might_sleep();
> -	synchronize_rcu();
> +	if (rtnl_is_locked())
> +		synchronize_rcu_expedited();
> +	else
> +		synchronize_rcu();
>  }
>  EXPORT_SYMBOL(synchronize_net);
> 
> 
>