netdev - Re: tun: Use netif_receive_skb instead of netif

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Message-ID: <20100519143103.GB26519@hmsreliant.think-freely.org>
Date:	Wed, 19 May 2010 10:31:03 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Neil Horman <nhorman@...hat.com>,
	Herbert Xu <herbert@...dor.hengli.com.au>,
	"David S. Miller" <davem@...emloft.net>,
	Thomas Graf <tgraf@...hat.com>, netdev@...r.kernel.org
Subject: Re: tun: Use netif_receive_skb instead of netif_rx

On Wed, May 19, 2010 at 04:10:29PM +0200, Eric Dumazet wrote:
> Le mercredi 19 mai 2010 à 08:05 -0400, Neil Horman a écrit :
> > On Wed, May 19, 2010 at 10:18:09AM +0200, Eric Dumazet wrote:
> > > Le mercredi 19 mai 2010 à 10:09 +0200, Eric Dumazet a écrit :
> > > 
> > > > Another concern I have is about RPS.
> > > > 
> > > > netif_receive_skb() must be called from process_backlog() context, or
> > > > there is no guarantee the IPI will be sent if this skb is enqueued for
> > > > another cpu.
> > > 
> > > Hmm, I just checked again, and this is wrong.
> > > 
> > > In case we enqueue skb on a remote cpu backlog, we also
> > > do __raise_softirq_irqoff(NET_RX_SOFTIRQ); so the IPI will be done
> > > later.
> > > 
> > But if this happens, then we loose the connection between the packet being
> > received and the process doing the reception, so the network cgroup classifier
> > breaks again.
> > 
> > Performance gains are still a big advantage here of course.
> 
> RPS is enabled on a per device (or more precisely per subqueue) basis, and disabled
> by default, so if cgroup classifier is needed, it should work as is.
> 
> Speaking of net/sched/cls_cgroup.c, I am contemplating following
> sequence :
> 
> rcu_read_lock();
> classid = task_cls_state(current)->classid;
> rcu_read_unlock();
> 
Yeah, I'd noticed there was no write side to that, but hadn't quite gotten to
investigating  that further :)

> RCU is definitly so special (should I say magic ?), that we use it even
> if not needed. It makes us happy...
> 
> 
> 
> 
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