netdev - Re: tun: Use netif_receive_skb instead of netif

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Date:	Wed, 19 May 2010 14:00:53 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	Neil Horman <nhorman@...hat.com>
Cc:	Eric Dumazet <eric.dumazet@...il.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 08:55:43AM -0400, Neil Horman wrote:
> On Wed, May 19, 2010 at 08:05:47AM -0400, Neil Horman wrote:
> > 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.
> > Neil
> > 
> Scratch what I said here, Herbert corrected me on this, and we're ok, as tun has
> no rps map.
> 
> I'll test this patch out in just a bit
> Neil
> 

I'm currently testing this, unfortunately, and its not breaking anything, but it
doesn't allow cgroups to classify frames comming from tun interfaces.  I'm still
investigating, but I think the issue is that, because we call local_bh_disable
with this patch, we wind up raising the count at SOFTIRQ_OFFSET in preempt_count
for the task.  Since the cgroup classifier has this check:

if (softirq_count() != SOFTIRQ_OFFSET))
	return -1;

We still fail to classify the frame.  the cgroup classifier is assuming that any
frame arriving with a softirq count of 1 means we came directly from the
dev_queue_xmit routine and is safe to check current().  Any less than that, and
something is wrong (as we at least need the local_bh_disable in dev_queue_xmit),
and any more implies that we have nested calls to local_bh_disable, meaning
we're really handling a softirq context.

Neil

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