netdev - Re: tun: Use netif_receive_skb instead of netif

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Date:	Wed, 19 May 2010 16:24:01 -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 02:00:53PM -0400, Neil Horman wrote:
> 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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> 


Just out of curiosity, how unsavory would it be if we were to dedicate the upper
bit in the SOFTIRQ_BITS range to be an indicator of weather we were actually
executing softirqs?  As noted above, we're tripping over the ambiguity here
between running in softirq context and actually just having softirqs disabled.
Would it be against anyones sensibilities if we were to dedicate the upper bit
in the softirq_count range to disambiguate the two conitions (or use a separate
flag for that matter)?

Neil

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