netdev - Re: [RFC] gro: Is it ok to share a single napi from several devs ?

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Date:	Sat, 28 Aug 2010 16:48:29 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jarek Poplawski <jarkao2@...il.com>
Cc:	Stephen Hemminger <shemminger@...tta.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [RFC] gro: Is it ok to share a single napi from several devs ?

Le samedi 28 août 2010 à 16:31 +0200, Jarek Poplawski a écrit :
> On Sat, Aug 28, 2010 at 12:54:06PM +0200, Eric Dumazet wrote:
> > In commit f2bde7328633269ee935d9ed96535ade15cc348f
> > Author: Stephen Hemminger <shemminger@...tta.com>
> > 
> >     net: allow multiple dev per napi with GRO
> >     
> >     GRO assumes that there is a one-to-one relationship between NAPI
> >     structure and network device. Some devices like sky2 share multiple
> >     devices on a single interrupt so only have one NAPI handler. Rather than
> >     split GRO from NAPI, just have GRO assume if device changes that
> >     it is a different flow.
> >    
> > 
> > It was assumed a napi could be shared by several devs, but I dont really
> > understand, since we have an unique ->dev pointer inside "napi_struct",
> > this one is set once, and never change.
> > 
> > This pointer is currently used from napi_get_frags() [but that could be
> > avoided], and from netpoll_poll_lock().
> > 
> > The netpoll_poll_lock() case is problematic.
> > 
> > static inline void *netpoll_poll_lock(struct napi_struct *napi)
> > {
> >         struct net_device *dev = napi->_dev;
> > 
> >         if (dev && dev->npinfo) {
> >                 spin_lock(&napi->poll_lock);
> > 
> > Maybe we should remove 'dev' field from napi_struct and replace it by a
> > npinfo pointer ?
> 
> Sky2 seems to work like a single netdev (with an internal sub-netdev),
> so I can't see your concern: what is the main aim of this replacement?

I am trying to understand why this commit was needed then.

It adds an extra test in the main loop, testing skb->dev against p->dev,
it must me for something...

I am trying to say that the one to one relationship between NAPI
structure and a device is not only a GRO thing, but also a netpoll one.

So either we completely remove this one to one relationship, either
Stephen commit was not needed.

Some clarification is needed.


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