netdev - Re: [PATCH v3 net-next 1/4] net: implement support for low latency socket polling

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Message-Id: <20130521.114921.460497085402760002.davem@davemloft.net>
Date:	Tue, 21 May 2013 11:49:21 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	priikone@....fi
Cc:	eric.dumazet@...il.com, eliezer.tamir@...ux.intel.com,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	jesse.brandeburg@...el.com, donald.c.skidmore@...el.com,
	e1000-devel@...ts.sourceforge.net, willemb@...gle.com,
	andi@...stfloor.org, hpa@...or.com, eliezer@...ir.org.il
Subject: Re: [PATCH v3 net-next 1/4] net: implement support for low latency
 socket polling

From: Pekka Riikonen <priikone@....fi>
Date: Tue, 21 May 2013 19:02:19 +0200 (CEST)

> On Tue, 21 May 2013, Eric Dumazet wrote:
> 
> : > > Alternatively, use a napi_id instead of a pointer.
> : > 
> : > I'm not sure I understand what you propose.
> : 
> : Oh well.
> : 
> : To get a pointer to a struct net_device, we can use ifindex, and do a
> : rcu lookup into a hash table to get the net_device. We do not need
> : {pointer,ifindex} but {ifindex} is enough
> : 
> : My suggestion is to not have skb->skb_ref but skb->napi_index : Its safe
> : to copy its value from skb->napi_index to sk->napi_index without
> : refcounting.
> : 
> : All NAPI need to get a unique napi_index, and be inserted in a hash
> : table for immediate/fast lookup.
> : 
> Maybe even that's not needed.  Couldn't skb->queue_mapping give the 
> correct NAPI instance in multiqueue nics?  The NAPI instance could be made 
> easily available from skb->dev.  In any case an index is much better than 
> a new pointer.

Queue mapping is more volatile, and consider layered devices.
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