netdev - Re: [RFC,PATCH] loopback: calls netif_receive_skb() instead of netif

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Message-Id: <20080226.182120.183405235.davem@davemloft.net>
Date:	Tue, 26 Feb 2008 18:21:20 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	dada1@...mosbay.com
Cc:	netdev@...r.kernel.org
Subject: Re: [RFC,PATCH] loopback: calls netif_receive_skb() instead of
 netif_rx()

From: Eric Dumazet <dada1@...mosbay.com>
Date: Thu, 21 Feb 2008 19:51:52 +0100

> Following patch directly calls netif_receive_skb() and avoids lot of 
> atomic operations.
> (atomic_inc(&dev->refcnt), set_and_set_bit(NAPI_STATE_SCHED, &n->state), ...
>   atomic_dec(&dev->refcnt)...), cache line ping-pongs on device refcnt, 
> but also softirq overhead.
> 
> This gives a nice boost on tbench for example (5 % on my machine)

My only concern is stack usage.

Note that packet reception can elicit a response and go all the way
back into this driver and all the way down into netif_receive_skb()
again.  And so on and so forth.

If there is some bug in the stack (ACK'ing ACKs, stuff like that) we
could get into a loop and overrun the kernel stack in no time at all.

So, if anything, this change could make inconvenient errors become
catastrophic and hard to diagnose.
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