netdev - Re: [RFC] restore netdev

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Date:	Fri, 17 Aug 2007 16:04:09 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	shemminger@...ux-foundation.org
Cc:	netdev@...r.kernel.org
Subject: Re: [RFC] restore netdev_priv optimization

From: Stephen Hemminger <shemminger@...ux-foundation.org>
Date: Fri, 17 Aug 2007 15:40:22 -0700

> Compile tested only!!!

Obviously.  The first loopback transmit is guarenteed to crash.

> Fix optimization of netdev_priv() lost by the  addition of multiqueue.
> Move the variable size subqueues to after the constant size priv area.
> 
> When putting back the old netdev_priv() code, I tried to make it
> clearer by using roundup() and ALIGN() macros.
> 
> --- a/include/linux/netdevice.h	2007-08-17 12:08:51.000000000 -0400
> +++ b/include/linux/netdevice.h	2007-08-17 12:48:03.000000000 -0400
> @@ -575,16 +575,15 @@ struct net_device
>  
>  	/* The TX queue control structures */
>  	unsigned int			egress_subqueue_count;
> -	struct net_device_subqueue	egress_subqueue[1];
> +	struct net_device_subqueue	*egress_subqueue;
>  };

This just trades off the dev->priv dereference for a
dev->egress_subqueue dereference.  I bet they occur about equally in
the data paths, at least on transmit.

And this also breaks loopback again, which uses a static struct netdev
in the kernel image, it doesn't use alloc_netdev(), so egress_subqueue
of loopback will be NULL.

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