netdev - Re: [RFC] restore netdev

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Date:	Fri, 17 Aug 2007 16:19:28 -0700
From:	Stephen Hemminger <shemminger@...ux-foundation.org>
To:	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org
Subject: Re: [RFC] restore netdev_priv optimization

On Fri, 17 Aug 2007 16:04:09 -0700 (PDT)
David Miller <davem@...emloft.net> wrote:

> 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.

That is fixable.

> > 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.

The subqueue is only referenced in start/stop queue and that only happens
once per packet on normal tx, and only if multiqueue is used.

The existing multiqueue penalizes all devices, not just multiqueue devices.

> 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.

That can be overcome.

-- 
Stephen Hemminger <shemminger@...ux-foundation.org>
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