netdev - Re: [PATCH] netdev: handle setting transmit queue length on active device.

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Date:	Wed, 18 Mar 2009 18:15:38 +0100
From:	Patrick McHardy <kaber@...sh.net>
To:	Stephen Hemminger <shemminger@...tta.com>
CC:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH] netdev: handle setting transmit queue length on active
 device.

Stephen Hemminger wrote:
> Some virtual devices like VLAN's or bridges by default have not transmit queue.
> In order to do queuing disciplines on these devices a queue must be added
> by setting transmit queue length. The kernel allows doing this at any time, but
> setting the queue length value is not enough. Setting the queue length does
> not create the transmit queue (going from 0 to non-zero) or destroy it
> (setting to zero); the queue is only created (or destroyed) when device
> comes up (or goes down).
> 
> This patch handles this be doing the necessary activations.
> 
> Signed-off-by: Stephen Hemminger <shemminger@...tta.com>
> 
> +/**
> + *
> + *	@dev: device
> + *	@len: limit of frames in transmit queue
> + *
> + *	Set ifalias for a device,
> + */
> +int dev_set_tx_queue_len(struct net_device *dev, unsigned long len)
> +{
> +	if (dev->flags & IFF_UP)
> +		dev_deactivate(dev);
> +	dev->tx_queue_len = len;
> +	if (dev->flags & IFF_UP)
> +		dev_activate(dev);
> +	return 0;
> +}

That seems quite heavy-weight in case of hierarchical qdiscs, it will
walk and reset the entire tree and looses all state, including packets
as a side effect. Its also a bit inconsistent since it only affects
the default pfifos at the root, not the inner default qdiscs if I'm
not mistaken.

A less intrusive way would be to simply use dev->tx_queue_len directly
in pfifo when no limit is configured. That would require to make a
runtime decision for each packet though. Two more alternatives:

- add a special pfifo variant that always uses tx_queue_len. Duplication
   comes down to pfifo_enqueue (a few lines) and a new struct Qdisc_ops

- if you really only care about the root, reconfigure it explicitly
   using fifo_set_limit()

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