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

From: Stephen Hemminger <[email-masked]>
Date: Wed, 18 Mar 2009 11:02:36 -0700

On Wed, 18 Mar 2009 18:15:38 +0100
Patrick McHardy <[email-masked]> wrote:

> 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 <[email-masked]>
> > 
> > +/**
> > + *
> > + *	@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()
> 

The problem is when setting limit to 0 to force noop and setting it
to non-zero to force pfifo_fast back again.