netdev - Re: [PATCH v2 net-next] net: Functions to report space available in device TX queues

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Date:	Wed, 3 Sep 2014 15:49:38 +0200
From:	Jesper Dangaard Brouer <brouer@...hat.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Tom Herbert <therbert@...gle.com>, davem@...emloft.net,
	netdev@...r.kernel.org, brouer@...hat.com
Subject: Re: [PATCH v2 net-next] net: Functions to report space available in
 device TX queues

On Wed, 03 Sep 2014 06:41:57 -0700
Eric Dumazet <eric.dumazet@...il.com> wrote:

> On Wed, 2014-09-03 at 15:20 +0200, Jesper Dangaard Brouer wrote:
> > On Mon, 25 Aug 2014 08:27:56 -0700 (PDT)
> > Tom Herbert <therbert@...gle.com> wrote:
> > 
> > > This patch adds netdev_tx_avail_queue and netdev_avail_queue which are
> > > used to report number of bytes available in transmit queues per BQL. The
> > > functions call dql_avail which returns BQL limit minus number of
> > > inflight bytes. These functions can be called without txlock, for
> > > instance to ascertain how much data should be dequeued from a qdisc in
> > > a batch. When called without the tx_lock, the result is technically a
> > > hint, subsequently when the tx_lock is done for a transmit it is
> > > possible the availability has changed (for example a transmit
> > > completion may have freed up more space in the queue or changed the
> > > limit).
> > > 
> > > Signed-off-by: Tom Herbert <therbert@...gle.com>
> > > ---
> > >  include/linux/netdevice.h | 28 ++++++++++++++++++++++++++--
> > >  1 file changed, 26 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> > > index 0fac884..bdf6c85 100644
> > > --- a/include/linux/netdevice.h
> > > +++ b/include/linux/netdevice.h
> > > @@ -2544,6 +2544,30 @@ static inline void netdev_completed_queue(struct net_device *dev,
> > >  	netdev_tx_completed_queue(netdev_get_tx_queue(dev, 0), pkts, bytes);
> > >  }
> > >  
> > > +static inline int netdev_tx_avail_queue(struct netdev_queue *dev_queue)
> > > +{
> > > +#ifdef CONFIG_BQL
> > > +	return dql_avail(&dev_queue->dql);
> > > +#else
> > > +	return DQL_MAX_LIMIT;
> > > +#endif
> > > +}
> > > +
> > > +/**
> > > + *	netdev_avail_queue - report how much space is availble for xmit
> > > + *	@dev: network device
> > > + *
> > > + *	Report the amount of space available in the TX queue in terms of
> > > + *	number of bytes. This returns the number of bytes avaiable per
> > > + *	DQL. This function may be called without taking the txlock on
> > > + *	the device, however in that case the result should be taken as
> > > + *	a (strong) hint.
> > > + */
> > > +static inline int netdev_avail_queue(struct net_device *dev)
> > > +{
> > > +	return netdev_tx_avail_queue(netdev_get_tx_queue(dev, 0));
> > > +}
> > > +
> > 
> > How can this work, when you are always passing 0 to: netdev_get_tx_queue(dev, 0)
> > ???
> > 
> 
> This is the helper for drivers having a single TX queue.

Okay, so I should use:
 bytelimit = netdev_tx_avail_queue(txq);



-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Sr. Network Kernel Developer at Red Hat
  Author of http://www.iptv-analyzer.org
  LinkedIn: http://www.linkedin.com/in/brouer
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