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

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Message-ID: <1409751717.26422.31.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Wed, 03 Sep 2014 06:41:57 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jesper Dangaard Brouer <brouer@...hat.com>
Cc:	Tom Herbert <therbert@...gle.com>, davem@...emloft.net,
	netdev@...r.kernel.org
Subject: Re: [PATCH v2 net-next] net: Functions to report space available in
 device TX queues

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.



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