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Message-ID: <1307041789.2812.27.camel@bwh-desktop>
Date: Thu, 02 Jun 2011 20:09:49 +0100
From: Ben Hutchings <bhutchings@...arflare.com>
To: Neil Horman <nhorman@...driver.com>
Cc: netdev@...r.kernel.org, Jay Vosburgh <fubar@...ibm.com>,
Andy Gospodarek <andy@...yhouse.net>,
"David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH] bonding: reset queue mapping prior to transmission to
physical device
On Thu, 2011-06-02 at 14:56 -0400, Neil Horman wrote:
> On Thu, Jun 02, 2011 at 07:35:53PM +0100, Ben Hutchings wrote:
> > On Thu, 2011-06-02 at 14:03 -0400, Neil Horman wrote:
> > > The bonding driver is multiqueue enabled, in which each queue represents a slave
> > > to enable optional steering of output frames to given slaves against the default
> > > output policy. However, it needs to reset the skb->queue_mapping prior to
> > > queuing to the physical device or the physical slave (if it is multiqueue) could
> > > wind up transmitting on an unintended tx queue (one that was reserved for
> > > specific traffic classes for instance)
[...]
> > So far as I can see, this has no effect, because dev_queue_xmit() always
> > sets queue_mapping (in dev_pick_tx()).
> >
> it resets the queue mapping exactly as you would expect it to. bonding is a
> multiqueue enabled device and selects a potentially non-zero queue based on the
> output of bond_select_queue.
>
> > What is the problem you're seeing?
> >
> The problem is exctly that. dev_pick_tx() on the bond device sets the
> queue_mapping as per the result of bond_select_queue (the ndo_select_queue
> method for the bonding driver). The implementation there is based on the use of
> tc with bonding, so that output slaves can be selected for certain types of
> traffic. But when that mechanism is used, skb->queue_mapping is preserved when
> the bonding driver queues the frame to the underlying slave. This denies the
> slave (if its also a multiqueue device) the opportunity to reselect the queue
> properly, because of this call path:
>
> bond_queue_xmit
> dev_queue_xmit(slave_dev)
> dev_pick_tx()
> skb_tx_hash()
> __skb_tx_hash()
>
> __skb_tx_hash sees that skb_queue_recorded returns true, and assigns a hardware queue mapping
> based on what the bonding driver chose using its own internal logic. Since
> bonding uses the multiqueue infrastructure to do slave output selection without
> any regard for slave output queue selection, it seems to me we should really
> reset the queue mapping to zero so the slave device can pick its own tx queue.
So you're effectively clearing the *RX queue* number (as this is before
dev_pick_tx()) in order to influence TX queue selection.
Here, the bonding device seems to be behaving as a forwarding device.
If TX queue selection can go wrong for certain combinations of queue
configuration when forwarding, then this is a problem for IP forwarding
and bridging as well, isn't it?
Ben.
--
Ben Hutchings, Senior Software Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.
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