netdev - Re: [PATCH 0/2] net: introduce IFF_NO_QUEUE as successor of zero tx_queue

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Message-Id: <20150817.115104.47255555484332113.davem@davemloft.net>
Date:	Mon, 17 Aug 2015 11:51:04 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	phil@....cc
Cc:	netdev@...r.kernel.org, brouer@...hat.com,
	alexei.starovoitov@...il.com, fw@...len.de, cwang@...pensource.com,
	eric.dumazet@...il.com
Subject: Re: [PATCH 0/2] net: introduce IFF_NO_QUEUE as successor of zero
 tx_queue_len

From: Phil Sutter <phil@....cc>
Date: Thu, 13 Aug 2015 19:01:05 +0200

> This series adds a new private net_device flag indicating that a device may
> (and probably should) be used without a queueing discipline attached to it.
> This is already common practice for many virtual device types like e.g.
> loopback, VLAN (802.1Q) or bridges (802.1D). The reason for this is that these
> devices lack an underlying layer which could impose back pressure and therefore
> making a TX queue necessary to not slow down senders.
> 
> Up to now, drivers being aware of the above applying to them set
> dev->tx_queue_len to zero to indicate no qdisc should be attached to the
> interface they drive and the kernel reacts upon this by assigning the noop
> qdisc instead of the default pfifo_fast. This implicit agreement though leads
> to an inconvenient situation once a user tries to attach a real qdisc to these
> devices, as the formerly special tx_queue_len value becomes a regular one,
> limiting the queue to zero packets and thus prevents any TX from happening. To
> overcome this, practically all qdisc implementations intercept and sanitize the
> malicious value.
> 
> With this series applied, drivers may signal the lack of need for a qdisc
> without having to tamper with tx_queue_len, making fallbacks in qdiscs and
> caveats in userspace unnecessary.
> 
> Upon upstream acceptance, this series will be followed up by a set of patches
> converting device drivers, adding a warning so out-of-tree driver authors get
> aware of this change and dropping all special handling of tx_queue_len in
> net/sched/.

Series applied, thanks.

Since the VRF changes went in right before this, I had to bump the IFF_NO_QUEUE
value to use bit 26 instead of bit 25.
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