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

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Date:	Fri, 14 Aug 2015 10:41:53 +0200
From:	Phil Sutter <phil@....cc>
To:	Stephen Hemminger <stephen@...workplumber.org>
Cc:	Jesper Dangaard Brouer <brouer@...hat.com>, netdev@...r.kernel.org,
	alexei.starovoitov@...il.com, davem@...emloft.net, 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

On Thu, Aug 13, 2015 at 12:11:57PM -0700, Stephen Hemminger wrote:
> On Thu, 13 Aug 2015 20:40:37 +0200
> Jesper Dangaard Brouer <brouer@...hat.com> wrote:
> 
> > On Thu, 13 Aug 2015 10:49:50 -0700
> > Stephen Hemminger <stephen@...workplumber.org> wrote:
> > 
> > > On Thu, 13 Aug 2015 19:01:05 +0200
> > > Phil Sutter <phil@....cc> wrote:
> > > 
> > > > 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,
> > > 
> > > So this is a workaround for user ignorance by introducing kernel API complexity.
> > > Before user sets qdisc, why don't they set tx queue length?
> > 
> > Please don't insist on keeping this broke interface... how should users
> > know that BEFORE adding a qdisc they MUST change the _device_ tx queue
> > length (not zero).
> 
> Before setting any qdisc, they should set queue length anyway.

Probably, yes. But if they don't, it depends on the interface driver
whether they're screwed or not. In my opinion, this inconsistency alone
is worth fixing.

> >  Getting "back" to the original state, they MUST
> > change the device tx queue len back to zero BEFORE deleting the qdisc,
> > such that when assigning the default queue qdisc the system detects
> > this device can work without a qdisc.  Changing the tx queue len to
> > zero after the qdisc is deleted will have not effect. 
> > 
> > Listen to the description, that interface is broken. The kernel really
> > needs to hide these details from userspace.
> > 
> > It even allows you to misconfigure the kernel, by tricking the kernel
> > into assigning noqueue to physical devices that really need it.
> 
> But adding a flag risks breaking external scripts.

Could you please elaborate on this? As far as I can tell, introducing a
separate flag is the only solution *not* breaking existing scripts. So
if you see the rub, I would like to know where exactly it is.

Cheers, Phil
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