netdev - RE: problem with noop and pfifo

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Date:	Thu, 19 Jan 2012 14:44:52 +0100
From:	Hemant-vilas RAMDASI <hemant.ramdasi@...ricsson.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"netdev-owner@...r.kernel.org" <netdev-owner@...r.kernel.org>
Subject: RE: problem with noop and pfifo_fast Qdisc switching

Hi,

> -----Original Message-----
> From: Eric Dumazet [mailto:eric.dumazet@...il.com]
> Sent: Thursday, January 19, 2012 6:46 PM
> To: Hemant-vilas RAMDASI
> Cc: netdev@...r.kernel.org; netdev-owner@...r.kernel.org
> Subject: Re: problem with noop and pfifo_fast Qdisc switching
> 
> Le jeudi 19 janvier 2012 à 13:47 +0100, Hemant-vilas RAMDASI a écrit :
> > Hi,
> >
> > I have a situation where one of important message (phonet pipe
> message) gets dropped because of 'noop' Qdisc.
> > This situation mostly happens when coming out of sleep and there is a
> message to send.
> > What I have observed is this.
> > In normal case the qdisc used id pfifo_fast. When CPU goes to sleep,
> it changes the qdisc to noop.
> > When CPU wakes up (because of a message from lower layers), phonet
> tries to send a control packet to driver and finds qdisc is still
> 'noop'.
> > Because the concerned message is a control message, this packet drop
> is creating deadlock situation.
> >
> > How the switch between pfifo_fast and noop is handled? Is this
> something solved in 3.0?
> > We are running a older version of kernel (2.6.35 based).
> 
> If message is important, you should check return code from xmit
> (NET_XMIT_CN is returned by noop_enqueue), and maybe resend it later.
> 
> Not sure by what you mean by 'cpu goes to sleep' or 'cpu wakes up' ?
> 
> Is it the machine going to suspend mode ?
> 
Yes, I mean machine going to suspend..
The problem with this message is that it can be retried only a couple of time.
We saw a code where dev_deactivate is called which changes the Qdisc to 'noop'. 
I hope it will restore it in dev_activate() again when machine resumes. We are checking this.
Is there any race between when dev_queue_xmit is called and dev_activate is still not complete?

Thanks,
Hemant