netdev - Re: An inconsistency/bug in ingress netem timestamps

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Date:	Wed, 15 Apr 2009 13:29:18 -0700
From:	Stephen Hemminger <shemminger@...tta.com>
To:	Jarek Poplawski <jarkao2@...il.com>
Cc:	Alex Sidorenko <alexandre.sidorenko@...com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: An inconsistency/bug in ingress netem timestamps

On Wed, 15 Apr 2009 22:26:20 +0200
Jarek Poplawski <jarkao2@...il.com> wrote:

> On Wed, Apr 15, 2009 at 04:10:43PM -0400, Alex Sidorenko wrote:
> > On April 15, 2009 03:50:22 pm Jarek Poplawski wrote:
> > 
> > > I agree there is an inconsistency, but it seems 100 ms isn't the
> > > "right" thing to show here. It shows an internal delay added on ifb by
> > > any packet scheduler, so probably not what a user usually expects.
> > 
> > Hi Jarek,
> > 
> > thank you for your comments. Yes, I understand that it just looked OK in this 
> > case even though technically the value was not quite correct.
> > 
> > > > The strange thing is that as soon as there is any ptype_all handler
> > > > installed, skb->tstamp is updated properly. Unfortunately, my knowledge
> > > > of TC internals is not good enough to find how exactly this happens.
> > >
> > > Isn't it when act_mirred calls dev_queue_xmit with dev_queue_xmit_nit?
> > > But, as above mentioned, I doubt it's "updated properly" in this case.
> > 
> > I can see that dev_queue_xmit_nit calls net_timestamp(skb) unconditionally. I 
> > agree that to fix this properly we need to update tstamp in another place 
> > explicitly (in ifb or netem?).
> 
> Hmm... I'm not sure how "popular" is netem on ifb, but we could try
> Stephen's opinion (Cc-ed).

If you are putting on netem on ingress, the timestamps could happen before
or after the added delay. As long as it is consistent, then I have no problem
with the existing behavior; ie. it is not a bug, it just works that way.
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