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

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Date:	Wed, 15 Apr 2009 22:26:20 +0200
From:	Jarek Poplawski <jarkao2@...il.com>
To:	Alex Sidorenko <alexandre.sidorenko@...com>
Cc:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Stephen Hemminger <shemminger@...tta.com>
Subject: Re: An inconsistency/bug in ingress netem timestamps

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).

Thanks,
Jarek P.
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