netdev - Re: [PATCH net] veth: Add updating of trans

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Date:   Tue, 21 Jun 2022 12:52:39 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Jay Vosburgh <jay.vosburgh@...onical.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org,
        Jonathan Toppins <jtoppins@...hat.com>,
        Veaceslav Falico <vfalico@...il.com>,
        Andy Gospodarek <andy@...yhouse.net>
Subject: Re: [PATCH net] veth: Add updating of trans_start

On Fri, 17 Jun 2022 17:55:50 -0700 Jakub Kicinski wrote:
> > >I presume it needs it to check if the device has transmitted anything
> > >in the last unit of time, can we look at the device stats for LLTX for
> > >example?    
> > 
> > 	Yes, that's the use case.  
> > 
> > 	Hmm.  Polling the device stats would likely work for software
> > devices, although the unit of time varies (some checks are fixed at one
> > unit, but others can be N units depending on the missed_max option
> > setting).
> > 
> > 	Polling hardware devices might not work; as I recall, some
> > devices only update the statistics on timespans on the order of seconds,
> > e.g., bnx2 and tg3 appear to update once per second.  But those do
> > update trans_start.  
> 
> Right, unfortunately.
> 
> > 	The question then becomes how to distinguish a software LLTX
> > device from a hardware LLTX device.  
> 
> If my way of thinking about trans_start is correct then we can test 
> for presence of ndo_tx_timeout. Anything that has the tx_timeout NDO
> must be maintaining trans_start.

So what's your thinking Jay? Keep this as an immediate small fix 
for net but work on using a different approach in net-next?