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

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Date:   Fri, 17 Jun 2022 12:44:13 -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 09:42:55 -0700 Jay Vosburgh wrote:
> 	In this case, it's to permit the bonding ARP / ND monitor to
> function if that software device (veth in this case) is added to a bond
> using the ARP / ND monitor (which relies on trans_start, and has done so
> since at least 2.6.0).  I'll agree it's a niche case; this was broken
> for veth for quite some time, but veth + netns is handy for software
> only test cases, so it seems worth doing.

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?

> 	I didn't exhaustively check all LLTX drivers, but, e.g., tun
> does update trans_start:
> 
> drivers/net/tun.c:
> 
>        /* NETIF_F_LLTX requires to do our own update of trans_start */
>         queue = netdev_get_tx_queue(dev, txq);
>         txq_trans_cond_update(queue);

Well, it is _an_ example, but the only one I can find. And the
justification is the same as yours now -- make bonding work a31d27fb.
Because of that I don't think we can use tun as a proof that trans 
start should be updated on LLTX devices as a general, stack-wide rule.
There's a lot more LLTX devices than veth and tun.