netdev - Re: [PATCH net-next v2 2/3] net: dev: Makes sure netif

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Date:   Thu, 10 Feb 2022 10:13:30 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Toke Høiland-Jørgensen 
        <toke@...e.dk>
Subject: Re: [PATCH net-next v2 2/3] net: dev: Makes sure netif_rx() can be
 invoked in any context.

On Thu, 10 Feb 2022 13:22:32 +0100 Sebastian Andrzej Siewior wrote:
> On 2022-02-07 08:47:17 [-0800], Jakub Kicinski wrote:
> > On Sat, 5 Feb 2022 21:36:05 +0100 Sebastian Andrzej Siewior wrote:  
> > > Don't we end up in the same situation as netif_rx() vs netix_rx_ni()?  
> > 
> > Sort of. TBH my understanding of the motivation is a bit vague.
> > IIUC you want to reduce the API duplication so drivers know what to
> > do[1]. I believe the quote from Eric you put in the commit message
> > pertains to HW devices, where using netif_rx() is quite anachronistic. 
> > But software devices like loopback, veth or tunnels may want to go via
> > backlog for good reasons. Would it make it better if we called
> > netif_rx() netif_rx_backlog() instead? Or am I missing the point?  
> 
> So we do netif_rx_backlog() with the bh disable+enable and
> __netif_rx_backlog() without it and export both tree wide?

At a risk of confusing people about the API we could also name the
"non-super-optimized" version netif_rx(), like you had in your patch.
Grepping thru the drivers there's ~250 uses so maybe we don't wanna
touch all that code. No strong preference, I just didn't expect to 
see __netif_rx_backlog(), but either way works.

> It would make it more obvious indeed. Could we add
> 	WARN_ON_ONCE(!(hardirq_count() | softirq_count()))
> to the shortcut to catch the "you did it wrong folks"? This costs me
> about 2ns.

Modulo lockdep_..(), so we don't have to run this check on prod kernels?

> TL;DR
> 
> The netix_rx_ni() is problematic on RT and I tried to do something about
> it. I remembered from the in_atomic() cleanup that a few drivers got it
> wrong (one way or another). We added also netif_rx_any_context() which
> is used by some of the drivers (which is yet another entry point) while
> the few other got fixed.
> Then I stumbled over the thread where the entry (netif_rx() vs
> netif_rx_ni()) was wrong and Dave suggested to have one entry point for
> them all. This sounded like a good idea since it would eliminate the
> several API entry points where things can go wrong and my RT trouble
> would vanish in one go.
> The part with deprecated looked promising but I didn't take into account
> that the overhead for legitimate users (like the backlog or the software
> tunnels you mention) is not acceptable.

I see. So IIUC primary motivation is replacing preempt disable with bh
disable but the cleanup seemed like a good idea.