netdev - Re: [PATCH net-next 3/4] net: dev: Makes sure netif

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Date:   Thu, 3 Feb 2022 17:44:32 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     bpf <bpf@...r.kernel.org>, netdev <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH net-next 3/4] net: dev: Makes sure netif_rx() can be
 invoked in any context.

On 2022-02-03 08:18:34 [-0800], Eric Dumazet wrote:
> > So we still end up with two interfaces. Do I move a few callers like the
> > one you already mentioned over to the __netif_rx() interface or will it
> > be the one previously mentioned for now?
> 
> 
> I would say vast majority of drivers would use netif_rx()
> 
> Only the one we consider critical (loopback traffic) would use
> __netif_rx(), after careful inspection.
> 
> As we said modern/high performance NIC are using NAPI and GRO these days.
> 
> Only virtual drivers might still use legacy netif_rx() and be in critical paths.

Let me then update something to the documentation so it becomes obvious.

> >  static inline void local_bh_enable(void)
> >  {
> > -       __local_bh_enable_ip(_THIS_IP_, SOFTIRQ_DISABLE_OFFSET);
> > +       if (unlikely(softirq_count() == SOFTIRQ_DISABLE_OFFSET)) {
> > +               __local_bh_enable_ip(_THIS_IP_, SOFTIRQ_DISABLE_OFFSET);
> > +       } else {
> > +               preempt_count_sub(SOFTIRQ_DISABLE_OFFSET);
> > +               barrier();
> > +       }
> >  }
> >
> >  #ifdef CONFIG_PREEMPT_RT
> >
> > lower the overhead to acceptable range? (I still need to sell this to
> > peterz first).
> 
> I guess the cost of the  local_bh_enable()/local_bh_disable() pair
> will be roughly the same, please measure it :)

We would avoid that branch maybe that helps. Will measure.

Sebastian