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

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Date:   Thu, 3 Feb 2022 08:18:34 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
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 Thu, Feb 3, 2022 at 7:40 AM Sebastian Andrzej Siewior
<bigeasy@...utronix.de> wrote:
>
> On 2022-02-03 07:25:01 [-0800], Eric Dumazet wrote:
> >
> > No, the loopback device (ifconfig log) I am referring to is in
> > drivers/net/loopback.c
> >
> > loopback_xmit() calls netif_rx() directly, while bh are already disabled.
>
> ah okay. Makes sense.
>
> > Instead of adding a local_bh_disable()/local_bh_enable() in netif_rx()
> > I suggested
> > to rename current netif_rx() to __netif_rx() and add a wrapper, eg :
>
> 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.

>
> Would something like
>
> diff --git a/include/linux/bottom_half.h b/include/linux/bottom_half.h
> index fc53e0ad56d90..561cbca431ca6 100644
> --- a/include/linux/bottom_half.h
> +++ b/include/linux/bottom_half.h
> @@ -30,7 +30,12 @@ static inline void local_bh_enable_ip(unsigned long ip)
>
>  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 :)

>
> Sebastian