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

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Date:   Wed, 2 Feb 2022 09:43:14 -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>
Subject: Re: [PATCH net-next 3/4] net: dev: Makes sure netif_rx() can be
 invoked in any context.

On Wed, Feb 2, 2022 at 4:28 AM Sebastian Andrzej Siewior
<bigeasy@...utronix.de> wrote:
>
> Dave suggested a while ago (eleven years by now) "Let's make netif_rx()
> work in all contexts and get rid of netif_rx_ni()". Eric agreed and
> pointed out that modern devices should use netif_receive_skb() to avoid
> the overhead.
> In the meantime someone added another variant, netif_rx_any_context(),
> which behaves as suggested.
>
> netif_rx() must be invoked with disabled bottom halves to ensure that
> pending softirqs, which were raised within the function, are handled.
> netif_rx_ni() can be invoked only from process context (bottom halves
> must be enabled) because the function handles pending softirqs without
> checking if bottom halves were disabled or not.
> netif_rx_any_context() invokes on the former functions by checking
> in_interrupts().
>
> netif_rx() could be taught to handle both cases (disabled and enabled
> bottom halves) by simply disabling bottom halves while invoking
> netif_rx_internal(). The local_bh_enable() invocation will then invoke
> pending softirqs only if the BH-disable counter drops to zero.
>
> Add a local_bh_disable() section in netif_rx() to ensure softirqs are
> handled if needed. Make netif_rx_ni() and netif_rx_any_context() invoke
> netif_rx() so they can be removed once they are no more users left.
>
> Link: https://lkml.kernel.org/r/20100415.020246.218622820.davem@davemloft.net
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>

Maybe worth mentioning this commit will show a negative impact, for
network traffic
over loopback interface.

My measure of the cost of local_bh_disable()/local_bh_enable() is ~6
nsec on one of my lab x86 hosts.

Perhaps we could have a generic netif_rx(), and a __netif_rx() for the
virtual drivers (lo and maybe tunnels).

void __netif_rx(struct sk_buff *skb);

static inline int netif_rx(struct sk_buff *skb)
{
   int res;
    local_bh_disable();
    res = __netif_rx(skb);
  local_bh_enable();
  return res;
}