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

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Date:   Thu, 03 Feb 2022 13:19:18 +0100
From:   Toke Høiland-Jørgensen <toke@...e.dk>
To:     Eric Dumazet <edumazet@...gle.com>,
        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.

Eric Dumazet <edumazet@...gle.com> writes:

> 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;
> }

+1, this seems like a reasonable solution!

-Toke