Subject: Re: [PATCH net-next] net: use jump label patching for ingress qdisc in __netif_receive_skb_core

From: Alexei Starovoitov <[email-masked]>
Date: Fri, 10 Apr 2015 15:40:00 -0700

On 4/10/15 3:30 PM, Eric Dumazet wrote:
> On Fri, 2015-04-10 at 23:07 +0200, Daniel Borkmann wrote:
>> Even if we make use of classifier and actions from the egress
>> path, we're going into handle_ing() executing additional code
>> on a per-packet cost for ingress qdisc, just to realize that
>> nothing is attached on ingress.
>>
>> Instead, this can just be blinded out as a no-op entirely with
>> the use of a static key. On input fast-path, we already make
>> use of static keys in various places, e.g. skb time stamping,
>> in RPS, etc. It makes sense to not waste time when we're assured
>> that no ingress qdisc is attached anywhere.
>>
>> Enabling/disabling of that code path is being done via two
>> helpers, namely net_{inc,dec}_ingress_queue(), that are being
>> invoked under RTNL mutex when a ingress qdisc is being either
>> initialized or destructed.
>>
>> Signed-off-by: Daniel Borkmann <[email-masked]>
>
> My concern about jump labels is they add a conditional on arches where
> CONFIG_JUMP_LABEL=n
>
> They look great on x86, but not sure about say MIPS.

mips has HAVE_ARCH_JUMP_LABEL, so it should be fast there.
I think all major archs are covered:
arch/arm/Kconfig:       select HAVE_ARCH_JUMP_LABEL if !XIP_KERNEL
arch/arm64/Kconfig:     select HAVE_ARCH_JUMP_LABEL
arch/mips/Kconfig:      select HAVE_ARCH_JUMP_LABEL
arch/powerpc/Kconfig:   select HAVE_ARCH_JUMP_LABEL
arch/s390/Kconfig:      select HAVE_ARCH_JUMP_LABEL if !MARCH_G5
arch/sparc/Kconfig:     select HAVE_ARCH_JUMP_LABEL if SPARC64
arch/x86/Kconfig:       select HAVE_ARCH_JUMP_LABEL