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

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Date:	Sat, 11 Apr 2015 00:34:46 +0200
From:	Daniel Borkmann <daniel@...earbox.net>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	davem@...emloft.net, jhs@...atatu.com, ast@...mgrid.com,
	edumazet@...gle.com, netdev@...r.kernel.org
Subject: Re: [PATCH net-next] net: use jump label patching for ingress qdisc
 in __netif_receive_skb_core

On 04/11/2015 12:30 AM, 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 <daniel@...earbox.net>
>
> 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.
>
> The cost of the extra conditional on MIPS was OK to avoid a call to
> __net_timestamp(), but maybe not to avoid a simple dereference ?

Thanks for the review! I could change that test into a macro and
test if CONFIG_JUMP_LABEL=n to make that if (1) effectively.
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