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

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Message-ID: <1428705014.25985.331.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Fri, 10 Apr 2015 15:30:14 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Daniel Borkmann <daniel@...earbox.net>
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 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 ?



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