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

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Message-ID: <CAHA+R7PXy_kOn3pUOdGvU4L3n-EgH8cXNCmGQfaDQTUNaP2S4w@mail.gmail.com>
Date:	Fri, 10 Apr 2015 18:14:29 -0700
From:	Cong Wang <cwang@...pensource.com>
To:	Daniel Borkmann <daniel@...earbox.net>
Cc:	David Miller <davem@...emloft.net>,
	Jamal Hadi Salim <jhs@...atatu.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Eric Dumazet <edumazet@...gle.com>,
	netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next] net: use jump label patching for ingress qdisc
 in __netif_receive_skb_core

On Fri, Apr 10, 2015 at 2:07 PM, Daniel Borkmann <daniel@...earbox.net> 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.

So the following code is slow enough to deserve a static key
optimization?

struct netdev_queue *rxq = rcu_dereference(skb->dev->ingress_queue);

if (!rxq || rcu_access_pointer(rxq->qdisc) == &noop_qdisc)
    goto out;


>
> 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.


Since static key can't be embedded into net_device, I doubt
it is useful to have a static key here, since qdisc is per net device.
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