netdev - Re: [patch net-next v2] net: core: introduce mini_Qdisc and eliminate usage of tp->q for clsact fastpath

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Date:   Thu, 26 Oct 2017 08:54:56 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Jiri Pirko <jiri@...nulli.us>
Cc:     netdev@...r.kernel.org, davem@...emloft.net, jhs@...atatu.com,
        xiyou.wangcong@...il.com, mlxsw@...lanox.com, edumazet@...gle.com,
        daniel@...earbox.net, alexander.h.duyck@...el.com,
        willemb@...gle.com, john.fastabend@...il.com
Subject: Re: [patch net-next v2] net: core: introduce mini_Qdisc and
 eliminate usage of tp->q for clsact fastpath

On Thu, Oct 26, 2017 at 11:41:03AM +0200, Jiri Pirko wrote:
> From: Jiri Pirko <jiri@...lanox.com>
> 
> In sch_handle_egress and sch_handle_ingress tp->q is used only in order
> to update stats. So stats and filter list are the only things that are
> needed in clsact qdisc fastpath processing. Introduce new mini_Qdisc
> struct to hold those items. This removes need for tp->q usage without
> added overhead.

that is false claim. The patch adds run-time overhead instead.

> Signed-off-by: Jiri Pirko <jiri@...lanox.com>
> ---
> v1->v2:
> - Use dev instead of skb->dev in sch_handle_egress as pointed out by Daniel
> - Fixed synchronize_rcu_bh() in mini_qdisc_disable and commented
...
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 24ac908..44ea1c3 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -3274,14 +3274,16 @@ EXPORT_SYMBOL(dev_loopback_xmit);
>  static struct sk_buff *
>  sch_handle_egress(struct sk_buff *skb, int *ret, struct net_device *dev)
>  {
> -	struct tcf_proto *cl = rcu_dereference_bh(dev->egress_cl_list);
> +	struct mini_Qdisc *miniq = rcu_dereference_bh(dev->miniq_egress);
>  	struct tcf_result cl_res;
> +	struct tcf_proto *cl;
>  
> -	if (!cl)
> +	if (!miniq)
>  		return skb;
> +	cl = rcu_dereference_bh(miniq->filter_list);

Just like Daniel pointed out in the earlier version of the patch
it adds another rcu dereference to critical path of execution
for every packet...

> @@ -4189,16 +4191,19 @@ sch_handle_ingress(struct sk_buff *skb, struct packet_type **pt_prev, int *ret,
>  		   struct net_device *orig_dev)
>  {
>  #ifdef CONFIG_NET_CLS_ACT
> -	struct tcf_proto *cl = rcu_dereference_bh(skb->dev->ingress_cl_list);
> +	struct mini_Qdisc *miniq = rcu_dereference_bh(skb->dev->miniq_ingress);
>  	struct tcf_result cl_res;
> +	struct tcf_proto *cl;
>  
>  	/* If there's at least one ingress present somewhere (so
>  	 * we get here via enabled static key), remaining devices
>  	 * that are not configured with an ingress qdisc will bail
>  	 * out here.
>  	 */
> -	if (!cl)
> +	if (!miniq)
>  		return skb;
> +	cl = rcu_dereference_bh(miniq->filter_list);

... both ingress and egress paths.
There gotta be very strong reasons to penalize all users for this
and I don't see them in commit log.