netdev - Re: [patch net-next v3 2/2] net: core: introduce mini_Qdisc and eliminate usage of tp->q for clsact fastpath

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Message-ID: <20171101161112.hpzl2a5oi5tdvauo@ast-mbp>
Date:   Wed, 1 Nov 2017 09:11:14 -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 v3 2/2] net: core: introduce mini_Qdisc and
 eliminate usage of tp->q for clsact fastpath

On Wed, Nov 01, 2017 at 09:18:03AM +0100, Jiri Pirko wrote:
> Wed, Nov 01, 2017 at 03:12:50AM CET, alexei.starovoitov@...il.com wrote:
> >On Tue, Oct 31, 2017 at 04:12:22PM +0100, 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. Also, introduce a helper to swap the
> >> mini_Qdisc structures in case filter list head changes.
> >> 
> >> This removes need for tp->q usage without added overhead.
> >> 
> >> Signed-off-by: Jiri Pirko <jiri@...lanox.com>
> >> ---
> >> v2->v3:
> >> - Using head change callback to replace miniq pointer every time tp head
> >>   changes. This eliminates one rcu dereference and makes the claim "without
> >>   added overhead" valid.
> >
> >you kidding, right?
> >It's still two loads.
> 
> I'm not.
> I replace:
> 
> one rcu_dereference_bh(dev->egress_cl_list)
> 	by one rcu_dereference_bh(dev->miniq_egress)
> 
> one dereference cl->q
> 	by one dereference miniq->filter_list
> 
> What do I miss?

that in assembler code they are the same when debug is off ?