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: <20171102112728.GB2024@nanopsycho>
Date:   Thu, 2 Nov 2017 12:27:28 +0100
From:   Jiri Pirko <jiri@...nulli.us>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
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

Wed, Nov 01, 2017 at 05:11:14PM CET, alexei.starovoitov@...il.com wrote:
>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 ?

Pardon my ignorance, but I don't get your point :( Could you please
elaborate a bit more? Thanks!