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

[Jiri Pirko <jiri@resnulli.us>]

Wed, Nov 01, 2017 at 05:11:14PM CET, alexei.starovoitov@gmail.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@gmail.com wrote:
>> >On Tue, Oct 31, 2017 at 04:12:22PM +0100, Jiri Pirko wrote:
>> >> From: Jiri Pirko <jiri@mellanox.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@mellanox.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!