netdev - Re: [PATCH v2 net-next] net: sched: run ingress qdisc without locks

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Date:	Mon, 04 May 2015 18:27:36 -0700
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	Jesper Dangaard Brouer <brouer@...hat.com>
CC:	"David S. Miller" <davem@...emloft.net>,
	John Fastabend <john.r.fastabend@...el.com>,
	Jamal Hadi Salim <jhs@...atatu.com>,
	Daniel Borkmann <daniel@...earbox.net>, netdev@...r.kernel.org
Subject: Re: [PATCH v2 net-next] net: sched: run ingress qdisc without locks

On 5/4/15 4:04 AM, Jesper Dangaard Brouer wrote:
>
> How do you activate the ingress code path?
>
> I'm just doing (is this enough?):
>   export DEV=eth4
>   tc qdisc add dev $DEV handle ffff: ingress

yes. plus my numbers also include u32 classifier.

> I re-ran the experiment, and I can also only show a 2.68ns
> improvement.  This is rather strange, and I cannot explain it.
>
> The lock clearly shows up in perf report[1] with 12.23% raw_spin_lock,
> and perf report[2] it clearly gone, but we don't see a 12% improvement
> in performance, but around 4.7%.

It's indeed puzzling. Hopefully intel experts can chime in.

> The "cost" of activating the ingress qdisc is also interesting:
>   (1/25398057*10^9)-(1/16989315*10^9) = -19.49 ns
>   (1/25398057*10^9)-(1/17800496*10^9) = -16.81 ns

yep, we're working hard on reducing it.
btw the cost of enabling rps without using it is ~8ns.
Our line rate goal is still a bit far, but hopefully getting closer :)
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