netdev - Re: [PATCH RFC net-next 0/3] Multi-CPU DSA support

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Message-ID: <20210412221721.3gszur3hbrkhe76m@skbuf>
Date:   Tue, 13 Apr 2021 01:17:21 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Marek Behun <marek.behun@....cz>
Cc:     DENG Qingfang <dqfext@...il.com>,
        Ansuel Smith <ansuelsmth@...il.com>, netdev@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andriin@...com>,
        Eric Dumazet <edumazet@...gle.com>,
        Wei Wang <weiwan@...gle.com>,
        Cong Wang <cong.wang@...edance.com>,
        Taehee Yoo <ap420073@...il.com>,
        Björn Töpel <bjorn@...nel.org>,
        zhang kai <zhangkaiheb@....com>,
        Weilong Chen <chenweilong@...wei.com>,
        Roopa Prabhu <roopa@...ulusnetworks.com>,
        Di Zhu <zhudi21@...wei.com>,
        Francis Laniel <laniel_francis@...vacyrequired.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC net-next 0/3] Multi-CPU DSA support

On Tue, Apr 13, 2021 at 12:04:57AM +0200, Marek Behun wrote:
> On Mon, 12 Apr 2021 19:32:11 +0300
> Vladimir Oltean <olteanv@...il.com> wrote:
>
> > On Mon, Apr 12, 2021 at 11:00:45PM +0800, DENG Qingfang wrote:
> > > On Sun, Apr 11, 2021 at 09:50:17PM +0300, Vladimir Oltean wrote:
> > > >
> > > > So I'd be tempted to say 'tough luck' if all your ports are not up, and
> > > > the ones that are are assigned statically to the same CPU port. It's a
> > > > compromise between flexibility and simplicity, and I would go for
> > > > simplicity here. That's the most you can achieve with static assignment,
> > > > just put the CPU ports in a LAG if you want better dynamic load balancing
> > > > (for details read on below).
> > > >
> > >
> > > Many switches such as mv88e6xxx only support MAC DA/SA load balancing,
> > > which make it not ideal in router application (Router WAN <--> ISP BRAS
> > > traffic will always have the same DA/SA and thus use only one port).
> >
> > Is this supposed to make a difference? Choose a better switch vendor!
>
> :-) Are you saying that we shall abandon trying to make the DSA
> subsystem work with better performace for our routers, in order to
> punish ourselves for our bad decision to use Marvell switches?

No, not at all, I just don't understand what is the point you and
Qingfang are trying to make. LAG is useful in general for load balancing.
With the particular case of point-to-point links with Marvell Linkstreet,
not so much. Okay. With a different workload, maybe it is useful with
Marvell Linkstreet too. Again okay. Same for static assignment,
sometimes it is what is needed and sometimes it just isn't.
It was proposed that you write up a user space program that picks the
CPU port assignment based on your favorite metric and just tells DSA to
reconfigure itself, either using a custom fancy static assignment based
on traffic rate (read MIB counters every minute) or simply based on LAG.
All the data laid out so far would indicate that this would give you the
flexibility you need, however you didn't leave any comment on that,
either acknowledging or explaining why it wouldn't be what you want.