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From: Tobias Waldekranz <tobias@waldekranz.com>
To: Marek Behun <marek.behun@nic.cz>
Cc: "Vladimir Oltean" <olteanv@gmail.com>,
	"Ansuel Smith" <ansuelsmth@gmail.com>,
	netdev@vger.kernel.org, "David S. Miller" <davem@davemloft.net>,
	"Jakub Kicinski" <kuba@kernel.org>,
	"Andrew Lunn" <andrew@lunn.ch>,
	"Vivien Didelot" <vivien.didelot@gmail.com>,
	"Florian Fainelli" <f.fainelli@gmail.com>,
	"Alexei Starovoitov" <ast@kernel.org>,
	"Daniel Borkmann" <daniel@iogearbox.net>,
	"Andrii Nakryiko" <andriin@fb.com>,
	"Eric Dumazet" <edumazet@google.com>,
	"Wei Wang" <weiwan@google.com>,
	"Cong Wang" <cong.wang@bytedance.com>,
	"Taehee Yoo" <ap420073@gmail.com>,
	"Björn Töpel" <bjorn@kernel.org>,
	"zhang kai" <zhangkaiheb@126.com>,
	"Weilong Chen" <chenweilong@huawei.com>,
	"Roopa Prabhu" <roopa@cumulusnetworks.com>,
	"Di Zhu" <zhudi21@huawei.com>,
	"Francis Laniel" <laniel_francis@privacyrequired.com>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH RFC net-next 0/3] Multi-CPU DSA support
Date: Mon, 12 Apr 2021 23:22:45 +0200	[thread overview]
Message-ID: <8735vvkxju.fsf@waldekranz.com> (raw)
In-Reply-To: <20210412213045.4277a598@thinkpad>

On Mon, Apr 12, 2021 at 21:30, Marek Behun <marek.behun@nic.cz> wrote:
> On Mon, 12 Apr 2021 14:46:11 +0200
> Tobias Waldekranz <tobias@waldekranz.com> wrote:
>
>> I agree. Unless you only have a few really wideband flows, a LAG will
>> typically do a great job with balancing. This will happen without the
>> user having to do any configuration at all. It would also perform well
>> in "router-on-a-stick"-setups where the incoming and outgoing port is
>> the same.
>
> TLDR: The problem with LAGs how they are currently implemented is that
> for Turris Omnia, basically in 1/16 of configurations the traffic would
> go via one CPU port anyway.
>
>
>
> One potencial problem that I see with using LAGs for aggregating CPU
> ports on mv88e6xxx is how these switches determine the port for a
> packet: only the src and dst MAC address is used for the hash that
> chooses the port.
>
> The most common scenario for Turris Omnia, for example, where we have 2
> CPU ports and 5 user ports, is that into these 5 user ports the user
> plugs 5 simple devices (no switches, so only one peer MAC address for
> port). So we have only 5 pairs of src + dst MAC addresses. If we simply
> fill the LAG table as it is done now, then there is 2 * 0.5^5 = 1/16
> chance that all packets would go through one CPU port.
>
> In order to have real load balancing in this scenario, we would either
> have to recompute the LAG mask table depending on the MAC addresses, or
> rewrite the LAG mask table somewhat randomly periodically. (This could
> be in theory offloaded onto the Z80 internal CPU for some of the
> switches of the mv88e6xxx family, but not for Omnia.)

I thought that the option to associate each port netdev with a DSA
master would only be used on transmit. Are you saying that there is a
way to configure an mv88e6xxx chip to steer packets to different CPU
ports depending on the incoming port?

The reason that the traffic is directed towards the CPU is that some
kind of entry in the ATU says so, and the destination of that entry will
either be a port vector or a LAG. Of those two, only the LAG will offer
any kind of balancing. What am I missing?

Transmit is easy; you are already in the CPU, so you can use an
arbitrarily fancy hashing algo/ebpf classifier/whatever to load balance
in that case.

  reply	other threads:[~2021-04-12 21:22 UTC|newest]

Thread overview: 68+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-10 13:34 [PATCH RFC net-next 0/3] Multi-CPU DSA support Ansuel Smith
2021-04-10 13:34 ` [PATCH RFC net-next 1/3] net: dsa: allow for multiple CPU ports Ansuel Smith
2021-04-12  3:35   ` DENG Qingfang
2021-04-12  4:41     ` Ansuel Smith
2021-04-12 15:30       ` DENG Qingfang
2021-04-12 16:17         ` Frank Wunderlich
2021-04-10 13:34 ` [PATCH RFC net-next 2/3] net: add ndo for setting the iflink property Ansuel Smith
2021-04-10 13:34 ` [PATCH RFC net-next 3/3] net: dsa: implement ndo_set_netlink for chaning port's CPU port Ansuel Smith
2021-04-10 13:34 ` [PATCH RFC iproute2-next] iplink: allow to change iplink value Ansuel Smith
2021-04-11 17:04   ` Stephen Hemminger
2021-04-11 17:09     ` Vladimir Oltean
2021-04-11 18:01 ` [PATCH RFC net-next 0/3] Multi-CPU DSA support Marek Behun
2021-04-11 18:08   ` Ansuel Smith
2021-04-11 18:39   ` Andrew Lunn
2021-04-12  2:07     ` Florian Fainelli
2021-04-12  4:53     ` Ansuel Smith
2021-04-11 18:50   ` Vladimir Oltean
2021-04-11 23:53     ` Vladimir Oltean
2021-04-12  2:10       ` Florian Fainelli
2021-04-12  5:04     ` Ansuel Smith
2021-04-12 12:46     ` Tobias Waldekranz
2021-04-12 14:35       ` Vladimir Oltean
2021-04-12 21:06         ` Tobias Waldekranz
2021-04-12 19:30       ` Marek Behun
2021-04-12 21:22         ` Tobias Waldekranz [this message]
2021-04-12 21:34           ` Vladimir Oltean
2021-04-12 21:49             ` Tobias Waldekranz
2021-04-12 21:56               ` Marek Behun
2021-04-12 22:06               ` Vladimir Oltean
2021-04-12 22:26                 ` Tobias Waldekranz
2021-04-12 22:48                   ` Vladimir Oltean
2021-04-12 23:04                     ` Marek Behun
2021-04-12 21:50           ` Marek Behun
2021-04-12 22:05             ` Tobias Waldekranz
2021-04-12 22:55               ` Marek Behun
2021-04-12 23:09                 ` Tobias Waldekranz
2021-04-12 23:13                   ` Tobias Waldekranz
2021-04-12 23:54                     ` Marek Behun
2021-04-13  0:27                       ` Marek Behun
2021-04-13  0:31                         ` Marek Behun
2021-04-13 14:46                         ` Tobias Waldekranz
2021-04-13 15:14                           ` Marek Behun
2021-04-13 18:16                             ` Tobias Waldekranz
2021-04-14 15:14                               ` Marek Behun
2021-04-14 18:39                                 ` Tobias Waldekranz
2021-04-14 23:39                                   ` Vladimir Oltean
2021-04-15  9:20                                     ` Tobias Waldekranz
2021-04-13 14:40                       ` Tobias Waldekranz
2021-04-12 15:00     ` DENG Qingfang
2021-04-12 16:32       ` Vladimir Oltean
2021-04-12 22:04         ` Marek Behun
2021-04-12 22:17           ` Vladimir Oltean
2021-04-12 22:47             ` Marek Behun
  -- strict thread matches above, loose matches on Subject: below --
2019-08-24  2:42 Marek Behún
2019-08-24 15:24 ` Andrew Lunn
2019-08-24 17:45   ` Marek Behun
2019-08-24 17:54     ` Andrew Lunn
2019-08-25  4:19   ` Marek Behun
2019-08-24 15:40 ` Vladimir Oltean
2019-08-24 15:44   ` Vladimir Oltean
2019-08-24 17:55     ` Marek Behun
2019-08-24 15:56   ` Andrew Lunn
2019-08-24 17:58     ` Marek Behun
2019-08-24 20:04 ` Florian Fainelli
2019-08-24 21:01   ` Marek Behun
2019-08-25  4:08     ` Marek Behun
2019-08-25  7:13   ` Marek Behun
2019-08-25 15:00     ` Florian Fainelli

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