Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
From:   DENG Qingfang <dqfext@gmail.com>
Date:   Tue, 2 Mar 2021 13:51:42 +0800
Message-ID: <CALW65jatBuoE=NDRqccfiMVugPh5eeYSf-9a9qWYhvvszD2Jiw@mail.gmail.com>
Subject: dsa_master_find_slave()'s time complexity and potential performance hit
To:     netdev <netdev@vger.kernel.org>, Andrew Lunn <andrew@lunn.ch>,
        Jakub Kicinski <kuba@kernel.org>,
        David Miller <davem@davemloft.net>,
        Florian Fainelli <f.fainelli@gmail.com>,
        Vivien Didelot <vivien.didelot@gmail.com>,
        Vladimir Oltean <olteanv@gmail.com>,
        linux-kernel@vger.kernel.org,
        Russell King - ARM Linux admin <linux@armlinux.org.uk>
Cc:     Birger Koblitz <git@birger-koblitz.de>,
        =?UTF-8?Q?Bj=C3=B8rn_Mork?= <bjorn@mork.no>,
        Stijn Segers <foss@volatilesystems.org>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

Since commit 7b9a2f4bac68 ("net: dsa: use ports list to find slave"),
dsa_master_find_slave() has been iterating over a linked list instead
of accessing arrays, making its time complexity O(n).
The said function is called frequently in DSA RX path, so it may cause
a performance hit, especially for switches that have many ports (20+)
such as RTL8380/8390/9300 (There is a downstream DSA driver for it,
see https://github.com/openwrt/openwrt/tree/openwrt-21.02/target/linux/realtek/files-5.4/drivers/net/dsa/rtl83xx).
I don't have one of those switches, so I can't test if the performance
impact is huge or not.