netdev - Re: [RFC PATCH net-next 2/4] net: dsa: remove the "dsa_to_port in a loop" antipattern from the core

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Message-ID: <20210810113532.tvu5dk5g7lbnrdjn@skbuf>
Date:   Tue, 10 Aug 2021 14:35:32 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Florian Fainelli <f.fainelli@...il.com>
Cc:     DENG Qingfang <dqfext@...il.com>,
        Vladimir Oltean <vladimir.oltean@....com>,
        netdev@...r.kernel.org, Jakub Kicinski <kuba@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Kurt Kanzenbach <kurt@...utronix.de>,
        Woojung Huh <woojung.huh@...rochip.com>,
        UNGLinuxDriver@...rochip.com, Sean Wang <sean.wang@...iatek.com>,
        Landen Chao <Landen.Chao@...iatek.com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Claudiu Manoil <claudiu.manoil@....com>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        George McCollister <george.mccollister@...il.com>
Subject: Re: [RFC PATCH net-next 2/4] net: dsa: remove the "dsa_to_port in a
 loop" antipattern from the core

On Tue, Aug 10, 2021 at 02:41:07AM -0700, Florian Fainelli wrote:
> On 8/9/2021 8:33 PM, DENG Qingfang wrote:
> > On Mon, Aug 09, 2021 at 10:03:18PM +0300, Vladimir Oltean wrote:
> > > Ever since Vivien's conversion of the ds->ports array into a dst->ports
> > > list, and the introduction of dsa_to_port, iterations through the ports
> > > of a switch became quadratic whenever dsa_to_port was needed.
> > 
> > So, what is the benefit of a linked list here? Do we allow users to
> > insert/delete a dsa_port at runtime? If not, how about using a
> > dynamically allocated array instead?
> 
> The goal was to flatten the space while doing cross switch operations, which
> would have otherwise required iterating over dsa_switch instances within a
> dsa_switch_tree, and then over dsa_port within each dsa_switch.

To expand on that: technically dsa_port_touch() _does_ happen at
runtime, since multiple switches in a cross-chip tree probe
asynchronously. To use a dynamically allocated array would mean to
preallocate the sum of all DSA switch ports' worth of memory, and to
preallocate an index for each DSA switch within that single array.
Overall a list is simpler.