netdev - Re: [PATCH net-next] net: dsa: Remove indirect function call for flow dissection

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Message-ID: <CA+h21hqEnFjPHyK9ZanzwXdvkcdTA3uZzJMf0eo0FZWRTFzouw@mail.gmail.com>
Date:   Fri, 3 Jan 2020 23:28:47 +0200
From:   Vladimir Oltean <olteanv@...il.com>
To:     Florian Fainelli <f.fainelli@...il.com>
Cc:     netdev <netdev@...r.kernel.org>,
        Alexander Lobakin <alobakin@...nk.ru>,
        Russell King <rmk+kernel@...linux.org.uk>,
        Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Hauke Mehrtens <hauke@...ke-m.de>,
        Woojung Huh <woojung.huh@...rochip.com>,
        Microchip Linux Driver Support <UNGLinuxDriver@...rochip.com>,
        Sean Wang <sean.wang@...iatek.com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Vladimir Oltean <vladimir.oltean@....com>,
        Claudiu Manoil <claudiu.manoil@....com>,
        Stanislav Fomichev <sdf@...gle.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Song Liu <songliubraving@...com>,
        Petar Penkov <ppenkov@...gle.com>,
        Matteo Croce <mcroce@...hat.com>,
        Jakub Sitnicki <jakub@...udflare.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Paul Blakey <paulb@...lanox.com>,
        Yoshiki Komachi <komachi.yoshiki@...il.com>,
        open list <linux-kernel@...r.kernel.org>,
        "moderated list:ARM/Mediatek SoC support" 
        <linux-arm-kernel@...ts.infradead.org>,
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        <linux-mediatek@...ts.infradead.org>
Subject: Re: [PATCH net-next] net: dsa: Remove indirect function call for flow dissection

On Fri, 3 Jan 2020 at 22:50, Florian Fainelli <f.fainelli@...il.com> wrote:
>
> The call path is the following on TX (e.g.: when you run a DHCP client),

Oh, it gets called on TX too, ok.
In that case, static proto_off information won't work for asymmetric
taggers such as ocelot which may have an independently configurable
prefix length on RX and TX.
I want to get rid of the RX tag prefix in ocelot though, but just saying.

> I don't think your formula works for EDSA which has an EtherType, but

Why doesn't it work with edsa?

> this would probably work for all tags we currently support except trailer.
>
> proto = (__be16 *)(skb->data)[overhead / 2 - 1];
>

I wasn't suggesting to do this exact calculation in flow_dissector.c,
but rather to pre-populate proto_off with a value statically derived
from it on a piece of paper, with the trailer exception where it would
be -2 in bytes or -1 in shorts, but nonetheless a negative and valid
value.

>
> I don't think anyone except Alexander did serious investigation this.
> For now, what I am interested in is reducing the amount of technical
> debt and expensive function calls.

Does the change bring any measurable improvement?

> --
> Florian

Regards,
-Vladimir