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

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Date:   Wed, 8 Jan 2020 10:03:36 -0800
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Vladimir Oltean <olteanv@...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>,
        "moderated list:ARM/Mediatek SoC support" 
        <linux-mediatek@...ts.infradead.org>
Subject: Re: [PATCH net-next] net: dsa: Remove indirect function call for flow
 dissection

On 1/3/20 1:28 PM, Vladimir Oltean wrote:
> 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?

It would, my bad.

> 
>> 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.

With the trailer, the EtherType is actually at the expected location,
that is 12 bytes from the beginning of the Ethernet frame, so we can
simplify things even more.

> 
>>
>> 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?

I did not implement flow dissection for Broadcom tags largely because it
did not show up as a performance problem with the different customers
but I will try to collect some numbers. At any rate, the patch is not
meant to be a performance improvement (though it might provide some
improvements) but ease maintenance and make it more straight forward for
future protocols to automatically gain dissection without having to
provide a function pointer.
-- 
Florian