netdev - Re: [RFC PATCH net-next 13/15] net/sched: taprio: automatically calculate queueMaxSDU based on TC gate durations

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Message-ID: <87wn56ual9.fsf@kurt>
Date:   Sat, 28 Jan 2023 13:06:58 +0100
From:   Kurt Kanzenbach <kurt@...utronix.de>
To:     Vladimir Oltean <vladimir.oltean@....com>, netdev@...r.kernel.org
Cc:     Vinicius Costa Gomes <vinicius.gomes@...el.com>
Subject: Re: [RFC PATCH net-next 13/15] net/sched: taprio: automatically
 calculate queueMaxSDU based on TC gate durations

On Sat Jan 28 2023, Vladimir Oltean wrote:
> taprio today has a huge problem with small TC gate durations, because it
> might accept packets in taprio_enqueue() which will never be sent by
> taprio_dequeue().
>
> Since not much infrastructure was available, a kludge was added in
> commit 497cc00224cf ("taprio: Handle short intervals and large
> packets"), which segmented large TCP segments, but the fact of the
> matter is that the issue isn't specific to large TCP segments (and even
> worse, the performance penalty in segmenting those is absolutely huge).
>
> In commit a54fc09e4cba ("net/sched: taprio: allow user input of per-tc
> max SDU"), taprio gained support for queueMaxSDU, which is precisely the
> mechanism through which packets should be dropped at qdisc_enqueue() if
> they cannot be sent.
>
> After that patch, it was necessary for the user to manually limit the
> maximum MTU per TC. This change adds the necessary logic for taprio to
> further limit the values specified (or not specified) by the user to
> some minimum values which never allow oversized packets to be sent.
>
> Signed-off-by: Vladimir Oltean <vladimir.oltean@....com>

Reviewed-by: Kurt Kanzenbach <kurt@...utronix.de>

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