netdev - Re: [PATCH net-next v1 0/9] ethtool: Add support for frame preemption

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Message-ID: <20201207231230.3avhe6yqklsbxsiz@skbuf>
Date:   Mon, 7 Dec 2020 23:12:31 +0000
From:   Vladimir Oltean <vladimir.oltean@....com>
To:     Vinicius Costa Gomes <vinicius.gomes@...el.com>
CC:     Jakub Kicinski <kuba@...nel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "jhs@...atatu.com" <jhs@...atatu.com>,
        "xiyou.wangcong@...il.com" <xiyou.wangcong@...il.com>,
        "jiri@...nulli.us" <jiri@...nulli.us>,
        "m-karicheri2@...com" <m-karicheri2@...com>,
        "Jose.Abreu@...opsys.com" <Jose.Abreu@...opsys.com>,
        Po Liu <po.liu@....com>,
        "intel-wired-lan@...ts.osuosl.org" <intel-wired-lan@...ts.osuosl.org>,
        "anthony.l.nguyen@...el.com" <anthony.l.nguyen@...el.com>
Subject: Re: [PATCH net-next v1 0/9] ethtool: Add support for frame preemption

On Mon, Dec 07, 2020 at 02:49:35PM -0800, Vinicius Costa Gomes wrote:
> Jakub Kicinski <kuba@...nel.org> writes:
>
> > On Tue,  1 Dec 2020 20:53:16 -0800 Vinicius Costa Gomes wrote:
> >> $ tc qdisc replace dev $IFACE parent root handle 100 taprio \
> >>       num_tc 3 \
> >>       map 2 2 1 0 2 2 2 2 2 2 2 2 2 2 2 2 \
> >>       queues 1@0 1@1 2@2 \
> >>       base-time $BASE_TIME \
> >>       sched-entry S 0f 10000000 \
> >>       preempt 1110 \
> >>       flags 0x2
> >>
> >> The "preempt" parameter is the only difference, it configures which
> >> queues are marked as preemptible, in this example, queue 0 is marked
> >> as "not preemptible", so it is express, the rest of the four queues
> >> are preemptible.
> >
> > Does it make more sense for the individual queues to be preemptible
> > or not, or is it better controlled at traffic class level?
> > I was looking at patch 2, and 32 queues isn't that many these days..
> > We either need a larger type there or configure this based on classes.
>
> I can set more future proof sizes for expressing the queues, sure, but
> the issue, I think, is that frame preemption has dimishing returns with
> link speed: at 2.5G the latency improvements are on the order of single
> digit microseconds. At greater speeds the improvements are even less
> noticeable.

You could look at it another way.
You can enable jumbo frames in your network, and your latency-sensitive
traffic would not suffer as long as the jumbo frames are preemptible.

> The only adapters that I see that support frame preemtion have 8 queues
> or less.
>
> The idea of configuring frame preemption based on classes is
> interesting. I will play with it, and see how it looks.

I admit I never understood why you insist on configuring TSN offloads
per hardware queue and not per traffic class.