netdev - Re: [PATCH net-next v2] net_sched: restore "mpu xxx" handling

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Date:   Wed, 12 Jan 2022 09:08:40 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     Kevin Bracey <kevin@...cey.fi>
Cc:     netdev <netdev@...r.kernel.org>, Jakub Kicinski <kuba@...nel.org>,
        Jiri Pirko <jiri@...nulli.us>, Vimalkumar <j.vimal@...il.com>
Subject: Re: [PATCH net-next v2] net_sched: restore "mpu xxx" handling

On Wed, Jan 12, 2022 at 9:02 AM Kevin Bracey <kevin@...cey.fi> wrote:
>
> commit 56b765b79e9a ("htb: improved accuracy at high rates") broke
> "overhead X", "linklayer atm" and "mpu X" attributes.
>
> "overhead X" and "linklayer atm" have already been fixed. This restores
> the "mpu X" handling, as might be used by DOCSIS or Ethernet shaping:
>
>     tc class add ... htb rate X overhead 4 mpu 64
>
> The code being fixed is used by htb, tbf and act_police. Cake has its
> own mpu handling. qdisc_calculate_pkt_len still uses the size table
> containing values adjusted for mpu by user space.
>
> iproute2 tc has always passed mpu into the kernel via a tc_ratespec
> structure, but the kernel never directly acted on it, merely stored it
> so that it could be read back by `tc class show`.
>
> Rather, tc would generate length-to-time tables that included the mpu
> (and linklayer) in their construction, and the kernel used those tables.
>
> Since v3.7, the tables were no longer used. Along with "mpu", this also
> broke "overhead" and "linklayer" which were fixed in 01cb71d2d47b
> ("net_sched: restore "overhead xxx" handling", v3.10) and 8a8e3d84b171
> ("net_sched: restore "linklayer atm" handling", v3.11).
>
> "overhead" was fixed by simply restoring use of tc_ratespec::overhead -
> this had originally been used by the kernel but was initially omitted
> from the new non-table-based calculations.
>
> "linklayer" had been handled in the table like "mpu", but the mode was
> not originally passed in tc_ratespec. The new implementation was made to
> handle it by getting new versions of tc to pass the mode in an extended
> tc_ratespec, and for older versions of tc the table contents were analysed
> at load time to deduce linklayer.
>
> As "mpu" has always been given to the kernel in tc_ratespec,
> accompanying the mpu-based table, we can restore system functionality
> with no userspace change by making the kernel act on the tc_ratespec
> value.
>
> Fixes: 56b765b79e9a ("htb: improved accuracy at high rates")
> Signed-off-by: Kevin Bracey <kevin@...cey.fi>
> Cc: Eric Dumazet <edumazet@...gle.com>
> Cc: Jiri Pirko <jiri@...nulli.us>
> Cc: Vimalkumar <j.vimal@...il.com>

Thanks for the nice changelog.

I do have a question related to HTB offload.

Is this mpu attribute considered there ?

Thanks.