netdev - Re: [PATCH net] net: sched: act_api: size RTM

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Message-ID: <CAM0EoMk75BJYQUXm7FDW=ZmRsUqib3L+9tEAL90q_+DreroeXQ@mail.gmail.com>
Date: Thu, 5 Feb 2026 14:23:00 -0500
From: Jamal Hadi Salim <jhs@...atatu.com>
To: Paul Moses <p@....org>
Cc: netdev@...r.kernel.org, xiyou.wangcong@...il.com, jiri@...nulli.us, 
	davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com, 
	horms@...nel.org, linux-kernel@...r.kernel.org, stable@...r.kernel.org, 
	Vladimir Oltean <vladimir.oltean@....com>, Vinicius Costa Gomes <vinicius.gomes@...el.com>
Subject: Re: [PATCH net] net: sched: act_api: size RTM_GETACTION reply by fill size

On Thu, Feb 5, 2026 at 10:13 AM Paul Moses <p@....org> wrote:
>
> Looks like pedit might also affected. Hopefully this makes it more clear. Going to wait on more input before doing anything else with this.
>
> NLMSG_GOODSIZE = SKB_WITH_OVERHEAD(min(PAGE_SIZE, 8192))
> SKB_WITH_OVERHEAD(X) = X - SKB_DATA_ALIGN(sizeof(struct skb_shared_info))
> nla_total_size(payload) = NLA_ALIGN(NLA_HDRLEN + payload), with NLA_HDRLEN = 4 and 4 byte alignment
>
> Per entry size for the gate list:
>
> Each entry is a nested TCA_GATE_ONE_ENTRY plus five attributes:
>
> TCA_GATE_ONE_ENTRY (nest, no payload) -> 4
> INDEX (u32) -> 8
> GATE (flag, no payload) -> 4
> INTERVAL (u32) -> 8
> MAX_OCTETS (s32) -> 8
> IPV (s32) -> 8
>
> So one entry is:
>
> entry_sz = 4 + 8 + 4 + 8 + 8 + 8 = 40 bytes
>
> Fixed overhead for one act_gate dump:
>
> 1. Action wrapper (RTM_GETACTION):
>
> NLMSG_HDRLEN + sizeof(struct tcamsg) + nla_total_size(0)
> = 16 + 4 + 4 = 24 bytes
>
> 2. Action shared attributes emitted by tcf_action_dump_1, baseline only
>    (no cookie, no HW stats, no flags):
>
> TCA_ACT_KIND (IFNAMSIZ) = 20
> TCA_ACT_STATS nest = 4
> TCA_STATS_BASIC = 20
> TCA_STATS_PKT64 = 12
> TCA_STATS_QUEUE = 24
> TCA_ACT_OPTIONS nest = 4
> TCA_GACT_TM = 36
> TCA_ACT_IN_HW_COUNT = 8
> action number nest = 4
>
> Total shared baseline = 156 bytes
>
> Optional shared attributes, only if present:
>
> TCA_ACT_HW_STATS = +12
> TCA_ACT_USED_HW_STATS = +12
> TCA_ACT_FLAGS = +12
> TCA_ACT_COOKIE = +nla_total_size(cookie_len)
>
> 3. Gate specific attributes inside options, fixed part including TM:
>
> TCA_GATE_PARMS = 24
> BASE_TIME = 12
> CYCLE_TIME = 12
> CYCLE_TIME_EXT = 12
> CLOCKID = 8
> FLAGS = 8
> PRIORITY = 8
> ENTRY_LIST nest = 4
> TCA_GATE_TM = 36
>
> Total gate baseline = 124 bytes
>
> 4. 64 bit alignment padding, only when
>    !CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
>
> There are 7 attributes that trigger the 64 bit padding:
> -three stats blocks, three time values and the gate TM
> -Each adds 4 bytes, so add 28 bytes in that case
>
> Putting it together:
>
> fixed = 24 (wrapper) + 156 (shared baseline) + 124 (gate baseline)
> fixed = 304 bytes
>
> opt = nla_total_size(cookie_len)
> + 12 for each of HW_STATS, USED_HW_STATS and FLAGS if present
> + 28 if unaligned access padding is required
>
> The maximum number of entries that fit in a single skb is:
>
> Nmax = floor((NLMSG_GOODSIZE - fixed - opt) / 40)
>
> If PAGE_SIZE = 4096 and sizeof(struct skb_shared_info) = 320:
>
> NLMSG_GOODSIZE = 4096 - 320 = 3776
> Nmax = floor((3776 - 304) / 40) = 86
>
> 8192:
>
> NLMSG_GOODSIZE = 8192 - 320 = 7872
> Nmax = floor((7872 - 304) / 40) = 189
>

Seems arbitrary and I was hoping you dont have to change iproute2
which restricts the total size to 1KB.
Earlier, unless i misread, you said you are looking at IEEE - what
does the spec say?
If i am not mistaken, the spec is   IEEE 802.1Qbv which unfortunately
is behind a paywall.
The closest i could find was a vendor talking about it here:
https://onlinedocs.microchip.com/oxy/GUID-82119957-1E11-4B69-84AC-EF0EA08F5595-en-US-5/GUID-7E7509A4-351E-4D82-8266-967681BA2644.html

And they seem to indicate you can only have _one_ off and one timer
per queue, for a max of 8 queues.
Since Po is AWOL, +Cc the taprio folks (Vinicius, Vladmir).

cheers,
jamal