netdev - Re: [PATCH net-next v5 1/2] net sched actions: dump more than TCA_ACT_MAX

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Date:   Mon, 24 Apr 2017 08:49:00 -0400
From:   Jamal Hadi Salim <jhs@...atatu.com>
To:     Simon Horman <simon.horman@...ronome.com>,
        Jiri Pirko <jiri@...nulli.us>
Cc:     davem@...emloft.net, xiyou.wangcong@...il.com,
        eric.dumazet@...il.com, netdev@...r.kernel.org,
        Tom Herbert <tom@...bertland.com>,
        Pablo Neira Ayuso <pablo@...filter.org>
Subject: Re: [PATCH net-next v5 1/2] net sched actions: dump more than
 TCA_ACT_MAX_PRIO actions per batch

On 17-04-24 05:14 AM, Simon Horman wrote:
[..]

> Jamal, I am confused about why are you so concerned about the space
> consumed by this attribute, it's per-message, right? Is it the bigger
> picture you are worried about - a similar per-entry flag at some point in
> the future?

To me the two worries are one and the same.

Jiri strongly believes (from a big picture view) we must use
TLVs for extensibility.
While I agree with him in general i have strong reservations
in this case because i can get both extensibility and
build for performance with using a flag bitmask as the
content of the TLV.

A TLV consumes 64 bits minimum. It doesnt matter if we decide
to use a u8 or a u16, we are still sending 64 bits on that
TLV with the rest being PADding. Not to be melodramatic, but
the worst case scenario of putting everything in a TLV for 32
flags is using about 30x more space than using a bitmask.

Yes, space is important and if i can express upto 32 flags
with one TLV rather than 32 TLVs i choose one TLV.
I am always looking for ways to filter out crap i dont need
when i do stats collection. I have numerous wounds from fdb
entries which decided to use a TLV per flag.

The design approach we have used in netlink is: flags start
as a bitmap (whether they are on main headers or TLVs); they may be
complemented with a bitmask/selector (refer to IFLINK messages).

Lets look at this specific patch I have sending. I have already
changed it 3 times and involved a churn of 3 different flags.
If you asked me in the beggining i wouldve scratched my head
thinking for a near term use for bit #3, #4 etc,

I am fine with the counter-Postel view of having the kernel
validate that appropriate bits are set as long as we dont make
user space to now start learning how to play acrobatics.

cheers,
jamal