netdev - Re: traceability of wifi packet drops

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Date:   Wed, 29 Mar 2023 20:57:26 +0200
From:   Johannes Berg <johannes@...solutions.net>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Eric Dumazet <edumazet@...gle.com>, netdev@...r.kernel.org,
        linux-wireless@...r.kernel.org
Subject: Re: traceability of wifi packet drops

On Wed, 2023-03-29 at 11:02 -0700, Jakub Kicinski wrote:
> 
> No, no what I was trying to say is that instead of using the upper bits
> to identify the space (with 0 being the current enum skb_drop_reason)
> we could use entries in enum skb_drop_reason. In hope that it'd make
> the fine grained subsystem reason seem more like additional information
> than a completely parallel system.

Ah! Looking at your code example ... right, so you'd see "mac80211 drop
unusable" or "mac80211 drop to monitor", and fine-grained in the higher
bits.

> But it's just a thought, all of the approaches seem acceptable.

I _think_ I like the one I prototyped this morning better, I'm not sure
I like the subsystem == existing reason part _that_ much. It ultimately
doesn't matter much, it just feels odd that you'd be allowed to have a,
I don't know picking a random example, SKB_DROP_REASON_DUP_FRAG with a
fine-grained higher bits value?

Not that we'll ever be starved for space ...

> Quick code change perhaps illustrates it best:
> 

Yeah, that ends up really looking very similar :-)

Then again thinking about the implementation, we'd not be able to use a
simple array for the sub-reasons, or at least that'd waste a bunch of
space, since there are already quite a few 'main' reasons and we'd
want/need to add the mac80211 ones (with sub-reason) at the end. So that
makes a big array for the sub-reasons that's very sparsely populated (*)
Extending with a high 'subsystem' like I did this morning is more
compact here.

(*) or put the sub-reasons pointer/num with the 'main' reasons into the
drop_reasons[] array but that would take the same additional space

So ... which one do _you_ like better? I think I somewhat prefer the one
with adding a high bits subsystem, but I can relatively easily rejigger
my changes from this morning to implement the semantics you had here
too.

Anyone else have an opinion? :)

johannes