Re: traceability of wifi packet drops

[Jakub Kicinski <kuba@kernel.org>]

On Wed, 29 Mar 2023 20:57:26 +0200 Johannes Berg wrote:
> On Wed, 2023-03-29 at 11:02 -0700, Jakub Kicinski wrote:
> > 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 ...

Ack, for most drop_reasons having higher order bits would make no sense.

> > 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

Yup, the only difference is that the collector side is simpler if the
subsystem is a valid drop reason. For those who don't expect to care
about subsystem drop details the aggregate stats are still (bpftrace
notation):

	@stats[reason & 0xffff] = count();

With the higher bits we have to add a layer of stats to the collection?

	$grp = reason >> 24;
	if ($grp != 0)
		@groups[$grp] = count();
	else
		@stats[reason] = count();

That said, I'm probably over-thinking because most will do:

	@stats[reason] = count();

... which works the same regardless.

> 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.

No preference. You're coding it up so you're in the best position 
to pick :)