netdev - Re: traceability of wifi packet drops

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Date:   Tue, 28 Mar 2023 10:18:51 +0200
From:   Johannes Berg <johannes@...solutions.net>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        linux-wireless@...r.kernel.org
Subject: Re: traceability of wifi packet drops

On Tue, 2023-03-28 at 01:16 -0700, Eric Dumazet wrote:
> 
> About visibility: Even before 'drop reasons' developers can always use
> the call graph .
> 
> perf record -a -g -e skb:kfree_skb ...

Yeah, except right now it doesn't work in mac80211 because we have these
"RX handlers" that just return a status, and then a single place drops
the skb.

> Really drop reasons are nice when you want filtering and convenience.
> But they are a lot of work to add/maintain.
> This all comes at a cost (both maintenance but also run time cost
> because we need to propagate reasons )

Right ... I don't think the runtime cost would be much of an issue here
(we just get them as a return value from one function to pass to
kfree_skb), but the maintenance is hard.

I guess we could also restructure the code to call kfree_skb() in all
those places that want drop it and then we have the call trace, but I
doubt that would actually be cheaper both in the maintenance sense
(always need to pay attention to it) or in the runtime sense.

johannes