On Mon, 27 Mar 2023 16:19:34 +0200 Johannes Berg wrote: > So I just ran into this problem with a colleague again; we don't have > good visibility into why in the wifi stack a packet is dropped. > > In the network stack we have skb drop reasons for (part of?) this, but > we don't really use this in wifi/mac80211 yet. > > Unfortunately we have probably >100 distinct drop reasons in the wifi > stack, so annotating those is not only tedious, it would also double the > list of SKB drop reasons from currently ~75. > > Any good ideas? I even thought about just encoding the line number > wherever we use RX_DROP_UNUSABLE / RX_DROP_MONITOR, but that's kind of > awkward too. Obviously we could change the internal API to completely > get rid of enum ieee80211_rx_result and use enum skb_drop_reason > instead, but then we'd probably need to carve out some space to also > differentiate DROP_MONITOR and DROP_UNUSABLE, perhaps something like > > > SKB_DROP_REASON_MAC80211_MASK 0x03ff0000 > SKB_DROP_REASON_MAC80211_TYPE_MASK 0x03000000 > SKB_DROP_REASON_MAC80211_TYPE_UNUSABLE 0x01000000 > SKB_DROP_REASON_MAC80211_TYPE_MONITOR 0x02000000 > > SKB_DROP_REASON_MAC80211_DUP (SKB_DROP_REASON_MAC80211_TYPE_UNUSABLE | 1) > SKB_DROP_REASON_MAC80211_BAD_BIP_KEYIDX (SKB_DROP_REASON_MAC80211_TYPE_MONITOR | 1) > > > etc. > > > That'd be a LOT of annotations (and thus work) though, and a lot of new > IDs/names, for something that's not really used all that much, i.e. a > file number / line number within mac80211 would be completely > sufficient, so the alternative could be to just have a separate > tracepoint inside mac80211 with a line number or so? > > Anyone have any great ideas? We need something that'd scale to more subsystems, so I don't think having all the definitions in enum skb_drop_reason directly is an option. My knee jerk idea would be to either use the top 8 bits of the skb reason enum to denote the space. And then we'd say 0 is core 1 is wifi (enum ieee80211_rx_result) etc. Within the WiFi space you can use whatever encoding you like. On a quick look nothing is indexed by the reason directly, so no problems with using the high bits. Option #2 is to add one main drop reason called SKB_DROP_REASON_MAC80211 and have a separate tracepoint which exposes the detailed wifi reason and any necessary context. mac80211 would then have its own wrapper around kfree_skb_reason() which triggers the tracepoint. Those are perhaps fairly obvious and unimaginative. Adding Eric, since he has been filling in a lot of the drop reasons lately.
