Re: [WT PATCH 4/6] mac80211: Add per-sdata station hash, and sdata hash.

[Felix Fietkau <nbd@openwrt.org>]

On 2013-07-26 5:22 PM, Ben Greear wrote:
> On 07/26/2013 02:56 AM, Felix Fietkau wrote:
>> On 2013-07-26 10:53 AM, Johannes Berg wrote:
>>> On Thu, 2013-07-11 at 08:29 -0700, Ben Greear wrote:
>>>
>>>>> I also don't like maintaining two separate hash tables and all that.
>>>>>
>>>>> I'd reconsider if you actually remove the hash entirely, but that'll be
>>>>> tricky to walk the station list and will quite possibly make the RX path
>>>>> there more expensive?
>>>>
>>>> Remove local->sta_hash ?
>>>
>>> To be honest, I'm undecided. Yes, I was thinking that, but I also think
>>> having a huge hashtable like that for each virtual interface is way
>>> overkill, in particular for station interfaces that usually have one
>>> peer (the AP) and maybe a few TDLS peers. Or P2P-Device interfaces that
>>> have no peers at all ...
>>>
>>> I don't see a good way to improve the hash either, since we don't always
>>> (e.g. in RX path) have the interface address.
>> How about mixing in the interface address into the hash. Theoretically
>> you should always have that available, even in the rx path. Multicast
>> data packets contain the BSSID, so you can get the address from there.
>> You just need to be careful about checking the DS bits to figure out
>> which address to use ;)
>> I think this is a much better solution than duplicating the hash, or
>> moving it into sdata entirely.
> 
> I think I could probably get rid of the big global per wiphy hash and
> use the per-wiphy sdata-hash and per-sdata station hashes.
> 
> To me, that is cleanest because it gives a nice ownership relationship
> between wiphy, sdata, and stations.
> 
> For what it's worth, my hashing scheme has been working well on highly
> loaded APs and Station machines.
The global hash (with added vif-addr mixing) not only completely fixes
the many-STA-vif case, also has some other advantages compared to the
per-sdata hash:
- Lookup is easier in setups with multiple AP VLANs
- Better cache footprint (especially important for small embedded devices).
- You don't need a separate sdata lookup before the sta lookup.

I'm not convinced that keeping separate hashes is cleaner. Especially in
the AP_VLAN case, ownership is not clear in any way, since there's some
overlap between multiple sdata entities (belonging to the same BSS).

- Felix