Friday, July 20, 2012

Reading rate control information from userland..

I've begun working on exporting the ath(4) driver rate control information. The "current" way to extract this information has been to issue a sysctl to the rate control module, which dumps debugging state to the kernel console. This is quite suboptimal. Instead, I'm now packaging that up via a driver ioctl() call. It's temporary (hah, famous last words in the software industry :) and it's driver specific for now. The thirty second summary:

  • There's a new ioctl to ath(4) to query the rate control module for a single associated MAC address (or the BSS MAC when running as a STA);
  • Since the rate control is currently done at the driver level rather than at the VAP level, the call is to the driver rather than via the VAP (wlanX) interface;
  • There's no easy way to get "all" station details whilst maintaining correct locking.

The last point deserves a little more explanation. I've introduced (well, _using_ now) a per-node lock when doing rate control updates. I acquire this lock when copying the rate control data out, so the snapshot is consistent.

So to fetch the state for a node, the following occurs:

  • Call the net80211 layer to find an ieee80211_node for the given mac address - that involves locking the node table and getting a reference for the node (if found); 
  • Then locking the ath_node associated with it;
  • Copy the data out;
  • Unlock the ath_node;
  • Decrement the ieee80211_node reference counter (which requires the node table lock.)

Now, the node table lock only occurs whilst fetching the node reference. It isn't held whilst doing the actual rate control manipulation. Compare to what I'd do if I wanted to walk the node table. The net80211 API for doing this holds the node lock whilst waking the node list. This means that I'll end up holding the node table lock whilst acquiring the ath_node lock. Now, that's fine - however, if I then decide somewhere else to try and do any ieee80211 operation whilst holding the ath_node lock, I may find myself with a lock ordering problem.

So for now the API will just support doing a single lookup for a given MAC, rather than trying to pull all of the rate control table entries down at once.

Here's an example output from the command:

adrian@marilyn:~/work/freebsd/ath/head/src/tools/tools/ath/athratestats]> ./athratestats -i ath1 -m 06:16:16:03:40:d0
static_rix (-1) ratemask 0xf
[ 250] cur rate 5  Mb since switch: packets 1 ticks 43028655
[ 250] last sample (11  Mb) cur sample (0 ) packets sent 10708
[ 250] packets since sample 16 sample tt 6275
[1600] cur rate 11  Mb since switch: packets 15 ticks 43025720
[1600] last sample (5  Mb) cur sample (0 ) packets sent 2423
[1600] packets since sample 7 sample tt 12713
[ 2  Mb: 250]        9:9        (100%) (EWMA 100.0%) T       11 F    0 avg  2803 last 42176930
[ 5  Mb: 250]     3139:3139     (100%) (EWMA 100.0%) T     3273 F    0 avg  1433 last 43028656
[ 5  Mb:1600]       29:29       (100%) (EWMA 100.0%) T       39 F    0 avg  5303 last 42192044
[11  Mb: 250]     7560:7560     (100%) (EWMA 100.0%) T     7838 F    0 avg  1857 last 43026094
[11  Mb:1600]     2394:2394     (100%) (EWMA 100.0%) T     2581 F    0 avg  2919 last 43026411

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