Re: [Regression] rt2800usb - Wifi performance issues and connection drops

[Felix Fietkau <nbd@nbd.name>]

On 07.03.23 21:54, Alexander Wetzel wrote:
>>>
>> 
>> I just uploaded a test patch to bugzilla.
>> Please have a look if that fixes the issue.
>> 
>> If not I would be interested in the output of your iTXQ status.
>> Enable CONFIG_MAC80211_DEBUGFS and run this command when the connection 
>> is bad and send/share/upload to bugzilla the resulting debug.out:
>> 
>> k=1; while [ $k -lt 10 ]; do \
>> cat /sys/kernel/debug/ieee80211/phy?/netdev:*/stations/*/aqm; \
>> k=$(($k+1)); done >> debug.out
> 
> Thomas and I continued with some debugging in
> https://bugzilla.kernel.org/show_bug.cgi?id=217119
> 
> But the results so far are unexpected and we decided to continue the
> debugging with the round here. Hoping someone sees something I miss.
> 
> A very summary where we are:
> I can't reproduce the bug with a very similar card and kernel config so
> far. Thomas card stops the iTXQs for intervalls >30s. Mine operates
> normally.
> 
> A more useful but longer summary:
> 
> Thomas updated to a 6.2 kernel and reported "connection drops and
> bandwidth problems" with his rt2800usb wlan card. (6.1 is ok.) Asked for
> some more details he reported:
> "...slow bandwidth stuff works better, but the main problem/test case is
> to start a 8-16 mbit video stream, which sometimes runs for a few
> seconds and then stops or it doesn't start at all"
> 
> He bisected the issue and identified my commit 4444bc2116ae ("wifi:
> mac80211: Proper mark iTXQs for resumption") as culprit.
> 
> Checking the internal iTXQ status when the issue is ongoing shows, that
> TID zero is flagged as dirty and thus is not transmitting queued
> packets. Interesting line from
> /sys/kernel/debug/ieee80211/phy?/netdev:*/stations/*/aqm:
> tid ac backlog-bytes backlog-packets new-flows drops marks overlimit
> collisions tx-bytes tx-packets flags
> 0 2 619736 404 1681 0 0 0 1 4513965 3019 0xe(RUN AMPDU NO-AMSDU DIRTY)
> 
> --> The "normal" iTXQ handling IEEE80211_AC_BE has queued packets and is
> flagged as DIRTY. There even is a potential race setting the DIRTY flag,
> but the fix for that is not helping.
> 
> Thus Thomas applied two debug patches, to better understand why the
> DIRTY flag is not cleared.
> 
> And looking at the output from those we see that the driver stops Tx by
> calling ieee80211_stop_queue(). When ieee80211_wake_queue() mac80211
> correctly resumes TX but is getting stopped by the driver after a single
> packet again. (The start of the relevant log is missing, so that may be
> initially more).
> I assume TX is still ok at that stage. But after some singe Tx
> operations the driver stops the queues again. Here the relevant part of
> the log:
> [  179.584997] XXXX __ieee80211_wake_txqs: waking TID 0
> [  179.585022] XXXX drv_tx: TX
> [  179.585027] XXXX ieee80211_stop_queue: called
> [  179.585028] XXXX ieee80211_tx_dequeue: mark TID 0 dirty. Reason: 1
> [  179.585030] XXXX __ieee80211_wake_txqs: TID 3 NOT dirty
> [  179.585031] XXXX __ieee80211_wake_txqs: TID 8 NOT dirty
> [  179.585033] XXXX __ieee80211_wake_txqs: TID 11 NOT dirty
> [  179.585034] XXXX __ieee80211_wake_txqs: EXIT
> [  179.585035] XXXX __ieee80211_wake_txqs: ENTRY
> [  179.585036] XXXX __ieee80211_wake_txqs: TID 1 NOT dirty
> [  179.585037] XXXX __ieee80211_wake_txqs: TID 2 NOT dirty
> [  179.585038] XXXX __ieee80211_wake_txqs: TID 9 NOT dirty
> [  179.585040] XXXX __ieee80211_wake_txqs: TID 10 NOT dirty
> [  179.585041] XXXX __ieee80211_wake_txqs: EXIT
> [  179.585047] XXXX drv_tx: TX
> [  179.585056] XXXX ieee80211_tx_dequeue: mark TID 0 dirty. Reason: 1
> [  179.585271] XXXX ieee80211_tx_dequeue: mark TID 0 dirty. Reason: 1
> [  179.585868] XXXX ieee80211_tx_dequeue: mark TID 0 dirty. Reason: 1
> [  179.586120] XXXX ieee80211_tx_dequeue: mark TID 0 dirty. Reason: 1
> [  179.586544] XXXX ieee80211_tx_dequeue: mark TID 0 dirty. Reason: 1
> [  179.586792] XXXX ieee80211_tx_dequeue: mark TID 0 dirty. Reason: 1
> [  179.587317] XXXX ieee80211_tx_dequeue: mark TID 0 dirty. Reason: 1
> [  179.587591] XXXX ieee80211_tx_dequeue: mark TID 0 dirty. Reason: 1
> [  179.588569] XXXX ieee80211_tx_dequeue: mark TID 0 dirty. Reason: 1
> ....
> [  214.307617] XXXX ieee80211_wake_queue: called
> 
> 
> --> So the driver blocked TX for more than 30s. Which is a good
> explanation of what Thomas observes.
> 
> But there is nothing mac80211 can do differently here. Whatever is the
> real reason for the issue, it's nothing obvious I see.
> 
> Luckily I found a card using the same driver and nearly the same card:
> Thomas systems:Linux version 6.2.2-gentoo (root@foo) (gcc (Gentoo
> Hardened 12.2.1_p20230121-r1 p10) 12.2.1 20230121, GNU ld (Gentoo 2.39
> p5) 2.39.0) #2 SMP Fri Mar  3 16:59:02 CET 2023ieee80211 phy0:
> rt2x00_set_rt: Info - RT chipset 3070, rev 0201 detected
> ieee80211 phy0: rt2x00_set_rf: Info - RF chipset 0005 detected
> ieee80211 phy0: Selected rate control algorithm 'minstrel_ht'
> 
> My system, using the kernel config from Thomas with only minor
> modifications (different filesystems and initramfs settings and enabled
> mac80211 debug and developer options):
> Linux version 6.2.2-gentoo (root@Perry.mordor) (gcc (Gentoo
> 12.2.1_p20230121-r1 p10) 12.2.1 20230121, GNU ld (Gentoo 2.40 p2)
> 2.40.0) #2 SMP Tue Mar  7 18:18:47 CET 2023ieee80211 phy0:
> rt2x00_set_rt: Info - RT chipset 3070, rev 0200 detected
> ieee80211 phy0: rt2x00_set_rf: Info - RF chipset 0005 detected
> ieee80211 phy0: Selected rate control algorithm 'minstrel_ht'
> ieee80211 phy0: rt2x00lib_request_firmware: Info - Loading firmware file
> 'rt2870.bin'
> ieee80211 phy0: rt2x00lib_request_firmware: Info - Firmware detected -
> version: 0.36
> 
> But there is one big difference on my system: I can't reproduce the bug
> so far. It's working as it should... (I did not apply the debug patches
> myself so far)
> 
> I'm now planning to look a bit more into the rt2800usb driver and
> provide another debug patch for interesting looking code pieces in it.
> 
> @Thomas:
> I've also uploaded you my binary kernel I'm running at the moment here:
> https://www.awhome.eu/s/5FjqMS73rtCtSBM
> 
> That kernel should also be able to boot and operate your system. Can you
> try that and tell me, if that makes any difference?
> 
> I'm also planning to provide some more debug patches, to figuring out
> which part of commit 4444bc2116ae ("wifi: mac80211: Proper mark iTXQs
> for resumption") fixes the issue for you. Assuming my understanding
> above is correct the patch should not really fix/break anything for
> you...With the findings above I would have expected your git bisec to
> identify commit a790cc3a4fad ("wifi: mac80211: add wake_tx_queue
> callback to drivers") as the first broken commit...
I can't point to any specific series of events where it would go wrong, 
but I suspect that the problem might be the fact that you're doing tx 
scheduling from within ieee80211_handle_wake_tx_queue. I don't see how 
it's properly protected from potentially being called on different CPUs 
concurrently.

Back when I was debugging some iTXQ issues in mt76, I also had problems 
when tx scheduling could happen from multiple places. My solution was to 
have a single worker thread that handles tx, which is scheduled from the 
wake_tx_queue op.
Maybe you could do something similar in mac80211 for non-iTXQ drivers.

- Felix