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

[Felix Fietkau <nbd@nbd.name>]

On 08.03.23 12:41, Alexander Wetzel wrote:
> On 08.03.23 08:52, Felix Fietkau wrote:
> 
>>> 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.
>> 
> 
> I think it's already doing all of that:
> ieee80211_handle_wake_tx_queue() is the mac80211 implementation for the
> wake_tx_queue op. The drivers without native iTXQ support simply link it
> to this handler.
I know. The problem I see is that I can't find anything that guarantees 
that .wake_tx_queue_op is not being called concurrently from multiple 
different places. ieee80211_handle_wake_tx_queue is doing the scheduling 
directly, instead of deferring it to a single workqueue/tasklet/thread, 
and multiple concurrent calls to it could potentially cause issues.

- Felix