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

[Alexander Wetzel]

On 08.03.23 12:57, Felix Fietkau wrote:
> 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.

Good hint, thanks.
According to the latest debug log exactly that seems to be happening:

ieee80211_wake_queue() is called by the driver and wake_txqs_tasklet 
tasklet is started. But during execution of the drv_wake_tx_queue() from 
the tasklet userspace queues a new skb and also calls into 
drv_wake_tx_queue(), which is then run overlapping...

Not sure yet how that could cause the problem. But this breaks the 
assumption that drv_wake_tx_queue() are not overlapping. And TX fails 
directly after such an overlapping TX...

I'll probably just serialize the calls and then we verify if that helps...

Alexander