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

[Alexander Wetzel]

On 09.03.23 23:13, Alexander Wetzel wrote:
> 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...

Serialization helps. A (crude and in multiple ways incorrect) patch 
preventing two drv_wake_tx_queue() running for the same ac fixed the 
issue for Thomas:
https://bugzilla.kernel.org/show_bug.cgi?id=217119#c20

So it looks like we'll now have soon a fix for the issue.

The driver wakes the queue for IEEE80211_AC_BE often for only a single 
skb and then stops it again.
The short run time is insufficient for wake_txqs_tasklet to proper wake 
all queues itself and from time to time a new TX operation squeezes in 
after IEEE80211_AC_BE has been unblocked but prior of drv_wake_tx_queue 
being called from the wake_txqs_tasklet. When this happens 
drv_wake_tx_queue is called two times: Once from the tasklet, once from 
the userspace.

ieee80211_handle_wake_tx_queue is using ieee80211_txq_schedule_start, 
which has this documented requirement:
"The driver must not call multiple TXQ scheduling rounds concurrently."
Now I don't think that is causing the reported regression. Nevertheless 
we should prevent concurrent calls of ieee80211_handle_wake_tx_queue for 
that reason alone.

The real reason of the hangs is probably in the rt2800usb driver or 
hardware. I don't see anything in the driver code, so probably the HW 
itself has a problem with the two near-concurrent TX operations.

The real culprit of the regression should be commit a790cc3a4fad ("wifi: 
mac80211: add wake_tx_queue callback to drivers"), which switched 
rt2800usb over to iTXQs. But without the fix from commit 4444bc2116ae 
("wifi: mac80211: Proper mark iTXQs for resumption") mac80211 omitted to 
schedule the required run of the wake_txqs_tasklet. Thus thus instead of 
two concurrent drv_wake_tx_queue we only got one and the driver 
continued to work.

I asked Thomas on bugzilla to test the "best" solution I came up with.

There seems to be multiple ways. But I can't find a simple, low risk and 
complete fix. So I compromised...

When Thomas can confirm the fix we can soon discuss the fix on 
linux-wireless.


Alexander