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
