Messages in this thread | | | Date | Sun, 12 Mar 2023 09:58:23 +0100 | From | Felix Fietkau <> | Subject | Re: [Regression] rt2800usb - Wifi performance issues and connection drops |
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On 11.03.23 22:26, Alexander Wetzel wrote: > 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.
I would recommend the following approach for properly fixing this issue:
On init if the .wake_tx_queue op is set to ieee80211_handle_wake_tx_queue, create a single kthread that iterates over all hw queues and schedules each one of them like ieee80211_handle_wake_tx_queue does now. Change ieee80211_handle_wake_tx_queue to simply schedule the kthread without doing anything else. This is how mt76 handles tx scheduling in the driver, and it works quite well.
- Felix
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