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

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Date:   Sun, 12 Mar 2023 09:58:23 +0100
From:   Felix Fietkau <nbd@....name>
To:     Alexander Wetzel <alexander@...zel-home.de>,
        Linux regressions mailing list <regressions@...ts.linux.dev>
Cc:     "linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Mann <rauchwolke@....net>,
        Stanislaw Gruszka <stf_xl@...pl>,
        Helmut Schaa <helmut.schaa@...glemail.com>,
        Johannes Berg <johannes.berg@...el.com>
Subject: Re: [Regression] rt2800usb - Wifi performance issues and connection
 drops

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