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

[Alexander Wetzel]

On 08.03.23 13:21, Linux regression tracking (Thorsten Leemhuis) 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.
>
> Alexander, Felix, many thx for looking into this.
>
> This more and more sounds like something that might take a while to get
> fixed, which makes it harder to get this fixed within those time-frames
> Documentation/process/handling-regressions.rst outlines. So please allow
> me to ask:
>
> Is reverting the culprit (and reapplying it later once the real cause is
> found and fixed) an option, or would that cause other regressions?

This patch turned out to fix a (much worse) pre-release regression. See e.g.
https://lore.kernel.org/linux-wireless/7cff27f8-d363-bbfb-241e-8d6fc0009c40@xxxxxxxxxxxxx/T/#t

To fix both regressions will force us to revert more commits other 
patches depends on...

> Ciao, Thorsten (wearing his 'the Linux kernel's regression tracker' hat)
> --
> Everything you wanna know about Linux kernel regression tracking:
> https://linux-regtracking.leemhuis.info/about/#tldr
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