Message-ID: <ba784fc6-ad8f-e5da-ae4e-0db7ad09f7dc@wetzel-home.de>
Date:   Wed, 8 Mar 2023 17:50:05 +0100
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Subject: Re: [Regression] rt2800usb - Wifi performance issues and connection
 drops
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To:     Linux regressions mailing list <regressions@lists.linux.dev>,
        Felix Fietkau <nbd@nbd.name>
Cc:     "linux-wireless@vger.kernel.org" <linux-wireless@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Thomas Mann <rauchwolke@gmx.net>,
        Stanislaw Gruszka <stf_xl@wp.pl>,
        Helmut Schaa <helmut.schaa@googlemail.com>,
        Johannes Berg <johannes.berg@intel.com>
References: <b8efebc6-4399-d0b8-b2a0-66843314616b@leemhuis.info>
 <5a7cd098-1d83-6297-e802-ce998c8ec116@leemhuis.info>
 <6025e17e-4c29-6d36-6b9c-2fec543b21c4@wetzel-home.de>
 <debc7fe9-204d-63a7-aa61-91b20a46f385@wetzel-home.de>
 <4a02173f-3a60-0a7e-8962-3778e6c55bf3@nbd.name>
 <cfa5cc30-bf5a-bffd-4c2f-eec8a6522dd5@wetzel-home.de>
 <42185fa2-4191-fcf5-9c0f-fd7098bb856b@nbd.name>
 <2246a9d5-789d-08c9-f6a7-fb9db2edfe9f@leemhuis.info>
From:   Alexander Wetzel <alexander@wetzel-home.de>
In-Reply-To: <2246a9d5-789d-08c9-f6a7-fb9db2edfe9f@leemhuis.info>
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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@leemhuis.info/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
> If I did something stupid, please tell me, as explained on that page.