Message-ID: <18c406b6-ca57-b8e4-e60c-f4f0186ed392@leemhuis.info>
Date:   Thu, 9 Mar 2023 08:59:24 +0100
From:   "Linux regression tracking (Thorsten Leemhuis)" 
        <regressions@...mhuis.info>
To:     Alexander Wetzel <alexander@...zel-home.de>,
        Linux regressions mailing list <regressions@...ts.linux.dev>,
        Felix Fietkau <nbd@....name>
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 08.03.23 17:50, Alexander Wetzel wrote:
> 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

Uggh, thx for the update, that's unfortunate, but that's how it is
sometimes. I just asked because the culprit didn't have a Reported-by or
together with a Link: to the backstory, so it looked like it might be
fine to revert. But then it's not a option.

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.

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