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Date: Mon, 26 Feb 2024 12:54:03 +0200
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To: Linux regressions mailing list <regressions@lists.linux.dev>,
 Thomas Gleixner <tglx@linutronix.de>
Cc: "Christian A. Ehrhardt" <lk@c--e.de>, niklas.neronin@linux.intel.com,
 Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
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 linux-x86_64@vger.kernel.org, netdev@vger.kernel.org,
 Randy Dunlap <rdunlap@infradead.org>,
 Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
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From: Mathias Nyman <mathias.nyman@linux.intel.com>
Subject: Re: This is the fourth time I've tried to find what led to the
 regression of outgoing network speed and each time I find the merge commit
 8c94ccc7cd691472461448f98e2372c75849406c
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On 26.2.2024 11.51, Linux regression tracking (Thorsten Leemhuis) wrote:
> On 26.02.24 10:24, Mathias Nyman wrote:
>> On 26.2.2024 7.45, Linux regression tracking (Thorsten Leemhuis) wrote:
>>> On 21.02.24 14:44, Mathias Nyman wrote:
>>>> On 21.2.2024 1.43, Randy Dunlap wrote:
>>>>> On 2/20/24 15:41, Randy Dunlap wrote:
>>>>>> {+ tglx]
>>>>>> On 2/20/24 15:19, Mikhail Gavrilov wrote:
>>>>>>> On Mon, Feb 19, 2024 at 2:41 PM Mikhail Gavrilov
>>>>>>> <mikhail.v.gavrilov@gmail.com> wrote:
>>>>>>> I spotted network performance regression and it turned out, this was
>>>>>>> due to the network card getting other interrupt. It is a side effect
>>>>>>> of commit 57e153dfd0e7a080373fe5853c5609443d97fa5a.
>>>>>> That's a merge commit (AFAIK, maybe not so much). The commit in
>>>>>> mainline is:
>>>>>>
>>>>>> commit f977f4c9301c
>>>>>> Author: Niklas Neronin <niklas.neronin@linux.intel.com>
>>>>>> Date:   Fri Dec 1 17:06:40 2023 +0200
>>>>>>
>>>>>>        xhci: add handler for only one interrupt line
>>>>>>
>>>>>>> Installing irqbalance daemon did not help. Maybe someone experienced
>>>>>>> such a problem?
>>>>>>
>>>>>> Thomas, would you look at this, please?
>>>>>>
>>>>>> A network device and xhci (USB) driver are now sharing interrupts.
>>>>>> This causes a large performance decrease for the networking device.
>>>>
>>>> Short recap:
>>>
>>> Thx for that. As the 6.8 release is merely two or three weeks away while
>>> a fix is nowhere near in sight yet (afaics!) I start to wonder if we
>>> should consider a revert here and try reapplying the culprit in a later
>>> cycle when this problem is fixed.
> 
> Thx for the reply.
> 
>> I don't think reverting this series is a solution.
>>
>> This isn't really about those usb xhci patches.
>> This is about which interrupt gets assigned to which CPU.
> 
> I know, but from my understanding of Linus expectations wrt to handling
> regressions it does not matter much if a bug existed earlier or
> somewhere else: what counts is the commit that exposed the problem.
> 
> But I might be wrong here. Anyway, not CCing Linus for this; but I'll
> likely point him to this direction on Sunday in my next weekly report,
> unless some fix comes into sight.
> 
>> Mikhail got unlucky when the network adapter interrupts on that system was
>> assigned to CPU0, clearly a more "clogged" CPU, thus causing a drop in max
>> bandwidth.
> 
> But maybe others will be just as "unlucky". Or is there anything to
> believe otherwise? Maybe some aspect of the .config or local setup that
> is most likely unique to Mikhail's setup?

I believe this is a zero-sum case.

Others got equally lucky due to this change.
Their devices end up interrupting less clogged CPUs and see a similar
performance increase.
  
Thanks
Mathias