linux-kernel - 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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Date: Wed, 21 Feb 2024 15:44:47 +0200
From: Mathias Nyman <mathias.nyman@...ux.intel.com>
To: Randy Dunlap <rdunlap@...radead.org>,
 Mikhail Gavrilov <mikhail.v.gavrilov@...il.com>,
 Thomas Gleixner <tglx@...utronix.de>
Cc: "Christian A. Ehrhardt" <lk@...e.de>, niklas.neronin@...ux.intel.com,
 Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
 Greg KH <gregkh@...uxfoundation.org>, linux-usb@...r.kernel.org,
 linux-x86_64@...r.kernel.org, netdev@...r.kernel.org
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

On 21.2.2024 1.43, Randy Dunlap wrote:
> 
> 
> On 2/20/24 15:41, Randy Dunlap wrote:
>> {+ tglx]
> 
> (this time for real)
> 
>>
>> On 2/20/24 15:19, Mikhail Gavrilov wrote:
>>> On Mon, Feb 19, 2024 at 2:41 PM Mikhail Gavrilov
>>> <mikhail.v.gavrilov@...il.com> wrote:
>>>>
>>>> I installed irqbalance daemon and nothing changed.
>>>> So who is responsible for irq balancing?
>>>
>>> Sorry for the noise. Can anyone give me an answer?
>>> Who is responsible for distributing interrupts in Linux?
>>> 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@...ux.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:

xhci (USB) and network device didn't share interrupts, or even interrupt the
same CPU in either good or bad case.

A change in how many interrupts xhci driver requests changed which CPU
the network device interrupts.

In the bad case Mikhail Gavrilovs network device was interrupting CPU0
together with:
- IR-IO-APIC    2-edge      timer
- IR-PCI-MSIX-0000:07:00.0    1-edge      nvme1q1

In the good case network device was interrupting CPU27 together with:
- IR-PCI-MSIX-0000:04:00.0   27-edge      nvme0q27
- IR-PCI-MSIX-0000:07:00.0   28-edge      nvme1q28

Manually moving network device irq 87 from CPU0 to CPU23 helped.
(echo 800000 > /proc/irq/87/smp_affinity)

Thanks
-Mathias