linux-kernel - Re: [IRQ] IRQ affinity not working properly?

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Date:   Sun, 28 Mar 2021 20:45:34 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Chris Friesen <chris.friesen@...driver.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [IRQ] IRQ affinity not working properly?

On Fri, Jan 29 2021 at 13:17, Chris Friesen wrote:
> I have a CentOS 7 linux system with 48 logical CPUs and a number of

Kernel version?

> Intel NICs running the i40e driver.  It was booted with 
> irqaffinity=0-1,24-25 in the kernel boot args, resulting in 
> /proc/irq/default_smp_affinity showing "0000,03000003".   CPUs 2-11 are 
> set as "isolated" in the kernel boot args.  The irqbalance daemon is not 
> running.
>
> The problem I'm seeing is that /proc/interrupts shows iavf interrupts 
> (associated with physical devices running the i40e driver) on other CPUs 
> than the expected affinity.  For example, here are some iavf interrupts 
> on CPU 4 where I would not expect to see any interrupts given that "cat 
> /proc/irq/<NUM>/smp_affinity_list" reports "0-1,24-25" for all these 
> interrupts.  (Sorry for the line wrapping.)
>
> cat /proc/interrupts | grep -e CPU -e 941: -e 942: -e 943: -e 944: -e 
> 945: -e 961: -e 962: -e 963: -e 964: -e 965:
>
>               CPU0       CPU1       CPU2       CPU3 CPU4       CPU5
> 941:          0          0          0          0 28490          0 
> IR-PCI-MSI-edge iavf-0000:b5:03.6:mbx
> 942:          0          0          0          0 333832         0 
> IR-PCI-MSI-edge      iavf-net1-TxRx-0
> 943:          0          0          0          0 300842         0 
> IR-PCI-MSI-edge      iavf-net1-TxRx-1
> 944:          0          0          0          0 333845         0 
> IR-PCI-MSI-edge      iavf-net1-TxRx-2
> 945:          0          0          0          0 333822         0 
> IR-PCI-MSI-edge      iavf-net1-TxRx-3
> 961:          0          0          0          0 28492         0 
> IR-PCI-MSI-edge iavf-0000:b5:02.7:mbx
> 962:          0          0          0          0 435608         0 
> IR-PCI-MSI-edge      iavf-net1-TxRx-0
> 963:          0          0          0          0 394832         0 
> IR-PCI-MSI-edge      iavf-net1-TxRx-1
> 964:          0          0          0          0 398414         0 
> IR-PCI-MSI-edge      iavf-net1-TxRx-2
> 965:          0          0          0          0 192847         0 
> IR-PCI-MSI-edge      iavf-net1-TxRx-3
>
> There were IRQs coming in on the "iavf-0000:b5:02.7:mbx" interrupt at 
> roughly 1 per second without any traffic, while the interrupt rate on 
> the "iavf-net1-TxRx-<X>" seemed to be related to traffic.
>
> Is this expected?  It seems like the IRQ subsystem is not respecting the 
> configured SMP affinity for the interrupt in question.  I've also seen 
> the same behaviour with igb interrupts.

No it's not expected. Do you see the same behaviour with a recent
mainline kernel, i.e. 5.10 or 5.11?

Thanks,

        tglx