Re: IRQ affinity on Linux guest

[Mihai Neagu <mihai.neagu@ni.com>]

Radim,

Thanks for your answer. Indeed setting IRQ affinity to a specific core 
seems to be respected.

However, on software emulation and on the real machine, while IRQ 
affinity defaults to 3, all interrupts go on CPU0, while on KVM they go 
on CPU1.

I wonder why KVM would act differently than both the real machine and 
the software emulation in this particular aspect.

Is there a machine or processor that I can specify at KVM command line 
to make it behave like the real x86_64 processor which defaults 
interrupts to CPU0?

Thanks,
Mihai

On 08/21/2015 02:20 AM, Radim Krčmář wrote:
> 2015-08-20 17:16+0300, Mihai Neagu:
>> Here is how IRQ affinity is configured on guest at startup, in an init.d
>> script:
>>
>> echo 1 > /proc/irq/default_smp_affinity
>> for x in /proc/irq/*/smp_affinity;
>> do
>>    echo 1 > $x
>> done 2> /dev/null
>>
>> The command line for starting the hardware accelerated VM:
>> qemu-system-x86-64 -enable-kvm -kernel bzImage -hda rootfs.ext2 -append    \
>> "root=/dev/sda console=ttyS0 rw" -nographic -cpu qemu64 -snapshot -smp 2   \
>> -m 2048
>>
>> On the hardware accelerated guest, 'cat /proc/interrupts' shows:
>>             CPU0       CPU1
>>    0:         26          0   IO-APIC-edge      timer
>>    1:          7          4   IO-APIC-edge      i8042
>>    4:       1137        523   IO-APIC-edge      serial
>>    8:          0          1   IO-APIC-edge      rtc0
>>    9:          0          0   IO-APIC-fasteoi   acpi
>>   11:       4971          4   IO-APIC-fasteoi   eth0
>>   12:         66         64   IO-APIC-edge      i8042
>>   14:       1958        714   IO-APIC-edge      ata_piix
>>   15:       4512         63   IO-APIC-edge      ata_piix
>> ...
>> Interrupts are serviced on both cores, even though affinity is set to 1.
> KVM's APIC balances interrupts -- until you set the affinity (probably
> near the end of boot process), both CPUs are going to receive roughly
> the same amount but after directing subsequent interrupts to CPU0, CPU1
> shouldn't receive more.
>
> Please verify that CPU0 is not receiving all interrupts by doing a
> difference between two `cat /proc/interrupts` after the affinity was
> set.  (CPU0 has higher numbers in your excerpt, which makes me suspect
> that it works as expected.)
>
> Thanks.
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