Re: IRQ affinity on Linux guest

["Radim Krčmář" <rkrcmar@redhat.com>]

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.