Is it possible to change which core timer interrupts happen on?

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On my Debian 8 system, when I run the command watch -n0.1 --no-title cat /proc/interrupts, I get the output below.

           CPU0       CPU1       CPU2       CPU3       CPU4       CPU5       CPU6       CPU7                                                                                                                                                                                       [0/1808]
  0:         46          0          0      10215          0          0          0          0   IO-APIC-edge      timer
  1:          1          0          0          2          0          0          0          0   IO-APIC-edge      i8042
  8:          0          0          0          1          0          0          0          0   IO-APIC-edge      rtc0
  9:          0          0          0          0          0          0          0          0   IO-APIC-fasteoi   acpi
 12:          0          0          0          4          0          0          0          0   IO-APIC-edge      i8042
 18:          0          0          0          0          8          0          0          0   IO-APIC-fasteoi   i801_smbus
 19:       7337          0          0          0          0          0          0          0   IO-APIC-fasteoi   ata_piix, ata_piix
 21:          0         66          0          0          0          0          0          0   IO-APIC-fasteoi   ehci_hcd:usb1
 23:          0          0         35          0          0          0          0          0   IO-APIC-fasteoi   ehci_hcd:usb2
 40:     208677          0          0          0          0          0          0          0  HPET_MSI-edge      hpet2
 41:          0       4501          0          0          0          0          0          0  HPET_MSI-edge      hpet3
 42:          0          0       2883          0          0          0          0          0  HPET_MSI-edge      hpet4
 43:          0          0          0       1224          0          0          0          0  HPET_MSI-edge      hpet5
 44:          0          0          0          0       1029          0          0          0  HPET_MSI-edge      hpet6
 45:          0          0          0          0          0          0          0          0   PCI-MSI-edge      aerdrv, PCIe PME
 46:          0          0          0          0          0          0          0          0   PCI-MSI-edge      PCIe PME
 47:          0          0          0          0          0          0          0          0   PCI-MSI-edge      PCIe PME
 48:          0          0          0          0          0          0          0          0   PCI-MSI-edge      PCIe PME
 49:          0          0          0          0          0       8570          0          0   PCI-MSI-edge      eth0-rx-0
 50:          0          0          0          0          0          0       1684          0   PCI-MSI-edge      eth0-tx-0
 51:          0          0          0          0          0          0          0          2   PCI-MSI-edge      eth0
NMI:          8          2          2          2          1          2          1         49   Non-maskable interrupts
LOC:         36         31         29         26         21       7611        886       1390   Local timer interrupts
SPU:          0          0          0          0          0          0          0          0   Spurious interrupts
PMI:          8          2          2          2          1          2          1         49   Performance monitoring interrupts
IWI:          0          0          0          1          1          0          1          0   IRQ work interrupts
RTR:          7          0          0          0          0          0          0          0   APIC ICR read retries
RES:        473       1027       1530        739       1532       3567       1529       1811   Rescheduling interrupts
CAL:        846       1012       1122       1047        984       1008       1064       1145   Function call interrupts
TLB:          2          7          5          3         12         15         10          6   TLB shootdowns
TRM:          0          0          0          0          0          0          0          0   Thermal event interrupts
THR:          0          0          0          0          0          0          0          0   Threshold APIC interrupts
MCE:          0          0          0          0          0          0          0          0   Machine check exceptions
MCP:          4          4          4          4          4          4          4          4   Machine check polls
THR:          0          0          0          0          0          0          0          0   Hypervisor callback interrupts
ERR:          0
MIS:          0

Observe that the timer interrupt is firing mostly on CPU3.

Is it possible to move the timer interrupt to CPU0?

interrupt x86-64 linux-kernel linux
2 Answers

Use isolcpus. It may not reduce your timer interrupts to 0, but on our servers they are greatly reduced.

If you use isolcpus, then the kernel will not affine interrupts to your CPUs that it might otherwise do. For example, we have systems with 12 core dual CPUs. We noticed NVME interrupts on our CPU1 (the second CPU), even with the CPUs isolated via tuned and its cpu-partitioning scheme. nvme drives on our Dell systems are connected to the PCIe lanes on CPU1, hence the interrupts on those cores.

As per my ticket with Red Hat (and Margaret Bloom, who wrote an excellent answer here), if you don't want the interrupts to be affined to your CPUs, you need to use isolcpus on the kernel boot line. And lo and behold, I tried it and our interrupts went to 0 for the NVME drives on all isolated CPU cores.

I have not attempted to isolate ALL cores on CPU1; I don't know if they'll simply be affined to CPU0 or what.

And, in a short summary: any interrupt in /proc/interrupts with "MSI" in the name, is managed by the kernel.

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