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SubjectRE: timer interrupt stucks using tickless kernel
Date
>> I have configured my kernel (2.6.31.6) so that I get periodic ticks from
>> the timer interrupt (i.e. tickless is not configured) with a rate
determined
>> by the HZ value. I've also checked the "high resolution timer support"
>> and also SMP support. The posblem is that the timer is getting
incremented
>> very slowly (way slower than the HZ value), just like it would be if the
>> kernel was tickless. Is there an explanation for that? How can I get
>> periodic
>> timer ticks?
>>
>> AFAI understand, the HPET timer is used through the IO-APIC controller to
>> trigger the processors (N.B. if the "High Resolution Timer Support"
option
>> wasn't checked, the timer used would  be PIT, right?) and each
processor's
>> LAPIC timer is used for time keeping. Please correct me if I am wrong.
>
>Not quite, if High Resolution Timers support was disabled, you would
>still use the HPET hardware instead of the pit, but you'd stay in
>periodic mode. However, since you have Highres Timers on, the system
>is using oneshot mode (which is necessary to trigger interrupts faster
>then HZ), and likely has chosen the LAPIC timer as the interrupt
>source.
>
>In this case, the timer tick becomes an hrt event, that triggers every
>HZ, instead of being caused by a periodic interrupt.

OK, so I configured the kernel as follows, expecting to have periodic
interrupts by the HPET timer using the PIC controller using only a single
processor (UP configuration).

Unselected:
Tickless System (Dynamic Ticks)
High Resolution Timer Support
Symmetric multi-processing support
IO-APIC support on uniprocessors (NEW)

Selected:
Local APIC support on uniprocessors
HPET timer support

However, when I ran /proc/interrupts, I noticed many weird things:
1. The timer is still not getting incremented at a rate determined by
HZ.
2. When performing cat proc/interrupts, I see that:
0: 157 IO-APIC-edge timer
How the system timer is attached on IO-APIC at the time that the
IO-APIC
is disabled on kernel configurations? Basically, the timer is not the
only
peripheral attached to IO-APIC as can be seen by /proc/interrupts.

Here is the output of /proc/interrupts :

CPU0
0: 157 IO-APIC-edge timer
1: 2 IO-APIC-edge i8042
4: 544 IO-APIC-edge serial
8: 1 IO-APIC-edge rtc0
9: 0 IO-APIC-fasteoi acpi
12: 4 IO-APIC-edge i8042
14: 4 IO-APIC-edge ata_piix
15: 0 IO-APIC-edge ata_piix
16: 0 IO-APIC-fasteoi uhci_hcd:usb5
18: 0 IO-APIC-fasteoi uhci_hcd:usb4
19: 45 IO-APIC-fasteoi ata_piix, uhci_hcd:usb3
22: 149 IO-APIC-fasteoi HDA Intel
23: 72 IO-APIC-fasteoi ehci_hcd:usb1, uhci_hcd:usb2
27: 0 PCI-MSI-edge eth0
NMI: 0 Non-maskable interrupts
LOC: 1865130 Local timer interrupts
SPU: 0 Spurious interrupts
CNT: 0 Performance counter interrupts
PND: 0 Performance pending work
TRM: 0 Thermal event interrupts
THR: 0 Threshold APIC interrupts
MCE: 0 Machine check exceptions
MCP: 7 Machine check polls
ERR: 0
MIS: 0

Is it that difficult to have periodic timer interrupts using HZ rate?

Thanks,

John K.

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