linux-kernel - RE: timer interrupt stucks using tickless kernel

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Message-ID: <SNT111-DS1740657A3615A93A6C0EDFA78F0@phx.gbl>
Date:	Tue, 8 Dec 2009 00:18:41 +0000
From:	"Ioannis Kyriakopoulos" <johnkyr83@...mail.com>
To:	"'john stultz'" <johnstul@...ibm.com>
CC:	<linux-kernel@...r.kernel.org>
Subject: RE: timer interrupt stucks using tickless kernel

>> 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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