Many mfgpt timer interrupts with tickless kernel

[William Cohen]

Hi all,

I was trying to make sense of the data that I collected earlier with the 
idle1.stp probe. It displayed the percentage of time spent in halted mode, which 
the highest I saw on an idle machine was 17 percentage. This seemed rather low 
for a machine that was doing nothing, so I took a closer look at what was going 
on the idle machine. All this work is based on kernel built from dilinger's:

kernel-2.6.21-20070312.1.olpc.3eca75102a57502.src.rpm

I ran an experiment where the machine state idle for 10 seconds with the 
systemtap script running. I also compared the number of interrupts before and 
after the experiment. There is about 150 interrupts per second from the mfgpt 
timer. I thought with the tickless kernel that this would be a lot lower.
Below is the data from the experiment. I tried booting the machine in runlevel 
three and got the same results, about 150 interrupts per second. Why so many 
timer interrupts for tickless kernel?

-Will

  cat /proc/interrupts > /tmp/tone; staprun idle1.ko -c 'sleep 10'; cat 
/proc/interrupts > /tmp/tten; diff -u /tmp/tone /tmp/tten
Starting halt watch
Ran for 10117637 us
entered default_halt 1619
exited default_halt 1619
percent halted: 17
count: 1619
avg_time: 1083
min_time: 3
max_time: 2553
usec                distribution
value |-------------------------------------------------- count
     0 |                                                      0
     1 |                                                      0
     2 |                                                      1
     4 |                                                     18
     8 |                                                     19
    16 |@                                                    26
    32 |@                                                    37
    64 |@                                                    48
   128 |@                                                    37
   256 |@                                                    51
   512 |@@@                                                  97
  1024 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@  1284
  2048 |                                                      1
  4096 |                                                      0
  8192 |                                                      0

--- /tmp/tone   2007-03-22 13:37:38.931938793 -0400
+++ /tmp/tten   2007-03-22 13:37:49.932104719 -0400
@@ -6,9 +6,9 @@
    4:         12    XT-PIC-XT        serial
    5:          0    XT-PIC-XT        CS5535 Audio
    6:          0    XT-PIC-XT        DCON
-  7:     222470    XT-PIC-XT        mfgpt-timer
+  7:     224002    XT-PIC-XT        mfgpt-timer
    8:          1    XT-PIC-XT        rtc
- 10:      93831    XT-PIC-XT        ehci_hcd:usb1, ohci_hcd:usb2
+ 10:      94187    XT-PIC-XT        ehci_hcd:usb1, ohci_hcd:usb2
   11:        162    XT-PIC-XT        CAFE NAND, cafe-ccic, sdhci:slot0
   12:         97    XT-PIC-XT        i8042
  NMI:          0