Red Hat Bugzilla – Bug 81784
APIC interrupt handling not used on UP even with custom kernel
Last modified: 2007-04-18 12:49:45 EDT
User-Agent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; .NET CLR
Description of problem:
I am using an Asus P4B533 motherboard (Intel 845E chipset). The BIOS interrupt
handling setting is set to APIC mode. Windows XP is using APIC mode on this
machine (as seen from the IRQ numbers over 15 used for various devices)
The standard UP kernel does not seem to use any APIC features. I tried
compiling a custom kernel that includes these config settings:
When this kernel is booted, I get these messages:
Found and enabled local APIC!
Using local APIC timer interrupts.
calibrating APIC timer ...
..... CPU clock speed is 2019.1251 MHz.
..... host bus clock speed is 100.1917 MHz.
However, there is no indication that it is actually using the IO APIC which
would allow it to use actual APIC interrupt handling. /proc/interrupts shows
this as well:
0: 1115708 XT-PIC timer
1: 7 XT-PIC keyboard
2: 0 XT-PIC cascade
5: 0 XT-PIC usb-uhci
8: 1 XT-PIC rtc
9: 12629 XT-PIC ehci-hcd, eth0, cmpci
10: 0 XT-PIC usb-uhci
11: 216515 XT-PIC usb-uhci, nvidia
14: 14587 XT-PIC ide0
15: 54739 XT-PIC ide1
Is this something that has been intentionally disabled in the Red Hat version
of the UP kernel?
Version-Release number of selected component (if applicable):
Linux version 2.4.18-19.8.0custom (root@northbrook) (gcc version 3.2 20020903
(Red Hat Linux 8.0 3.2-7)) #2 Tue Jan 7 22:17:36 CST 2003
Steps to Reproduce:
1. Configure custom UP kernel as shown above
2. Check whether APIC mode is used on APIC-supporting system.
Actual Results: Kernel is still using XT-PIC style interrupt handling.
Expected Results: APIC interrupt handling mode is used like on Windows XP.
This would likely provide a small performance boost, among other things.
The use of the APIC for interrupt routing depends on what tables are available.
Most likely this board only handles APIC with ACPI