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I'm running Linux on an Intel Core 2 Duo processor. When I apply power to my computer, it boots into one core but appears to leave the other one idle. On the screen, it prints:
SMP: Allowing 2 CPUs, 1 hotplug CPUs
I can confirm that only 1 CPU is active by logging in and examining /proc/interrupts and /proc/cpuinfo. After issuing a "reboot" command, the screen shows:
SMP: Allowing 2 CPUs, 0 hotplug CPUs
So, when I do a cold boot, I one get 1 CPU. When I do a warm boot, I get both CPUs. This is a problem for me because I'm running Linux on an embedded board and I need both CPUs to be running.
I've played around with BIOS settings, the boot string, kernel configuration, and CPU hot plugging. I can't seem to find a way to get both processors running without booting the system twice.
I thought that the CPU hot plugging path showed some promise but I can't seem to get that feature working on my system. The first issue is that the Linux documentation doesn't seem to match with what I'm seeing. It states that you should be able to go into /sys/devices/system/cpu/cpuX/ and write a zero or a one to the file labeled 'online' to disable it or enable it. When I go to /sys/devices/system/cpu I only see a directory for cpu0. There isn't a directory for cpu1.
After a reboot, I do see two directories int /sys/devices/system/cpu (cpu0 and cpu1). At this point, it won't let me 'offline' the second processor. The echo command gives me an error. (By the Way: the 'online' file in each cpuX directory has read only permissions.) This error occurs after I've changed the permissions on the file to read/write.
I confirmed that the following parameters are set in my .config file: CONFIG_HOTPLUG, CONFIG_HOTPLUG_CPU, CONFIG_SMP, CONFIG_SUSPEND_SMP
I've done searches on google until my eyes popped out and can't find much information. I see others have had similar problems but they all seem to link back to ACPI in BIOS. I've already played with all of those settings and I didn't see any effect. One reason I don't think it's a BIOS problem is because a warm boot (reboot) seems to 'fix' it.
I've tried different kernels, different distributions, and different computers. I see similar behavior in all configurations. I feel like I'm missing something fundamental.
I emailed the manufacturer of my embedded board and gave them a detailed description about this problem. After some investigation, they determined that the BIOS is somehow getting confused.
On some cold boots, the BIOS thinks the board is coming out of a standby mode instead of a cold boot. When it comes out of standby, the BIOS doesn't reload the ACPI tables that are necessary for Linux to bring up both CPU cores. They are sending me an updated BIOS image.
Thanks for suggesting that I contact the manufacturer.