Multiple CPUs are useful for CPU-bound processing like compression and I do want to use compression to generate crash dump quickly. But now we cannot wakeup the 2nd and later cpus in the kdump 2nd kernel if crash happens on AP. If crash happens on AP, kexec enters the 2nd kernel with the AP, and there BSP in the 1st kernel is expected to be haling in the 1st kernel or possibly in any fatal system error state. To wake up AP, we use the method called INIT-INIT-SIPI. INIT causes BSP to jump into BIOS init code. A typical visible behaviour is hang or immediate reset, depending on the BIOS init code. AP can be initiated by INIT even in a fatal state: MP spec explains that processor-specific INIT can be used to recover AP from a fatal system error. On the other hand, there's no method for BSP to recover; it might be possible to do so by NMI plus any hand-coded reset code that is carefully designed, but at least I have no idea in this direction now. Therefore, the idea I do in this patch set is simply to disable BSP if vboot cpu is AP. My motivation is to use multiple CPUs in order to quickly generate crash dump on the machine with huge amount of memory. I assume such machine tends to also have a lot of CPUs. So disabling one CPU would be no problem. On most BIOSs, BSP is always assigned to cpu#1; on other BIOSs, BSP could probably be assigned to a fixed cpu number. Assuming this fact, it might be possible to choose an idea that waking up the cpus except for cpu#1, not waking up cpu#1 only. But I don't choose this in this patch set because: - It's ugly desgin to keep switch in sysfs that can unintentionally cause system to enter undefined behaviour. - Memory space for BSP is never used if BSP is not running. Amount of reserved memory for 2nd kernel is typically from 128MB to 512MB only, severely limited. If BSP is unused, I want to use the space for another AP instead. Note: recent upstream kernel fails reserving memory for kdump 2nd kernel. To run kdump, please apply the patch below on top of this patch set: https://lkml.org/lkml/2012/8/31/238 --- HATAYAMA Daisuke (2): x86, apic: Disable BSP if boot cpu is AP x86, apic: Introduce boot_cpu_is_bsp indicating whether boot cpu is BSP or not arch/x86/include/asm/mpspec.h | 5 ++++- arch/x86/kernel/acpi/boot.c | 10 +++++++++- arch/x86/kernel/apic/apic.c | 34 +++++++++++++++++++++++++++++++++- arch/x86/kernel/devicetree.c | 2 +- arch/x86/kernel/mpparse.c | 15 +++++++++++++-- arch/x86/kernel/setup.c | 2 ++ arch/x86/platform/sfi/sfi.c | 2 +- 7 files changed, 63 insertions(+), 7 deletions(-) -- Thanks. HATAYAMA, Daisuke
