本文主要是介绍cpu masks的初始化，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！
在内核中，有几个位图变量是用作标识cpu数量和状态的，它们分别是：
| 变量名称 | 用途 | 循环所使用的宏 |
| cpu_possible_mask | 系统中有多少个可以运行的cpu核 | for_each_possible_cpu |
| cpu_present_mask | 系统中有多少个可处于运行状态的cpu核 | for_each_present_cpu |
| cpu_online_mask | 系统中有多少个正在工作的cpu核 | for_each_online_cpu |
| cpu_active_mask | 系统中有多少个活跃的cpu核 |
本文主要介绍一下cpu_possible_mask和cpu_present_mask的初始化。
1， cpu_possible_mask的初始化过程：
start_kernel
--> setup_arch
--> smp_init_cpus
--> acpi_parse_and_init_cpus
smp_cpu_setup
-->set_cpu_possible
从上述过程中，内核从acpi中获取了可以运行的cpu的数量。
2，cpu_present_mask
start_kernel
-->reset_init
--> kernel_thread(kernel_thread)
kernel_init(thread)
-->kernel_init_freeable
-->smp_prepare_cpus
-->cpu_prepare
-->cpu_psci_cpu_prepare
-->set_cpu_present
-->smp_init
-->cpu_up
-->cpu_psci_cpu_boot
还是从start_kernel开始，这个时候cpu0已经启动完成，内核创建了一个线程kernel_init，其余的cpu和在这个函数里进行初始化工作。
内核首先根据cpu_possible_mask中的信息，为每一个possible cpu调用cpu_prepare，得到了固件的回复后，将该cpu设置到cpu_present_mask相对应的位置上，这样系统就获得了可以运行的cpu的位图。
然后内核调用smp_init-->cpu_up逐一初始化这些内核，关于这些内核的的启动流程，可以参考我的另一篇博客：
smp_init过程解析_slab_prepare_cpu-CSDN博客
为什么想到要看这两个变量的初始化呢，原因是最近正在处理的项目是一个双cpu的服务器，每个服务器有32个核，但我们无论是在GUI的系统信息里，还是在dmidecode中，得到的信息都是一个64核的cpu，其中dmidecode信息如下：
dmidecode -r processor# dmidecode 3.2
Getting SMBIOS data from sysfs.
SMBIOS 3.3.0 present.
# SMBIOS implementations newer than version 3.2.0 are not
# fully supported by this version of dmidecode.Handle 0x0008, DMI type 4, 48 bytes
Processor InformationSocket Designation: SOCKET 0Type: Central ProcessorFamily: ARMManufacturer: PHYTIUM LTDID: 10 08 00 00 00 00 00 00Signature: Implementor 0x00, Variant 0x0, Architecture 0, Part 0x081, Revision 0Version: Phytium S5000C 64 CoreVoltage: 0.9 VExternal Clock: UnknownMax Speed: 2300 MHzCurrent Speed: 2300 MHzStatus: Populated, EnabledUpgrade: NoneL1 Cache Handle: 0x0005L2 Cache Handle: 0x0006L3 Cache Handle: 0x0007Serial Number: KAP8160405050000Asset Tag: Not SetPart Number: Not SetCore Count: 64Core Enabled: 64Thread Count: 64Characteristics:64-bit capableMulti-CoreExecute ProtectionEnhanced Virtualization一开始以为是cpu_possible_mask或者cpu_present_mask有误，但根据dmidecode的源码分析后，看到这些信息是从内核的文件/sys/firmware/dmi/tables/smbios_entry_point中获取，dmidecode的过程如下，见dmidecode.c
static void dmi_decode(const struct dmi_header *h, u16 ver)
{const u8 *data = h->data;/** Note: DMI types 37 and 42 are untested*/switch (h->type){case 0: /* 7.1 BIOS Information */......case 4: /* 7.5 Processor Information */printf("Processor Information\n");if (h->length < 0x1A) break;printf("\tSocket Designation: %s\n",dmi_string(h, data[0x04]));printf("\tType: %s\n",dmi_processor_type(data[0x05]));printf("\tFamily: %s\n",dmi_processor_family(h, ver));printf("\tManufacturer: %s\n",dmi_string(h, data[0x07]));dmi_processor_id(h, "\t");printf("\tVersion: %s\n",dmi_string(h, data[0x10]));printf("\tVoltage:");dmi_processor_voltage(data[0x11]);printf("\n");printf("\tExternal Clock: ");dmi_processor_frequency(data + 0x12);printf("\n");printf("\tMax Speed: ");dmi_processor_frequency(data + 0x14);printf("\n");printf("\tCurrent Speed: ");dmi_processor_frequency(data + 0x16);printf("\n");if (data[0x18] & (1 << 6))printf("\tStatus: Populated, %s\n",dmi_processor_status(data[0x18] & 0x07));elseprintf("\tStatus: Unpopulated\n");printf("\tUpgrade: %s\n",dmi_processor_upgrade(data[0x19]));if (h->length < 0x20) break;if (!(opt.flags & FLAG_QUIET)){printf("\tL1 Cache Handle:");dmi_processor_cache(WORD(data + 0x1A), "L1", ver);printf("\n");printf("\tL2 Cache Handle:");dmi_processor_cache(WORD(data + 0x1C), "L2", ver);printf("\n");printf("\tL3 Cache Handle:");dmi_processor_cache(WORD(data + 0x1E), "L3", ver);printf("\n");}if (h->length < 0x23) break;printf("\tSerial Number: %s\n",dmi_string(h, data[0x20]));printf("\tAsset Tag: %s\n",dmi_string(h, data[0x21]));printf("\tPart Number: %s\n",dmi_string(h, data[0x22]));if (h->length < 0x28) break;if (data[0x23] != 0)printf("\tCore Count: %u\n",h->length >= 0x2C && data[0x23] == 0xFF ?WORD(data + 0x2A) : data[0x23]);if (data[0x24] != 0)printf("\tCore Enabled: %u\n",h->length >= 0x2E && data[0x24] == 0xFF ?WORD(data + 0x2C) : data[0x24]);if (data[0x25] != 0)printf("\tThread Count: %u\n",h->length >= 0x30 && data[0x25] == 0xFF ?WORD(data + 0x2E) : data[0x25]);printf("\tCharacteristics:");dmi_processor_characteristics(WORD(data + 0x26), "\t\t");break;......。cpu的信息包括socket都在dmicode信息中。
dmi信息在内核中的处理是在driver/firmware/dmi_scan.c中，代码如下：
void __init dmi_scan_machine(void)
{char __iomem *p, *q;char buf[32];if (efi_enabled(EFI_CONFIG_TABLES)) {if (efi.smbios3 != EFI_INVALID_TABLE_ADDR) {p = dmi_early_remap(efi.smbios3, 32);......dmidecode这些信息是由efi写到内存的efi.smbios3地址中，内核使用iomap将该地址映射到内核的虚拟地址空间，并且读出来放到了/sys/firmware/dmi/tables/smbios_entry_point中，工具dmidecode对其进行读取和解析。
所以这个问题是smbios信息有误，更新后就可以显示正确的信息了。
dmidecode的源码在如下地址：
Index of /releases/dmidecode/
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