张超《Linux内核分析》MOOC课程http://mooc.study.163.com/course/USTC-1000029000
我的代码可见https://www.shiyanlou.com/courses/reports/986221
在这里我们用的是linux-3.18.6版本，以下简写成linux。
start_kernel在 /linux/init/main.c中定义：
这个函数是内核由引导程序引导以后，由自解压程序解压以后执行的第一个函数，可以认为是整个内核的入口函数，以后我分析的代码全部从这个函数开始！
这个函数做的事情相对比较简单，就是线性的初始化一些内核的基础机制，如中断，内存管理，进程管理，信号，文件系统，KO等！最后就启动一个init线程，init线程再读取文件系统里的init程序，做为系统的第一个进程而存在！
其实，start_kernel函数是0是做为0号进程存在的，它在最后就是空转CPU：
500asmlinkage __visible void __init start_kernel(void) 501{ 502 char *command_line; 503 char *after_dashes; 504 505 /* 506 * Need to run as early as possible, to initialize the 507 * lockdep hash: 508 */ 509 lockdep_init(); 510 set_task_stack_end_magic(&init_task); 511 smp_setup_processor_id(); 512 debug_objects_early_init(); 513 514 /* 515 * Set up the the initial canary ASAP: 516 */ 517 boot_init_stack_canary(); 518 519 cgroup_init_early(); 520 521 local_irq_disable(); 522 early_boot_irqs_disabled = true; 523 524/* 525 * Interrupts are still disabled. Do necessary setups, then 526 * enable them 527 */ 528 boot_cpu_init(); 529 page_address_init(); 530 pr_notice("%s", linux_banner); 531 setup_arch(&command_line); 532 mm_init_cpumask(&init_mm); 533 setup_command_line(command_line); 534 setup_nr_cpu_ids(); 535 setup_per_cpu_areas(); 536 smp_prepare_boot_cpu(); /* arch-specific boot-cpu hooks */ 537 538 build_all_zonelists(NULL, NULL); 539 page_alloc_init(); 540 541 pr_notice("Kernel command line: %s\n", boot_command_line); 542 parse_early_param(); 543 after_dashes = parse_args("Booting kernel", 544 static_command_line, __start___param, 545 __stop___param - __start___param, 546 -1, -1, &unknown_bootoption); 547 if (!IS_ERR_OR_NULL(after_dashes)) 548 parse_args("Setting init args", after_dashes, NULL, 0, -1, -1, 549 set_init_arg); 550 551 jump_label_init(); 552 553 /* 554 * These use large bootmem allocations and must precede 555 * kmem_cache_init() 556 */ 557 setup_log_buf(0); 558 pidhash_init(); 559 vfs_caches_init_early(); 560 sort_main_extable(); 561 trap_init(); 562 mm_init(); 563 564 /* 565 * Set up the scheduler prior starting any interrupts (such as the 566 * timer interrupt). Full topology setup happens at smp_init() 567 * time - but meanwhile we still have a functioning scheduler. 568 */ 569 sched_init(); 570 /* 571 * Disable preemption - early bootup scheduling is extremely 572 * fragile until we cpu_idle() for the first time. 573 */ 574 preempt_disable(); 575 if (WARN(!irqs_disabled(), 576 "Interrupts were enabled *very* early, fixing it\n")) 577 local_irq_disable(); 578 idr_init_cache(); 579 rcu_init(); 580 context_tracking_init(); 581 radix_tree_init(); 582 /* init some links before init_ISA_irqs() */ 583 early_irq_init(); 584 init_IRQ(); 585 tick_init(); 586 rcu_init_nohz(); 587 init_timers(); 588 hrtimers_init(); 589 softirq_init(); 590 timekeeping_init(); 591 time_init(); 592 sched_clock_postinit(); 593 perf_event_init(); 594 profile_init(); 595 call_function_init(); 596 WARN(!irqs_disabled(), "Interrupts were enabled early\n"); 597 early_boot_irqs_disabled = false; 598 local_irq_enable(); 599 600 kmem_cache_init_late(); 601 602 /* 603 * HACK ALERT! This is early. We're enabling the console before 604 * we've done PCI setups etc, and console_init() must be aware of 605 * this. But we do want output early, in case something goes wrong. 606 */ 607 console_init(); 608 if (panic_later) 609 panic("Too many boot %s vars at `%s'", panic_later, 610 panic_param); 611 612 lockdep_info(); 613 614 /* 615 * Need to run this when irqs are enabled, because it wants 616 * to self-test [hard/soft]-irqs on/off lock inversion bugs 617 * too: 618 */ 619 locking_selftest(); 620 621#ifdef CONFIG_BLK_DEV_INITRD 622 if (initrd_start && !initrd_below_start_ok && 623 page_to_pfn(virt_to_page((void *)initrd_start)) < min_low_pfn) { 624 pr_crit("initrd overwritten (0x%08lx < 0x%08lx) - disabling it.\n", 625 page_to_pfn(virt_to_page((void *)initrd_start)), 626 min_low_pfn); 627 initrd_start = 0; 628 } 629#endif 630 page_cgroup_init(); 631 debug_objects_mem_init(); 632 kmemleak_init(); 633 setup_per_cpu_pageset(); 634 numa_policy_init(); 635 if (late_time_init) 636 late_time_init(); 637 sched_clock_init(); 638 calibrate_delay(); 639 pidmap_init(); 640 anon_vma_init(); 641 acpi_early_init(); 642#ifdef CONFIG_X86 643 if (efi_enabled(EFI_RUNTIME_SERVICES)) 644 efi_enter_virtual_mode(); 645#endif 646#ifdef CONFIG_X86_ESPFIX64 647 /* Should be run before the first non-init thread is created */ 648 init_espfix_bsp(); 649#endif 650 thread_info_cache_init(); 651 cred_init(); 652 fork_init(totalram_pages); 653 proc_caches_init(); 654 buffer_init(); 655 key_init(); 656 security_init(); 657 dbg_late_init(); 658 vfs_caches_init(totalram_pages); 659 signals_init(); 660 /* rootfs populating might need page-writeback */ 661 page_writeback_init(); 662 proc_root_init(); 663 cgroup_init(); 664 cpuset_init(); 665 taskstats_init_early(); 666 delayacct_init(); 667 668 check_bugs(); 669 670 sfi_init_late(); 671 672 if (efi_enabled(EFI_RUNTIME_SERVICES)) { 673 efi_late_init(); 674 efi_free_boot_services(); 675 } 676 677 ftrace_init(); 678 679 /* Do the rest non-__init'ed, we're now alive */ 680 rest_init(); 681}