专题：Linux进程管理专题
关键词：swapper、init_task、fork。
Linux内核通常把进程叫作任务，进程控制块(PCB Processing Control Block)用struct task_struct表示。
线程是轻量级进程，是操作系统做小调度单元，一个进程可以拥有多个线程。
线程之所以被称为轻量级，是因为共享进程的资源空间。线程和进程使用相同的进程PCB数据结构。
内核使用clone方法创建线程，类似于fork方法，但会确定哪些资源和父进程共享，哪些资源为线程独享。
1. init进程
init进程也称为swapper进程或者idle进程，是在Linux启动是的第一个进程。
idle进程在内核启动(start_kernel())时静态创建，所有的核心数据结构都静态赋值。
当系统没有进程需要调度时，调度器就会执行idle进程。
start_kernel ->rest_init ->cpu_startup_entry ->cpu_idle_loop
1.1 init_task
init_task进程的task_struct数据结构通过INIT_TASK宏来赋值。
/* Initial task structure */ struct task_struct init_task = INIT_TASK(init_task); EXPORT_SYMBOL(init_task);
INIT_TASK用来填充init_task数据结构。
#define INIT_TASK(tsk) \ { \ .state = 0, \ .stack = &init_thread_info, \-------#define init_thread_info (init_thread_union.thread_info) .usage = ATOMIC_INIT(2), \ .flags = PF_KTHREAD, \----------表明是一个内核线程 .prio = MAX_PRIO-20, \----------MAX_PRIO为140，此处prio为120，对应的nice值为0.关于prio和nice参考：prio和nice之间的关系。 .static_prio = MAX_PRIO-20, \ .normal_prio = MAX_PRIO-20, \ .policy = SCHED_NORMAL, \-------调度策略是SCHED_NORMAL。 .cpus_allowed = CPU_MASK_ALL, \ .nr_cpus_allowed= NR_CPUS, \ .mm = NULL, \ .active_mm = &init_mm, \------------idle进程的内存管理结构数据 .restart_block = { \ .fn = do_no_restart_syscall, \ }, \ .se = { \ .group_node = LIST_HEAD_INIT(tsk.se.group_node), \ }, \ .rt = { \ .run_list = LIST_HEAD_INIT(tsk.rt.run_list), \ .time_slice = RR_TIMESLICE, \ }, \ .tasks = LIST_HEAD_INIT(tsk.tasks), \ INIT_PUSHABLE_TASKS(tsk) \ INIT_CGROUP_SCHED(tsk) \ .ptraced = LIST_HEAD_INIT(tsk.ptraced), \ .ptrace_entry = LIST_HEAD_INIT(tsk.ptrace_entry), \ .real_parent = &tsk, \ .parent = &tsk, \ .children = LIST_HEAD_INIT(tsk.children), \ .sibling = LIST_HEAD_INIT(tsk.sibling), \ .group_leader = &tsk, \ RCU_POINTER_INITIALIZER(real_cred, &init_cred), \ RCU_POINTER_INITIALIZER(cred, &init_cred), \ .comm = INIT_TASK_COMM, \ .thread = INIT_THREAD, \ .fs = &init_fs, \ .files = &init_files, \ .signal = &init_signals, \ .sighand = &init_sighand, \ .nsproxy = &init_nsproxy, \ .pending = { \ .list = LIST_HEAD_INIT(tsk.pending.list), \ .signal = {{0}}}, \ .blocked = {{0}}, \ .alloc_lock = __SPIN_LOCK_UNLOCKED(tsk.alloc_lock), \ .journal_info = NULL, \ .cpu_timers = INIT_CPU_TIMERS(tsk.cpu_timers), \ .pi_lock = __RAW_SPIN_LOCK_UNLOCKED(tsk.pi_lock), \ .timer_slack_ns = 50000, /* 50 usec default slack */ \ .pids = { \ [PIDTYPE_PID] = INIT_PID_LINK(PIDTYPE_PID), \ [PIDTYPE_PGID] = INIT_PID_LINK(PIDTYPE_PGID), \ [PIDTYPE_SID] = INIT_PID_LINK(PIDTYPE_SID), \ }, \ .thread_group = LIST_HEAD_INIT(tsk.thread_group), \ .thread_node = LIST_HEAD_INIT(init_signals.thread_head), \ INIT_IDS \ INIT_PERF_EVENTS(tsk) \ INIT_TRACE_IRQFLAGS \ INIT_LOCKDEP \ INIT_FTRACE_GRAPH \ INIT_TRACE_RECURSION \ INIT_TASK_RCU_PREEMPT(tsk) \ INIT_TASK_RCU_TASKS(tsk) \ INIT_CPUSET_SEQ(tsk) \ INIT_RT_MUTEXES(tsk) \ INIT_PREV_CPUTIME(tsk) \ INIT_VTIME(tsk) \ INIT_NUMA_BALANCING(tsk) \ INIT_KASAN(tsk) \ }
1.2 thread_info、thread_union、task_struct关系
thread_union包括thread_info和内核栈；
task_struct的stack指向init_thread_union.thread_info。
1.2.1 init_thread_info
init_thread_info被__init_task_data修饰，所以它会被固定在.data..init_task段中。
/* * Initial thread structure. Alignment of this is handled by a special * linker map entry. */ union thread_union init_thread_union __init_task_data = { INIT_THREAD_INFO(init_task) }; #define __init_task_data __attribute__((__section__(".data..init_task")))
下面看看.data..init_task段，在vmlinux.lds.S链接文件中定义了大小和位置。
可以看出在_data开始的地方保留了一块2页大小的空间，存放init_task_info。
SECTIONS { ... .data : AT(__data_loc) { _data = .; /* address in memory */ _sdata = .; /* * first, the init task union, aligned * to an 8192 byte boundary. */ INIT_TASK_DATA(THREAD_SIZE)------------------------------存放在_data开始地方，2页大小，即8KB。 ... _edata = .; } _edata_loc = __data_loc + SIZEOF(.data); ... } #define INIT_TASK_DATA(align) \ . = ALIGN(align); \ *(.data..init_task) #define THREAD_SIZE_ORDER 1 #define THREAD_SIZE (PAGE_SIZE << THREAD_SIZE_ORDER) #define THREAD_START_SP (THREAD_SIZE - 8)
init_thread_info是thread_union联合体，被固定为8KB大小。
union thread_union {
struct thread_info thread_info;
unsigned long stack[THREAD_SIZE/sizeof(long)]; };
init_thread_info中包含了struct thread_info类型数据结构，它是由INIT_THREAD_INFO进行初始化。
struct thread_info { unsigned long flags; /* low level flags */ int preempt_count; /* 0 => preemptable, <0 => bug */ mm_segment_t addr_limit; /* address limit */ struct task_struct *task; /* main task structure */ struct exec_domain *exec_domain; /* execution domain */ __u32 cpu; /* cpu */ __u32 cpu_domain; /* cpu domain */ struct cpu_context_save cpu_context; /* cpu context */ __u32 syscall; /* syscall number */ __u8 used_cp[16]; /* thread used copro */ unsigned long tp_value[2]; /* TLS registers */ #ifdef CONFIG_CRUNCH struct crunch_state crunchstate; #endif union fp_state fpstate __attribute__((aligned(8))); union vfp_state vfpstate; #ifdef CONFIG_ARM_THUMBEE unsigned long thumbee_state; /* ThumbEE Handler Base register */ #endif }; #define INIT_THREAD_INFO(tsk) \ { \ .task = &tsk, \ .exec_domain = &default_exec_domain, \ .flags = 0, \ .preempt_count = INIT_PREEMPT_COUNT, \ .addr_limit = KERNEL_DS, \ .cpu_domain = domain_val(DOMAIN_USER, DOMAIN_MANAGER) | \ domain_val(DOMAIN_KERNEL, DOMAIN_MANAGER) | \ domain_val(DOMAIN_IO, DOMAIN_CLIENT), \ }
1.2.2 init_task内核栈
ARM32处理器从汇编跳转到C语言的入口点start_kernel()函数之前，设置了SP寄存器指向8KB内核栈顶部区域，其中预留了8B空洞。
/* * The following fragment of code is executed with the MMU on in MMU mode, * and uses absolute addresses; this is not position independent. * * r0 = cp#15 control register * r1 = machine ID * r2 = atags/dtb pointer * r9 = processor ID */ __INIT __mmap_switched: adr r3, __mmap_switched_data ldmia r3!, {r4, r5, r6, r7} ... ARM( ldmia r3, {r4, r5, r6, r7, sp}) THUMB( ldmia r3, {r4, r5, r6, r7} ) THUMB( ldr sp, [r3, #16] ) ... b start_kernel------------------------------------------------跳转到start_kernel函数 ENDPROC(__mmap_switched) .align 2 .type __mmap_switched_data, %object __mmap_switched_data: .long __data_loc @ r4 .long _sdata @ r5 .long __bss_start @ r6 .long _end @ r7 .long processor_id @ r4 .long __machine_arch_type @ r5 .long __atags_pointer @ r6 #ifdef CONFIG_CPU_CP15 .long cr_alignment @ r7 #else .long 0 @ r7 #endif .long init_thread_union + THREAD_START_SP @ sp-----------------定义了SP寄存器的值，指向8KB栈空间顶部。 .size __mmap_switched_data, . - __mmap_switched_data
1.2.3 从sp到current逆向查找
内核中用一个current常量获取当前进程task_structg数据结构，从sp到current的流程如下：
- 通过SP寄存器获取当前内核栈指针。
- 栈指针对齐后获取struct thread_info数据结构指针
- 通过thread_info->task成员获取task_struct数据结构
可以和内核栈示意图结合看。
#define get_current() (current_thread_info()->task) #define current get_current() /* * how to get the current stack pointer in C */ register unsigned long current_stack_pointer asm ("sp"); /* * how to get the thread information struct from C */ static inline struct thread_info *current_thread_info(void) __attribute_const__; static inline struct thread_info *current_thread_info(void) { return (struct thread_info *) (current_stack_pointer & ~(THREAD_SIZE - 1)); }
2. fork
Linux通过fork、vfork、clone等系统调用来建立线程或进程，在内核中这三个系统调用都通过一个函数来实现，即do_fork()。也包括内核线程kernel_thread。
do_fork定义在fork.c中，下面四个封装接口的区别就在于其传递的参数。
/* * Create a kernel thread. */ pid_t kernel_thread(int (*fn)(void *), void *arg, unsigned long flags) { return do_fork(flags|CLONE_VM|CLONE_UNTRACED, (unsigned long)fn, (unsigned long)arg, NULL, NULL); } SYSCALL_DEFINE0(fork) { return do_fork(SIGCHLD, 0, 0, NULL, NULL); } SYSCALL_DEFINE0(vfork) { return do_fork(CLONE_VFORK | CLONE_VM | SIGCHLD, 0, 0, NULL, NULL); } SYSCALL_DEFINE5(clone, unsigned long, clone_flags, unsigned long, newsp, int __user *, parent_tidptr, int, tls_val, int __user *, child_tidptr) { return do_fork(clone_flags, newsp, 0, parent_tidptr, child_tidptr); }
fork只使用用了SIGCHLD标志位在紫禁城终止后发送SIGCHLD信号通知父进程。fork是重量级应用，为子进程建立了一个基于父进程的完整副本，然后子进程基于此运行。
但是采用了COW技术，子进程只复制父进程页表，而不复制页面内容。当子进程需要写入内容时才触发写时复制机制，为子进程创建一个副本。
vfork比fork多了连个标志位：CLONE_VFORK表示父进程会被挂起，直至子进程释放虚拟内存资源；CLONE_VM表示父子进程运行在相同的内存空空间中。
在fork实现COW技术后，vfork意义已经不大。
clone用于创建线程，并且参数通过寄存器从用户空间传递下来，通常会指定新的栈地址newsp。借助clone_flags，clone给了用户更大的选择空间，他可以是fork/vfork，也可以和父进程共用资源。
kernel_thread用于创建内核线程，CLONE_VM表示和父进程共享内存资源；CLONE_UNTRACED表示线程不能被设置CLONE_PTRACE。
简单来说fork重，vfork趋淘汰，clone轻，kernel_thread内核。
2.1 do_fork及其参数解释
do_fork有5个参数：
- clone_flags：创建进程的标志位集合
- stack_start：用户态栈的起始地址
- stack_size：用户态栈的大小
- parent_tidptr和child_tidptr：指向用户空间地址的两个指针，分别指向父子进程PID。
其中clone_flags是影响do_fork行为的重要参数：
/* * cloning flags: */ #define CSIGNAL 0x000000ff /* signal mask to be sent at exit */ #define CLONE_VM 0x00000100 /* set if VM shared between processes */-------------------------父子进程运行在同一个虚拟空间 #define CLONE_FS 0x00000200 /* set if fs info shared between processes */--------------------父子进程共享文件系统信息 #define CLONE_FILES 0x00000400 /* set if open files shared between processes */--------------父子进程共享文件描述符表 #define CLONE_SIGHAND 0x00000800 /* set if signal handlers and blocked signals shared */-----父子进程共享信号处理函数表 #define CLONE_PTRACE 0x00002000 /* set if we want to let tracing continue on the child too */---------父进程被跟踪ptrace，子进程也会被跟踪。 #define CLONE_VFORK 0x00004000 /* set if the parent wants the child to wake it up on mm_release */----在创建子进程时启动完成机制completion，wait_for_completion()会使父进程进入睡眠等待，知道子进程调用execve()或exit()释放虚拟内存资源。 #define CLONE_PARENT 0x00008000 /* set if we want to have the same parent as the cloner */------------新创建的进程是兄弟关系，而不是父子关系。 #define CLONE_THREAD 0x00010000 /* Same thread group? */ #define CLONE_NEWNS 0x00020000 /* New mount namespace group */------------父子进程不共享mount namespace #define CLONE_SYSVSEM 0x00040000 /* share system V SEM_UNDO semantics */-- #define CLONE_SETTLS 0x00080000 /* create a new TLS for the child */ #define CLONE_PARENT_SETTID 0x00100000 /* set the TID in the parent */ #define CLONE_CHILD_CLEARTID 0x00200000 /* clear the TID in the child */ #define CLONE_DETACHED 0x00400000 /* Unused, ignored */ #define CLONE_UNTRACED 0x00800000 /* set if the tracing process can't force CLONE_PTRACE on this clone */ #define CLONE_CHILD_SETTID 0x01000000 /* set the TID in the child */ /* 0x02000000 was previously the unused CLONE_STOPPED (Start in stopped state) and is now available for re-use. */ #define CLONE_NEWUTS 0x04000000 /* New utsname namespace */ #define CLONE_NEWIPC 0x08000000 /* New ipc namespace */ #define CLONE_NEWUSER 0x10000000 /* New user namespace */----------子进程要创建新的User Namespace。 #define CLONE_NEWPID 0x20000000 /* New pid namespace */------------创建一个新的PID namespace。 #define CLONE_NEWNET 0x40000000 /* New network namespace */ #define CLONE_IO 0x80000000 /* Clone io context */
主要函数调用路径如下：
do_fork------------------------------------------ ->copy_process--------------------------------- ->dup_task_struct---------------------------- ->sched_fork--------------------------------- ->copy_files ->copy_fs ->copy_sighand ->copy_signal ->copy_mm------------------------------------ ->dup_mm----------------------------------- ->copy_namespaces ->copy_io ->copy_thread--------------------------------
do_fork()先对CLONE_UNTRACED进行简单检查，主要将工作交给copy_process进行处理，最后唤醒创建的进程。
/* * Ok, this is the main fork-routine. * * It copies the process, and if successful kick-starts * it and waits for it to finish using the VM if required. */ long do_fork(unsigned long clone_flags, unsigned long stack_start, unsigned long stack_size, int __user *parent_tidptr, int __user *child_tidptr) { struct task_struct *p; int trace = 0; long nr; /* * Determine whether and which event to report to ptracer. When * called from kernel_thread or CLONE_UNTRACED is explicitly * requested, no event is reported; otherwise, report if the event * for the type of forking is enabled. */ if (!(clone_flags & CLONE_UNTRACED)) { if (clone_flags & CLONE_VFORK) trace = PTRACE_EVENT_VFORK; else if ((clone_flags & CSIGNAL) != SIGCHLD) trace = PTRACE_EVENT_CLONE; else trace = PTRACE_EVENT_FORK; if (likely(!ptrace_event_enabled(current, trace))) trace = 0; } p = copy_process(clone_flags, stack_start, stack_size, child_tidptr, NULL, trace); /* * Do this prior waking up the new thread - the thread pointer * might get invalid after that point, if the thread exits quickly. */ if (!IS_ERR(p)) { struct completion vfork; struct pid *pid; trace_sched_process_fork(current, p); pid = get_task_pid(p, PIDTYPE_PID); nr = pid_vnr(pid); if (clone_flags & CLONE_PARENT_SETTID) put_user(nr, parent_tidptr); if (clone_flags & CLONE_VFORK) {------------------对于CLONE_VFORK标志位，初始化vfork完成量 p->vfork_done = &vfork; init_completion(&vfork); get_task_struct(p); } wake_up_new_task(p);------------------------------唤醒新创建的进程p，也即把进程加入调度器里接受调度执行。 /* forking complete and child started to run, tell ptracer */ if (unlikely(trace)) ptrace_event_pid(trace, pid); if (clone_flags & CLONE_VFORK) { if (!wait_for_vfork_done(p, &vfork))---------等待子进程释放p->vfork_done完成量 ptrace_event_pid(PTRACE_EVENT_VFORK_DONE, pid); } put_pid(pid); } else { nr = PTR_ERR(p); } return nr; }
2.2 copy_process
include/linux/sched.h中定义了进程标志位：
/* * Per process flags */ #define PF_EXITING 0x00000004 /* getting shut down */ #define PF_EXITPIDONE 0x00000008 /* pi exit done on shut down */ #define PF_VCPU 0x00000010 /* I'm a virtual CPU */ #define PF_WQ_WORKER 0x00000020 /* I'm a workqueue worker */ #define PF_FORKNOEXEC 0x00000040 /* forked but didn't exec */ #define PF_MCE_PROCESS 0x00000080 /* process policy on mce errors */ #define PF_SUPERPRIV 0x00000100 /* used super-user privileges */ #define PF_DUMPCORE 0x00000200 /* dumped core */ #define PF_SIGNALED 0x00000400 /* killed by a signal */ #define PF_MEMALLOC 0x00000800 /* Allocating memory */ #define PF_NPROC_EXCEEDED 0x00001000 /* set_user noticed that RLIMIT_NPROC was exceeded */ #define PF_USED_MATH 0x00002000 /* if unset the fpu must be initialized before use */ #define PF_USED_ASYNC 0x00004000 /* used async_schedule*(), used by module init */ #define PF_NOFREEZE 0x00008000 /* this thread should not be frozen */ #define PF_FROZEN 0x00010000 /* frozen for system suspend */ #define PF_FSTRANS 0x00020000 /* inside a filesystem transaction */ #define PF_KSWAPD 0x00040000 /* I am kswapd */ #define PF_MEMALLOC_NOIO 0x00080000 /* Allocating memory without IO involved */ #define PF_LESS_THROTTLE 0x00100000 /* Throttle me less: I clean memory */ #define PF_KTHREAD 0x00200000 /* I am a kernel thread */ #define PF_RANDOMIZE 0x00400000 /* randomize virtual address space */ #define PF_SWAPWRITE 0x00800000 /* Allowed to write to swap */ #define PF_NO_SETAFFINITY 0x04000000 /* Userland is not allowed to meddle with cpus_allowed */ #define PF_MCE_EARLY 0x08000000 /* Early kill for mce process policy */ #define PF_MUTEX_TESTER 0x20000000 /* Thread belongs to the rt mutex tester */ #define PF_FREEZER_SKIP 0x40000000 /* Freezer should not count it as freezable */ #define PF_SUSPEND_TASK 0x80000000 /* this thread called freeze_processes and should not be frozen */
copy_process借助current获取当前进程的task_struct数据结构，然后创建新进程数据结构task_struct并复制父进程内容，继续初始化进程主要部分，比如内存空间、文件句柄、文件系统、IO、等等。
/* * This creates a new process as a copy of the old one, * but does not actually start it yet. * * It copies the registers, and all the appropriate * parts of the process environment (as per the clone * flags). The actual kick-off is left to the caller. */ static struct task_struct *copy_process(unsigned long clone_flags, unsigned long stack_start, unsigned long stack_size, int __user *child_tidptr, struct pid *pid, int trace) { int retval; struct task_struct *p; if ((clone_flags & (CLONE_NEWNS|CLONE_FS)) == (CLONE_NEWNS|CLONE_FS)) return ERR_PTR(-EINVAL); if ((clone_flags & (CLONE_NEWUSER|CLONE_FS)) == (CLONE_NEWUSER|CLONE_FS))---------------CLONE_FS（父子进程共享文件系统）和CLONE_NEWNS/CLONE_NEWUSER（父子进程不共享mount/user namespace）冲突， return ERR_PTR(-EINVAL); /* * Thread groups must share signals as well, and detached threads * can only be started up within the thread group. */ if ((clone_flags & CLONE_THREAD) && !(clone_flags & CLONE_SIGHAND))--------------------线程组共享信号处理函数 return ERR_PTR(-EINVAL); /* * Shared signal handlers imply shared VM. By way of the above, * thread groups also imply shared VM. Blocking this case allows * for various simplifications in other code. */ if ((clone_flags & CLONE_SIGHAND) && !(clone_flags & CLONE_VM))----------------------共享信号处理函数需要共享内存空间 return ERR_PTR(-EINVAL); /* * Siblings of global init remain as zombies on exit since they are * not reaped by their parent (swapper). To solve this and to avoid * multi-rooted process trees, prevent global and container-inits * from creating siblings. */ if ((clone_flags & CLONE_PARENT) && current->signal->flags & SIGNAL_UNKILLABLE)-----------------------------init是所有用户空间进程父进程，如果和init兄弟关系，那么进程将无法被回收，从而变成僵尸进程。 return ERR_PTR(-EINVAL); /* * If the new process will be in a different pid or user namespace * do not allow it to share a thread group or signal handlers or * parent with the forking task. */ if (clone_flags & CLONE_SIGHAND) {---------------------------------------------------新的pid或user命名空间和共享信号处理以及线程组冲突，因为他们在namespace中访问隔离。 if ((clone_flags & (CLONE_NEWUSER | CLONE_NEWPID)) || (task_active_pid_ns(current) != current->nsproxy->pid_ns_for_children)) return ERR_PTR(-EINVAL); } retval = security_task_create(clone_flags); if (retval) goto fork_out; retval = -ENOMEM; p = dup_task_struct(current);-------------------------------------------------------分配一个task_struct实例，将当前进程current作为母板。 if (!p) goto fork_out; ftrace_graph_init_task(p); rt_mutex_init_task(p); #ifdef CONFIG_PROVE_LOCKING DEBUG_LOCKS_WARN_ON(!p->hardirqs_enabled); DEBUG_LOCKS_WARN_ON(!p->softirqs_enabled); #endif retval = -EAGAIN; if (atomic_read(&p->real_cred->user->processes) >= task_rlimit(p, RLIMIT_NPROC)) { if (p->real_cred->user != INIT_USER && !capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN)) goto bad_fork_free; } current->flags &= ~PF_NPROC_EXCEEDED; retval = copy_creds(p, clone_flags); if (retval < 0) goto bad_fork_free; /* * If multiple threads are within copy_process(), then this check * triggers too late. This doesn't hurt, the check is only there * to stop root fork bombs. */ retval = -EAGAIN; if (nr_threads >= max_threads)----------------------------------------------max_threads是系统允许最多线程个数，nr_threads是系统当前进程个数。 goto bad_fork_cleanup_count; if (!try_module_get(task_thread_info(p)->exec_domain->module)) goto bad_fork_cleanup_count; delayacct_tsk_init(p); /* Must remain after dup_task_struct() */ p->flags &= ~(PF_SUPERPRIV | PF_WQ_WORKER);---------------------------------告诉系统不使用超级用户权限，并且不是workqueue内核线程。 p->flags |= PF_FORKNOEXEC;--------------------------------------------------执行fork但不立即执行 INIT_LIST_HEAD(&p->children);-----------------------------------------------新进程的子进程链表 INIT_LIST_HEAD(&p->sibling);------------------------------------------------新进程的兄弟进程链表 rcu_copy_process(p); p->vfork_done = NULL; spin_lock_init(&p->alloc_lock); init_sigpending(&p->pending); p->utime = p->stime = p->gtime = 0; p->utimescaled = p->stimescaled = 0; #ifndef CONFIG_VIRT_CPU_ACCOUNTING_NATIVE p->prev_cputime.utime = p->prev_cputime.stime = 0; #endif #ifdef CONFIG_VIRT_CPU_ACCOUNTING_GEN seqlock_init(&p->vtime_seqlock); p->vtime_snap = 0; p->vtime_snap_whence = VTIME_SLEEPING; #endif #if defined(SPLIT_RSS_COUNTING) memset(&p->rss_stat, 0, sizeof(p->rss_stat)); #endif p->default_timer_slack_ns = current->timer_slack_ns; task_io_accounting_init(&p->ioac); acct_clear_integrals(p); posix_cpu_timers_init(p); p->start_time = ktime_get_ns(); p->real_start_time = ktime_get_boot_ns(); p->io_context = NULL; p->audit_context = NULL; if (clone_flags & CLONE_THREAD) threadgroup_change_begin(current); cgroup_fork(p); #ifdef CONFIG_NUMA p->mempolicy = mpol_dup(p->mempolicy); if (IS_ERR(p->mempolicy)) { retval = PTR_ERR(p->mempolicy); p->mempolicy = NULL; goto bad_fork_cleanup_threadgroup_lock; } #endif... #ifdef CONFIG_BCACHE p->sequential_io = 0; p->sequential_io_avg = 0; #endif /* Perform scheduler related setup. Assign this task to a CPU. */ retval = sched_fork(clone_flags, p);-----------------------------------------初始化进程调度相关数据结构，将进程指定到某一CPU上。 if (retval) goto bad_fork_cleanup_policy; retval = perf_event_init_task(p); if (retval) goto bad_fork_cleanup_policy; retval = audit_alloc(p); if (retval) goto bad_fork_cleanup_perf; /* copy all the process information */ shm_init_task(p); retval = copy_semundo(clone_flags, p); if (retval) goto bad_fork_cleanup_audit; retval = copy_files(clone_flags, p);-----------------------------------------复制父进程打开的文件信息 if (retval) goto bad_fork_cleanup_semundo; retval = copy_fs(clone_flags, p);--------------------------------------------复制父进程fs_struct信息 if (retval) goto bad_fork_cleanup_files; retval = copy_sighand(clone_flags, p); if (retval) goto bad_fork_cleanup_fs; retval = copy_signal(clone_flags, p); if (retval) goto bad_fork_cleanup_sighand; retval = copy_mm(clone_flags, p);--------------------------------------------复制父进程的内存管理相关信息 if (retval) goto bad_fork_cleanup_signal; retval = copy_namespaces(clone_flags, p); if (retval) goto bad_fork_cleanup_mm; retval = copy_io(clone_flags, p);--------------------------------------------复制父进程的io_context上下文信息 if (retval) goto bad_fork_cleanup_namespaces; retval = copy_thread(clone_flags, stack_start, stack_size, p); if (retval) goto bad_fork_cleanup_io; if (pid != &init_struct_pid) { retval = -ENOMEM; pid = alloc_pid(p->nsproxy->pid_ns_for_children); if (!pid) goto bad_fork_cleanup_io; } p->set_child_tid = (clone_flags & CLONE_CHILD_SETTID) ? child_tidptr : NULL; /* * Clear TID on mm_release()? */ p->clear_child_tid = (clone_flags & CLONE_CHILD_CLEARTID) ? child_tidptr : NULL; #ifdef CONFIG_BLOCK p->plug = NULL; #endif #ifdef CONFIG_FUTEX p->robust_list = NULL; #ifdef CONFIG_COMPAT p->compat_robust_list = NULL; #endif INIT_LIST_HEAD(&p->pi_state_list); p->pi_state_cache = NULL; #endif /* * sigaltstack should be cleared when sharing the same VM */ if ((clone_flags & (CLONE_VM|CLONE_VFORK)) == CLONE_VM) p->sas_ss_sp = p->sas_ss_size = 0; /* * Syscall tracing and stepping should be turned off in the * child regardless of CLONE_PTRACE. */ user_disable_single_step(p); clear_tsk_thread_flag(p, TIF_SYSCALL_TRACE); #ifdef TIF_SYSCALL_EMU clear_tsk_thread_flag(p, TIF_SYSCALL_EMU); #endif clear_all_latency_tracing(p); /* ok, now we should be set up.. */ p->pid = pid_nr(pid);-------------------------------------------------------获取新进程的pid if (clone_flags & CLONE_THREAD) { p->exit_signal = -1; p->group_leader = current->group_leader; p->tgid = current->tgid; } else { if (clone_flags & CLONE_PARENT) p->exit_signal = current->group_leader->exit_signal; else p->exit_signal = (clone_flags & CSIGNAL); p->group_leader = p; p->tgid = p->pid; } p->nr_dirtied = 0; p->nr_dirtied_pause = 128 >> (PAGE_SHIFT - 10); p->dirty_paused_when = 0; p->pdeath_signal = 0; INIT_LIST_HEAD(&p->thread_group); p->task_works = NULL; /* * Make it visible to the rest of the system, but dont wake it up yet. * Need tasklist lock for parent etc handling! */ write_lock_irq(&tasklist_lock); /* CLONE_PARENT re-uses the old parent */ if (clone_flags & (CLONE_PARENT|CLONE_THREAD)) { p->real_parent = current->real_parent; p->parent_exec_id = current->parent_exec_id; } else { p->real_parent = current; p->parent_exec_id = current->self_exec_id; } spin_lock(¤t->sighand->siglock); /* * Copy seccomp details explicitly here, in case they were changed * before holding sighand lock. */ copy_seccomp(p); /* * Process group and session signals need to be delivered to just the * parent before the fork or both the parent and the child after the * fork. Restart if a signal comes in before we add the new process to * it's process group. * A fatal signal pending means that current will exit, so the new * thread can't slip out of an OOM kill (or normal SIGKILL). */ recalc_sigpending(); if (signal_pending(current)) { spin_unlock(¤t->sighand->siglock); write_unlock_irq(&tasklist_lock); retval = -ERESTARTNOINTR; goto bad_fork_free_pid; } if (likely(p->pid)) { ptrace_init_task(p, (clone_flags & CLONE_PTRACE) || trace); init_task_pid(p, PIDTYPE_PID, pid); if (thread_group_leader(p)) { init_task_pid(p, PIDTYPE_PGID, task_pgrp(current)); init_task_pid(p, PIDTYPE_SID, task_session(current)); if (is_child_reaper(pid)) { ns_of_pid(pid)->child_reaper = p; p->signal->flags |= SIGNAL_UNKILLABLE; } p->signal->leader_pid = pid; p->signal->tty = tty_kref_get(current->signal->tty); list_add_tail(&p->sibling, &p->real_parent->children); list_add_tail_rcu(&p->tasks, &init_task.tasks); attach_pid(p, PIDTYPE_PGID); attach_pid(p, PIDTYPE_SID); __this_cpu_inc(process_counts); } else { current->signal->nr_threads++; atomic_inc(¤t->signal->live); atomic_inc(¤t->signal->sigcnt); list_add_tail_rcu(&p->thread_group, &p->group_leader->thread_group); list_add_tail_rcu(&p->thread_node, &p->signal->thread_head); } attach_pid(p, PIDTYPE_PID); nr_threads++;---------------------------------------------------------当前进程计数递增 } total_forks++; spin_unlock(¤t->sighand->siglock); syscall_tracepoint_update(p); write_unlock_irq(&tasklist_lock); proc_fork_connector(p); cgroup_post_fork(p); if (clone_flags & CLONE_THREAD) threadgroup_change_end(current); perf_event_fork(p); trace_task_newtask(p, clone_flags); uprobe_copy_process(p, clone_flags); return p;----------------------------------------------------------------成功返回新进程的task_struct。 ...return ERR_PTR(retval);---------------------------------------------------各种错误处理 }
dup_task_struct从父进程复制task_struct和thread_info。
static struct task_struct *dup_task_struct(struct task_struct *orig) { struct task_struct *tsk; struct thread_info *ti; int node = tsk_fork_get_node(orig); int err; tsk = alloc_task_struct_node(node);-------------------------------------------------分配一个task_struct结构体 if (!tsk) return NULL; ti = alloc_thread_info_node(tsk, node);---------------------------------------------分配一个thread_info结构体 if (!ti) goto free_tsk; err = arch_dup_task_struct(tsk, orig);----------------------------------------------将父进程的task_struct拷贝到新进程tsk if (err) goto free_ti; tsk->stack = ti;--------------------------------------------------------------------将新进程的栈指向创建的thread_info。 #ifdef CONFIG_SECCOMP /* * We must handle setting up seccomp filters once we're under * the sighand lock in case orig has changed between now and * then. Until then, filter must be NULL to avoid messing up * the usage counts on the error path calling free_task. */ tsk->seccomp.filter = NULL; #endif setup_thread_stack(tsk, orig);------------------------------------------------------将父进程的thread_info复制到子进程thread_info，并将子进程thread_info->task指向子进程 clear_user_return_notifier(tsk); clear_tsk_need_resched(tsk); set_task_stack_end_magic(tsk); ...return tsk; ...
}
进程相关运行状态有：
#define TASK_RUNNING 0
#define TASK_INTERRUPTIBLE 1 #define TASK_UNINTERRUPTIBLE 2 #define __TASK_STOPPED 4 #define __TASK_TRACED 8
sched_fork的主要任务交给__sched_fork()，然后根据优先级选择调度sched_class类，并执行其task_fork。
最后设置新进程运行的CPU，如果不是当前CPU则需要迁移过来。
/* * fork()/clone()-time setup: */ int sched_fork(unsigned long clone_flags, struct task_struct *p) { unsigned long flags; int cpu = get_cpu();-------------------------------------------------------首先关闭内核抢占，然后获取当前CPU id。 __sched_fork(clone_flags, p);----------------------------------------------填充sched_entity数据结构，初始化调度相关设置。 /* * We mark the process as running here. This guarantees that * nobody will actually run it, and a signal or other external * event cannot wake it up and insert it on the runqueue either. */ p->state = TASK_RUNNING;---------------------------------------------------设置为运行状态，虽然还没有实际运行。 /* * Make sure we do not leak PI boosting priority to the child. */ p->prio = current->normal_prio;--------------------------------------------继承父进程normal_prio作为子进程prio /* * Revert to default priority/policy on fork if requested. */ if (unlikely(p->sched_reset_on_fork)) { if (task_has_dl_policy(p) || task_has_rt_policy(p)) { p->policy = SCHED_NORMAL; p->static_prio = NICE_TO_PRIO(0); p->rt_priority = 0; } else if (PRIO_TO_NICE(p->static_prio) < 0) p->static_prio = NICE_TO_PRIO(0); p->prio = p->normal_prio = __normal_prio(p); set_load_weight(p); /* * We don't need the reset flag anymore after the fork. It has * fulfilled its duty: */ p->sched_reset_on_fork = 0; } if (dl_prio(p->prio)) {---------------------------------------------------SCHED_DEADLINE优先级应该是负值，即小于0。 put_cpu(); return -EAGAIN; } else if (rt_prio(p->prio)) {--------------------------------------------SCHED_RT优先级为0-99 p->sched_class = &rt_sched_class; } else {------------------------------------------------------------------SCHED_FAIR优先级为100-139 p->sched_class = &fair_sched_class; } if (p->sched_class->task_fork) p->sched_class->task_fork(p); /* * The child is not yet in the pid-hash so no cgroup attach races, * and the cgroup is pinned to this child due to cgroup_fork() * is ran before sched_fork(). * * Silence PROVE_RCU. */ raw_spin_lock_irqsave(&p->pi_lock, flags); set_task_cpu(p, cpu);------------------------------------------------------重要一点就是检查p->stack->cpu是不是当期CPU，如果不是则需要进行迁移。迁移函数使用之前确定的sched_class->migrate_task_rq。 raw_spin_unlock_irqrestore(&p->pi_lock, flags); #if defined(CONFIG_SCHEDSTATS) || defined(CONFIG_TASK_DELAY_ACCT) if (likely(sched_info_on())) memset(&p->sched_info, 0, sizeof(p->sched_info)); #endif #if defined(CONFIG_SMP) p->on_cpu = 0; #endif init_task_preempt_count(p); #ifdef CONFIG_SMP plist_node_init(&p->pushable_tasks, MAX_PRIO); RB_CLEAR_NODE(&p->pushable_dl_tasks); #endif put_cpu();-----------------------------------------------------------------再次允许内核抢占。 return 0; }
copy_mm首先设置MM相关参数，然后使用dup_mm来分配mm_struct数据结构，并从父进程复制到新进程mm_struct。
最后将创建的mm_struct复制给task_struct->mm。
static int copy_mm(unsigned long clone_flags, struct task_struct *tsk) { struct mm_struct *mm, *oldmm; int retval; tsk->min_flt = tsk->maj_flt = 0; tsk->nvcsw = tsk->nivcsw = 0; #ifdef CONFIG_DETECT_HUNG_TASK tsk->last_switch_count = tsk->nvcsw + tsk->nivcsw; #endif tsk->mm = NULL; tsk->active_mm = NULL; /* * Are we cloning a kernel thread? * * We need to steal a active VM for that.. */ oldmm = current->mm; if (!oldmm)-----------------------------------------------如果current->mm为NULL，表示是内核线程。 return 0; /* initialize the new vmacache entries */ vmacache_flush(tsk); if (clone_flags & CLONE_VM) {----------------------------CLONE_VM表示父子进程共享内存空间，依次没必要新建内存空间，直接使用oldmm。 atomic_inc(&oldmm->mm_users); mm = oldmm; goto good_mm; } retval = -ENOMEM; mm = dup_mm(tsk);---------------------------------------为子进程单独创建一个新的内存空间mm_struct。 if (!mm) goto fail_nomem; good_mm: tsk->mm = mm;-------------------------------------------对新进程内存空间进行赋值。 tsk->active_mm = mm; return 0; fail_nomem: return retval; }
dup_task从父进程复制mm_struct，然后进行初始化等操作，将完成的mm_struct返回给copy_mm。
/* * Allocate a new mm structure and copy contents from the * mm structure of the passed in task structure. */ static struct mm_struct *dup_mm(struct task_struct *tsk) { struct mm_struct *mm, *oldmm = current->mm; int err; mm = allocate_mm();-----------------------------------分配一个mm_struct数据结构 if (!mm) goto fail_nomem; memcpy(mm, oldmm, sizeof(*mm));-----------------------将父进程mm_struct复制到新进程mm_struct。 if (!mm_init(mm, tsk))--------------------------------主要对子进程的mm_struct成员进行初始化，虽然从父进程复制了相关数据，但是对于子进程需要重新进行初始化。 goto fail_nomem; dup_mm_exe_file(oldmm, mm); err = dup_mmap(mm, oldmm);----------------------------将父进程种所有VMA对应的pte页表项内容都复制到子进程对应的PTE页表项中。 if (err) goto free_pt; mm->hiwater_rss = get_mm_rss(mm); mm->hiwater_vm = mm->total_vm; if (mm->binfmt && !try_module_get(mm->binfmt->module)) goto free_pt; return mm; ... }
对ARM体系结构，Linux内核栈顶存放着ARM通用寄存器struct pt_regs。
struct pt_regs { unsigned long uregs[18]; }; #define ARM_cpsr uregs[16] #define ARM_pc uregs[15] #define ARM_lr uregs[14] #define ARM_sp uregs[13] #define ARM_ip uregs[12] #define ARM_fp uregs[11] #define ARM_r10 uregs[10] #define ARM_r9 uregs[9] #define ARM_r8 uregs[8] #define ARM_r7 uregs[7] #define ARM_r6 uregs[6] #define ARM_r5 uregs[5] #define ARM_r4 uregs[4] #define ARM_r3 uregs[3] #define ARM_r2 uregs[2] #define ARM_r1 uregs[1] #define ARM_r0 uregs[0] #define ARM_ORIG_r0 uregs[17]
关于pt_regs在内核栈的位置，可以看出首先通过task_stack_page(p)站到内核栈起始地址，即底部。
然后加上地址THREAD_START_SP，即THREAD_SIZE两个页面8KB减去8字节空洞。
所以childregs指向的位置是栈顶部。
#define task_pt_regs(p) \ ((struct pt_regs *)(THREAD_START_SP + task_stack_page(p)) - 1)
copy_thread首先获取栈顶pt_regs位置，然后填充thread_info->cpu_context进程上下文。
asmlinkage void ret_from_fork(void) __asm__("ret_from_fork"); int copy_thread(unsigned long clone_flags, unsigned long stack_start, unsigned long stk_sz, struct task_struct *p) { struct thread_info *thread = task_thread_info(p);--------------------------获取当前进程的thread_info。 struct pt_regs *childregs = task_pt_regs(p);-------------------------------获取当前进程的pt_regs memset(&thread->cpu_context, 0, sizeof(struct cpu_context_save));----------cpu_context中保存了进程上下文相关的通用寄存器。 if (likely(!(p->flags & PF_KTHREAD))) {------------------------------------内核线程处理 *childregs = *current_pt_regs(); childregs->ARM_r0 = 0; if (stack_start) childregs->ARM_sp = stack_start; } else {-------------------------------------------------------------------普通线程处理，r4等于stk_sz，r5指向start_start。 memset(childregs, 0, sizeof(struct pt_regs)); thread->cpu_context.r4 = stk_sz; thread->cpu_context.r5 = stack_start; childregs->ARM_cpsr = SVC_MODE; } thread->cpu_context.pc = (unsigned long)ret_from_fork;---------------------cpu_context中pc指向ret_from_fork thread->cpu_context.sp = (unsigned long)childregs;-------------------------cpu_context中sp指向新进程的内核栈 clear_ptrace_hw_breakpoint(p); if (clone_flags & CLONE_SETTLS) thread->tp_value[0] = childregs->ARM_r3; thread->tp_value[1] = get_tpuser(); thread_notify(THREAD_NOTIFY_COPY, thread); return 0; }
3. 关于fork()、vfork()、clone()测试
3.1 fork()嵌套打印
3.1.1 代码
#include <stdio.h> int main(void) { int i; for(i = 0; i<2; i++) { fork(); printf("_%d-%d-%d\n", getppid(), getpid(), i); } wait(NULL); wait(NULL); return 0; }
3.1.2 执行程序，记录log
执行输出结果如下：
sudo trace-cmd record -e all ./fork
/sys/kernel/tracing/events/*/filter
Current:4293-i=0
Current:4293-i=1
Current:4294-i=0
Current:4294-i=1
Current:4295-i=1
Current:4296-i=1
相关Trace记录在trace.dat中。
3.1.3 流程分析
使用kernelshark trace.dat，过滤sched_process_fork/sys_enter_write/sys_enter_wait4后结果如下。
其中sched_process_fork对应fork，sys_enter_write对应printf，sys_enter_wait4对应wait开始，sys_exit_wait4对应wait结束。
下图是不同进程的流程：
将fork进程关系流程图画出如下：
参考文档：《linux中fork（）函数详解（原创！！实例讲解）》
3.2 fork()、vfork()、clone()对比
对于fork()、vfork()、clone()三者的区别，前面已经有介绍，下面通过实例来看他们之间的区别。
3.2.1 fork()和vfork()对比
#include "stdio.h" int main() { int count = 1; int child; printf("Father, initial count = %d, pid = %d\n", count, getpid()); if(!(child = fork())) { int i; for(i = 0; i < 2; i++) { printf("Son, count = %d pid = %d\n", ++count, getpid()); } exit(1); } else {
sleep（1); printf("Father, count = %d pid = %d child = %d\n", count, getpid(), child); } } #include "stdio.h" int main() { int count = 1; int child; printf("Father, initial count = %d, pid = %d\n", count, getpid()); if(!(child = vfork())) { int i; for(i = 0; i < 2; i++) { printf("Son, count = %d pid = %d\n", ++count, getpid()); } exit(1); } else { printf("Father, count = %d pid = %d child = %d\n", count, getpid(), child); } }
fork输出结果如下：
Father, initial count = 1, pid = 4721
Father, count = 1 pid = 4721 child = 4722
Son, count = 2 pid = 4722
Son, count = 3 pid = 4722
vfork输出结果如下：
Father, initial count = 1, pid = 4726
Son, count = 2 pid = 4727
Son, count = 3 pid = 4727
Father, count = 3 pid = 4726 child = 4727
将fork代码加sleep(1);之后结果如下：
Father, initial count = 1, pid = 4858
Son, count = 2 pid = 4859
Son, count = 3 pid = 4859
Father, count = 1 pid = 4858 child = 4859
1. 可以看出vfork父进程在等待子进程结束，然后继续执行。
2. vfork父子进程之间共享地址空间，父进程的count被子进程修改。
3. fork将父进程打印延时后，可以看出主进程任然打印count=1，说明父子进程空间独立。
3.2.2 clone不同flag对比
clone的flag决定了clone的行为，比如是否共享空间、是否vfork等
#define _GNU_SOURCE #include "stdio.h" #include "sched.h" #include "signal.h" #define FIBER_STACK 8192 int count; void * stack; int do_something(){ int i; for(i = 0; i < 2; i++) { printf("Son, pid = %d, count = %d\n", getpid(), ++count); } free(stack); //这里我也不清楚，如果这里不释放，不知道子线程死亡后，该内存是否会释放，知情者可以告诉下,谢谢 exit(1); } int main() { void * stack; count = 1; stack = malloc(FIBER_STACK);//为子进程申请系统堆栈 if(!stack) { printf("The stack failed\n"); exit(0); } printf("Father, initial count = %d, pid = %d\n", count, getpid()); clone(&do_something, (char *)stack + FIBER_STACK, CLONE_VM|CLONE_VFORK, 0);//创建子线程 printf("Father, pid = %d count = %d\n", getpid(), count); exit(1); }
下面是不同flag组合的输出结果：
1. CLONE_VM|CLONE_VFORK
父子进程共享内存空间，并且父进程要等待子进程结束。
所以4968在4969结束之后才继续运行，并且count=3。
Father, initial count = 1, pid = 4968
Son, pid = 4969, count = 2
Son, pid = 4969, count = 3
Father, pid = 4968 count = 3
2. CLONE_VM
父子进程共享内存空间，但是父进程结束时强制子进程退出。
Father, initial count = 1, pid = 5017
Father, pid = 5017 count = 1
将父进程printf前加一个sleep(1)，可以看出父进程count=1。
Father, initial count = 1, pid = 5065
Son, pid = 5066, count = 2
Son, pid = 5066, count = 3
Father, pid = 5065 count = 3
3. CLONE_VFORK
这里没有共享内存空间，但是父进程要等待子进程结束。
所以父进程在子进程后打印，且count=3。
Father, initial count = 1, pid = 4998
Son, pid = 4999, count = 2
Son, pid = 4999, count = 3
Father, pid = 4998 count = 1
4. 0
父子进程不共享内存，但是父进程在结束时继续等待子进程退出。
这里看不出count是否共享。
Father, initial count = 1, pid = 5174
Father, pid = 5174 count = 1
Son, pid = 5175, count = 2
Son, pid = 5175, count = 3
在父进程printf之前加sleep(1)，结果如下：
和预期一样，主进程count是单独一份，而没有和子进程共用。
Father, initial count = 1, pid = 5257
Son, pid = 5258, count = 2
Son, pid = 5258, count = 3
Father, pid = 5257 count = 1
参考文档：linux系统调用fork, vfork, clone