Linux程序管理之核心執行緒
阿新 • • 發佈：2019-01-04
核心原始碼：linux-2.6.38.8.tar.bz2
目標平臺：ARM體系結構
在Linux系統中，程序和執行緒都使用task_struct結構體來表示，執行緒只不過是一種特殊（所謂的特殊也不過是在被建立時使用不同的clone標誌組合而已）的程序罷了。
核心執行緒只執行在核心態，只能使用大於PAGE_OFFSET的線性地址空間。
1、程序0
在Linux系統中，只有這個程序是靜態分配的。
/* linux-2.6.38.8/arch/arm/kernel/init_task.c */ struct task_struct init_task = INIT_TASK(init_task); union thread_union init_thread_union __init_task_data = { INIT_THREAD_INFO(init_task) };
然後，通過INIT_TASK巨集初始化init_task結構體，通過INIT_THREAD_INFO巨集初始化thread_info結構體。
/* linux-2.6.38.8/include/linux/init_task.h */ #define INIT_TASK(tsk) \ { \ .state = 0, \ .stack = &init_thread_info, \ .usage = ATOMIC_INIT(2), \ .flags = PF_KTHREAD, \ .lock_depth = -1, \ .prio = MAX_PRIO-20, \ .static_prio = MAX_PRIO-20, \ .normal_prio = MAX_PRIO-20, \ .policy = SCHED_NORMAL, \ .cpus_allowed = CPU_MASK_ALL, \ .mm = NULL, \ .active_mm = &init_mm, \ .se = { \ .group_node = LIST_HEAD_INIT(tsk.se.group_node), \ }, \ .rt = { \ .run_list = LIST_HEAD_INIT(tsk.rt.run_list), \ .time_slice = HZ, \ .nr_cpus_allowed = NR_CPUS, \ }, \ .tasks = LIST_HEAD_INIT(tsk.tasks), \ INIT_PUSHABLE_TASKS(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_INIT_POINTER(.real_cred, &init_cred), \ RCU_INIT_POINTER(.cred, &init_cred), \ .comm = "swapper", \ .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), \ .fs_excl = ATOMIC_INIT(0), \ .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), \ .dirties = INIT_PROP_LOCAL_SINGLE(dirties), \ INIT_IDS \ INIT_PERF_EVENTS(tsk) \ INIT_TRACE_IRQFLAGS \ INIT_LOCKDEP \ INIT_FTRACE_GRAPH \ INIT_TRACE_RECURSION \ INIT_TASK_RCU_PREEMPT(tsk) \ }
/* linux-2.6.38.8/arch/arm/include/asm/thread_info.h */ #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), \ .restart_block = { \ .fn = do_no_restart_syscall, \ }, \ }
2、程序1和kthreadd核心執行緒
/* linux-2.6.38.8/init/main.c */
kernel_thread(kernel_init, NULL, CLONE_FS | CLONE_SIGHAND);
pid = kernel_thread(kthreadd, NULL, CLONE_FS | CLONE_FILES);
kthreadd_task = find_task_by_pid_ns(pid, &init_pid_ns);/* linux-2.6.38.8/arch/arm/kernel/process.c */
pid_t kernel_thread(int (*fn)(void *), void *arg, unsigned long flags)
{
struct pt_regs regs;
memset(®s, 0, sizeof(regs));
regs.ARM_r4 = (unsigned long)arg;
regs.ARM_r5 = (unsigned long)fn;
regs.ARM_r6 = (unsigned long)kernel_thread_exit;
regs.ARM_r7 = SVC_MODE | PSR_ENDSTATE | PSR_ISETSTATE;
regs.ARM_pc = (unsigned long)kernel_thread_helper;
regs.ARM_cpsr = regs.ARM_r7 | PSR_I_BIT;
return do_fork(flags|CLONE_VM|CLONE_UNTRACED, 0, ®s, 0, NULL, NULL);
}當建立的程序被排程時，就會執行fn所指向的函式，引數arg用來給fn所指向的函式傳遞引數。引數flags用來儲存傳遞給do_fork函式的clone標誌。
核心執行緒kthreadd用來建立其它的核心執行緒。在Ubuntu 11.04中，所有其它的核心執行緒的父程序都是此執行緒。
$ ps -ef | grep "00 \[.*\]$" //在這裡，核心執行緒都用中括號括起來以示區別root 2 0 0 12:48 ? 00:00:00 [kthreadd]
root 3 2 0 12:48 ? 00:00:00 [ksoftirqd/0]
root 4 2 0 12:48 ? 00:00:00 [kworker/0:0]
root 6 2 0 12:48 ? 00:00:00 [migration/0]
root 7 2 0 12:48 ? 00:00:00 [cpuset]
root 8 2 0 12:48 ? 00:00:00 [khelper]
root 9 2 0 12:48 ? 00:00:00 [netns]
root 10 2 0 12:48 ? 00:00:00 [sync_supers]
root 11 2 0 12:48 ? 00:00:00 [bdi-default]
root 12 2 0 12:48 ? 00:00:00 [kintegrityd]
root 13 2 0 12:48 ? 00:00:00 [kblockd]
root 14 2 0 12:48 ? 00:00:00 [kacpid]
root 15 2 0 12:48 ? 00:00:00 [kacpi_notify]
root 16 2 0 12:48 ? 00:00:00 [kacpi_hotplug]
root 17 2 0 12:48 ? 00:00:00 [ata_sff]
root 18 2 0 12:48 ? 00:00:00 [khubd]
root 19 2 0 12:48 ? 00:00:00 [md]
root 22 2 0 12:48 ? 00:00:00 [khungtaskd]
root 23 2 0 12:48 ? 00:00:00 [kswapd0]
root 24 2 0 12:48 ? 00:00:00 [ksmd]
root 25 2 0 12:48 ? 00:00:00 [fsnotify_mark]
root 26 2 0 12:48 ? 00:00:00 [aio]
root 27 2 0 12:48 ? 00:00:00 [ecryptfs-kthrea]
root 28 2 0 12:48 ? 00:00:00 [crypto]
root 32 2 0 12:48 ? 00:00:00 [kthrotld]
root 34 2 0 12:48 ? 00:00:00 [scsi_eh_0]
root 35 2 0 12:48 ? 00:00:00 [scsi_eh_1]
root 36 2 0 12:48 ? 00:00:00 [kworker/u:3]
root 38 2 0 12:48 ? 00:00:00 [kworker/0:2]
root 39 2 0 12:48 ? 00:00:00 [kmpathd]
root 40 2 0 12:48 ? 00:00:00 [kmpath_handlerd]
root 41 2 0 12:48 ? 00:00:00 [kondemand]
root 42 2 0 12:48 ? 00:00:00 [kconservative]
root 192 2 0 12:48 ? 00:00:00 [mpt_poll_0]
root 193 2 0 12:48 ? 00:00:00 [mpt/0]
root 194 2 0 12:48 ? 00:00:00 [scsi_eh_2]
root 210 2 0 12:48 ? 00:00:00 [kjournald]
root 448 2 0 12:50 ? 00:00:00 [kpsmoused]
root 486 2 0 12:50 ? 00:00:00 [vmmemctl]
root 642 2 0 12:50 ? 00:00:00 [rpciod]
root 659 2 0 12:50 ? 00:00:00 [nfsiod]
root 703 2 0 12:50 ? 00:00:00 [flush-8:0]
root 876 2 0 12:50 ? 00:00:00 [lockd]
root 877 2 0 12:50 ? 00:00:00 [nfsd4]
root 878 2 0 12:50 ? 00:00:00 [nfsd4_callbacks]
root 879 2 0 12:50 ? 00:00:00 [nfsd]
root 880 2 0 12:50 ? 00:00:00 [nfsd]
root 881 2 0 12:50 ? 00:00:00 [nfsd]
root 882 2 0 12:50 ? 00:00:00 [nfsd]
root 883 2 0 12:50 ? 00:00:00 [nfsd]
root 884 2 0 12:50 ? 00:00:00 [nfsd]
root 885 2 0 12:50 ? 00:00:00 [nfsd]
root 886 2 0 12:50 ? 00:00:00 [nfsd]其中用來建立其它核心執行緒的函式如下：
/* linux-2.6.38.8/kernel/kthread.c */
int kthreadd(void *unused)
{
struct task_struct *tsk = current;
/* Setup a clean context for our children to inherit. */
set_task_comm(tsk, "kthreadd");
ignore_signals(tsk);
set_cpus_allowed_ptr(tsk, cpu_all_mask);
set_mems_allowed(node_states[N_HIGH_MEMORY]);
current->flags |= PF_NOFREEZE | PF_FREEZER_NOSIG;
for (;;) {
set_current_state(TASK_INTERRUPTIBLE);
if (list_empty(&kthread_create_list))
schedule();
__set_current_state(TASK_RUNNING);
spin_lock(&kthread_create_lock);
while (!list_empty(&kthread_create_list)) {
struct kthread_create_info *create;
create = list_entry(kthread_create_list.next,
struct kthread_create_info, list);
list_del_init(&create->list);
spin_unlock(&kthread_create_lock);
create_kthread(create);
spin_lock(&kthread_create_lock);
}
spin_unlock(&kthread_create_lock);
}
return 0;
}kthreadd函式通過死迴圈不斷地檢查kthread_create_list連結串列是否為空，如果不為空，則呼叫create_kthread函式來建立新的核心執行緒。
/* linux-2.6.38.8/kernel/kthread.c */
static void create_kthread(struct kthread_create_info *create)
{
int pid;
/* We want our own signal handler (we take no signals by default). */
pid = kernel_thread(kthread, create, CLONE_FS | CLONE_FILES | SIGCHLD);
if (pid < 0) {
create->result = ERR_PTR(pid);
complete(&create->done);
}
}在Linux核心中，可通過呼叫kthread_create函式來發起核心執行緒的建立。
/* linux-2.6.38.8/kernel/kthread.c */
struct task_struct *kthread_create(int (*threadfn)(void *data),
void *data,
const char namefmt[],
...)
{
struct kthread_create_info create;
create.threadfn = threadfn;
create.data = data;
init_completion(&create.done);
spin_lock(&kthread_create_lock);
list_add_tail(&create.list, &kthread_create_list);
spin_unlock(&kthread_create_lock);
wake_up_process(kthreadd_task); //喚醒kthreadd來建立核心執行緒
wait_for_completion(&create.done); //等待核心執行緒建立完畢
if (!IS_ERR(create.result)) {
static const struct sched_param param = { .sched_priority = 0 };
va_list args;
va_start(args, namefmt);
vsnprintf(create.result->comm, sizeof(create.result->comm),
namefmt, args);
va_end(args);
/*
* root may have changed our (kthreadd's) priority or CPU mask.
* The kernel thread should not inherit these properties.
*/
sched_setscheduler_nocheck(create.result, SCHED_NORMAL, ¶m);
set_cpus_allowed_ptr(create.result, cpu_all_mask);
}
return create.result;
}