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进程：是指计算机中已运行的程序。进程本身不是基本的运行单位，而是线程的容器。程序本身只是指令、数据及其组织形式的描述，进程才是程序（那些指令和数据）的真正运行实例。
进程是程序动态的形式。
进程生命周期Linux操作系统属于多任务操作系统，系统中的每个进程能够分时复用CPU时间片，通过有效的进程调度策略实现多任务并行执行。而进程在被CPU调度运行，等待CPU资源分配以及等待外部事件时会属于不同的状态。进程之间的状态关系：
运行：该进程此刻正在执行。
等待：进程能够运行，但没有得到许可，因为CPU分配给另一个进程。调度器可以在
下一次任务切换时选择该进程。
睡眠：进程正在睡眠无法运行，因为它在等待一个外部事件。调度器无法在下一次任
务切换时选择该进程。
Linux内核涉及进程和程序的所有算法都围绕一个名为task_struct的数据结构建立，该结构定义在include/linux/sched.h中。这是系统中主要的一个结构。在阐述调度器的实现之前，了解一下Linux管理进程的方式是很有必要的。task_struct包含很多成员，将进程与各个内核子系统联系，task_struct定义如下：
1 struct task_struct {
进程状态 2 volatile long state; /* -1 unrunnable, 0 runnable, >0 stopped */ 3 void *stack; 内核栈 4 atomic_t usage; 有几个进程在使用此结构 5 unsigned int flags; /* per process flags, defined below */ 6 unsigned int ptrace; 断点调试使用 7
多处理使用 8 #ifdef CONFIG_SMP 9 struct llist_node wake_entry; 10 int on_cpu; 11 unsigned int wakee_flips; 12 unsigned long wakee_flip_decay_ts; 13 struct task_struct *last_wakee; 14 15 int wake_cpu; 16 #endif 17 int on_rq; 18 19 int prio, static_prio, normal_prio; 20 unsigned int rt_priority; 优先级
//进程调度
21 const struct sched_class *sched_class; 22 struct sched_entity se; 23 struct sched_rt_entity rt; 24 #ifdef CONFIG_CGROUP_SCHED 25 struct task_group *sched_task_group; 26 #endif 27 struct sched_dl_entity dl; 28 29 #ifdef CONFIG_PREEMPT_NOTIFIERS 30 /* list of struct preempt_notifier: */ 31 struct hlist_head preempt_notifiers; 32 #endif 33 34 #ifdef CONFIG_BLK_DEV_IO_TRACE 35 unsigned int btrace_seq; 36 #endif 37 38 unsigned int policy; 39 int nr_cpus_allowed; 40 cpumask_t cpus_allowed; 41 42 #ifdef CONFIG_PREEMPT_RCU 43 int rcu_read_lock_nesting; 44 union rcu_special rcu_read_unlock_special; 45 struct list_head rcu_node_entry; 46 struct rcu_node *rcu_blocked_node; 47 #endif /* #ifdef CONFIG_PREEMPT_RCU */ 48 #ifdef CONFIG_TASKS_RCU 49 unsigned long rcu_tasks_nvcsw; 50 bool rcu_tasks_holdout; 51 struct list_head rcu_tasks_holdout_list; 52 int rcu_tasks_idle_cpu; 53 #endif /* #ifdef CONFIG_TASKS_RCU */ 54 55 #ifdef CONFIG_SCHED_INFO 56 struct sched_info sched_info; 57 #endif 58 59 struct list_head tasks; 60 #ifdef CONFIG_SMP 61 struct plist_node pushable_tasks; 62 struct rb_node pushable_dl_tasks; 63 #endif 64 65 struct mm_struct *mm, *active_mm; 66 /* per-thread vma caching */ 67 u32 vmacache_seqnum; 68 struct vm_area_struct *vmacache[VMACACHE_SIZE]; 69 #if defined(SPLIT_RSS_COUNTING) 70 struct task_rss_stat rss_stat; 71 #endif 72 /* task state */ 73 int exit_state; 74 int exit_code, exit_signal; 75 int pdeath_signal; /* The signal sent when the parent dies */ 76 unsigned long jobctl; /* JOBCTL_*, siglock protected */ 77 78 /* Used for emulating ABI behavior of previous Linux versions */ 79 unsigned int personality; 80 81 /* scheduler bits, serialized by scheduler locks */ 82 unsigned sched_reset_on_fork:1; 83 unsigned sched_contributes_to_load:1; 84 unsigned sched_migrated:1; 85 unsigned :0; /* force alignment to the next boundary */ 86 87 /* unserialized, strictly 'current' */ 88 unsigned in_execve:1; /* bit to tell LSMs we're in execve */ 89 unsigned in_iowait:1; 90 #ifdef CONFIG_MEMCG 91 unsigned memcg_may_oom:1; 92 #endif 93 #ifdef CONFIG_MEMCG_KMEM 94 unsigned memcg_kmem_skip_account:1; 95 #endif 96 #ifdef CONFIG_COMPAT_BRK 97 unsigned brk_randomized:1; 98 #endif 99 100 unsigned long atomic_flags; /* Flags needing atomic access. */ 101 102 struct restart_block restart_block; 103 104 pid_t pid; 进程和父进程的pid 105 pid_t tgid; 106 107 #ifdef CONFIG_CC_STACKPROTECTOR 108 /* Canary value for the -fstack-protector gcc feature */ 109 unsigned long stack_canary; 110 #endif 111 /* 112 * pointers to (original) parent process, youngest child, younger sibling, 113 * older sibling, respectively. (p->father can be replaced with 114 * p->real_parent->pid) 115 */
父进程
116 struct task_struct __rcu *real_parent; /* real parent process */
接收终止进程
117 struct task_struct __rcu *parent; /* recipient of SIGCHLD, wait4() reports */ 118 /* 119 * children/sibling forms the list of my natural children 120 */
子进程链表
121 struct list_head children; /* list of my children */
兄弟进程链表
122 struct list_head sibling; /* linkage in my parent's children list */
线程组组织
123 struct task_struct *group_leader; /* threadgroup leader */ 124 125 /* 126 * ptraced is the list of tasks this task is using ptrace on. 127 * This includes both natural children and PTRACE_ATTACH targets. 128 * p->ptrace_entry is p's link on the p->parent->ptraced list. 129 */
系统调用调试使用
130 struct list_head ptraced; 131 struct list_head ptrace_entry; 132 133 /* PID/PID hash table linkage. */ 134 struct pid_link pids[PIDTYPE_MAX]; 135 struct list_head thread_group; 136 struct list_head thread_node; 137 138 struct completion *vfork_done; /* for vfork() */ 139 int __user *set_child_tid; /* CLONE_CHILD_SETTID */ 140 int __user *clear_child_tid; /* CLONE_CHILD_CLEARTID */ 141
utime 用户态执行时间 stime 内核态执行时间 142 cputime_t utime, stime, utimescaled, stimescaled; 143 cputime_t gtime; 144 struct prev_cputime prev_cputime; 145 #ifdef CONFIG_VIRT_CPU_ACCOUNTING_GEN 146 seqlock_t vtime_seqlock; 147 unsigned long long vtime_snap; 148 enum { 149 VTIME_SLEEPING = 0, 150 VTIME_USER, 151 VTIME_SYS, 152 } vtime_snap_whence; 153 #endif 154 unsigned long nvcsw, nivcsw; /* context switch counts */ 155 u64 start_time; /* monotonic time in nsec */ 156 u64 real_start_time; /* boot based time in nsec */ 157 /* mm fault and swap info: this can arguably be seen as either mm-specific or thread-specific */ 158 unsigned long min_flt, maj_flt; 159 160 struct task_cputime cputime_expires; 161 struct list_head cpu_timers[3]; 162 163 /* process credentials */ 164 const struct cred __rcu *real_cred; /* objective and real subjective task 165 * credentials (COW) */ 166 const struct cred __rcu *cred; /* effective (overridable) subjective task 167 * credentials (COW) */ 168 char comm[TASK_COMM_LEN]; /* executable name excluding path 169 - access with [gs]et_task_comm (which lock 170 it with task_lock()) 171 - initialized normally by setup_new_exec */ 172 /* file system info */ 173 struct nameidata *nameidata; 174 #ifdef CONFIG_SYSVIPC 175 /* ipc stuff */ 176 struct sysv_sem sysvsem; 177 struct sysv_shm sysvshm; 178 #endif 179 #ifdef CONFIG_DETECT_HUNG_TASK 180 /* hung task detection */ 181 unsigned long last_switch_count; 182 #endif 183 /* filesystem information */ 184 struct fs_struct *fs; 185 /* open file information */ 186 struct files_struct *files; 187 /* namespaces */ 188 struct nsproxy *nsproxy; 189 /* signal handlers */ 190 struct signal_struct *signal; 191 struct sighand_struct *sighand; 192 193 sigset_t blocked, real_blocked; 194 sigset_t saved_sigmask; /* restored if set_restore_sigmask() was used */ 195 struct sigpending pending; 196 197 unsigned long sas_ss_sp; 198 size_t sas_ss_size; 199 200 struct callback_head *task_works; 201 202 struct audit_context *audit_context; 203 #ifdef CONFIG_AUDITSYSCALL 204 kuid_t loginuid; 205 unsigned int sessionid; 206 #endif 207 struct seccomp seccomp; 208 209 /* Thread group tracking */ 210 u32 parent_exec_id; 211 u32 self_exec_id; 212 /* Protection of (de-)allocation: mm, files, fs, tty, keyrings, mems_allowed, 213 * mempolicy */ 214 spinlock_t alloc_lock; 215 216 /* Protection of the PI data structures: */ 217 raw_spinlock_t pi_lock; 218 219 struct wake_q_node wake_q; 220 221 #ifdef CONFIG_RT_MUTEXES 222 /* PI waiters blocked on a rt_mutex held by this task */ 223 struct rb_root pi_waiters; 224 struct rb_node *pi_waiters_leftmost; 225 /* Deadlock detection and priority inheritance handling */ 226 struct rt_mutex_waiter *pi_blocked_on; 227 #endif 228 229 #ifdef CONFIG_DEBUG_MUTEXES 230 /* mutex deadlock detection */ 231 struct mutex_waiter *blocked_on; 232 #endif 233 #ifdef CONFIG_TRACE_IRQFLAGS 234 unsigned int irq_events; 235 unsigned long hardirq_enable_ip; 236 unsigned long hardirq_disable_ip; 237 unsigned int hardirq_enable_event; 238 unsigned int hardirq_disable_event; 239 int hardirqs_enabled; 240 int hardirq_context; 241 unsigned long softirq_disable_ip; 242 unsigned long softirq_enable_ip; 243 unsigned int softirq_disable_event; 244 unsigned int softirq_enable_event; 245 int softirqs_enabled; 246 int softirq_context; 247 #endif 248 #ifdef CONFIG_LOCKDEP 249 # define MAX_LOCK_DEPTH 48UL 250 u64 curr_chain_key; 251 int lockdep_depth; 252 unsigned int lockdep_recursion; 253 struct held_lock held_locks[MAX_LOCK_DEPTH]; 254 gfp_t lockdep_reclaim_gfp; 255 #endif 256 257 /* journalling filesystem info */ 258 void *journal_info; 259 260 /* stacked block device info */ 261 struct bio_list *bio_list; 262 263 #ifdef CONFIG_BLOCK 264 /* stack plugging */ 265 struct blk_plug *plug; 266 #endif 267 268 /* VM state */ 269 struct reclaim_state *reclaim_state; 270 271 struct backing_dev_info *backing_dev_info; 272 273 struct io_context *io_context; 274 275 unsigned long ptrace_message; 276 siginfo_t *last_siginfo; /* For ptrace use. */ 277 struct task_io_accounting ioac; 278 #if defined(CONFIG_TASK_XACCT) 279 u64 acct_rss_mem1; /* accumulated rss usage */ 280 u64 acct_vm_mem1; /* accumulated virtual memory usage */ 281 cputime_t acct_timexpd; /* stime + utime since last update */ 282 #endif 283 #ifdef CONFIG_CPUSETS 284 nodemask_t mems_allowed; /* Protected by alloc_lock */ 285 seqcount_t mems_allowed_seq; /* Seqence no to catch updates */ 286 int cpuset_mem_spread_rotor; 287 int cpuset_slab_spread_rotor; 288 #endif 289 #ifdef CONFIG_CGROUPS 290 /* Control Group info protected by css_set_lock */ 291 struct css_set __rcu *cgroups; 292 /* cg_list protected by css_set_lock and tsk->alloc_lock */ 293 struct list_head cg_list; 294 #endif 295 #ifdef CONFIG_FUTEX 296 struct robust_list_head __user *robust_list; 297 #ifdef CONFIG_COMPAT 298 struct compat_robust_list_head __user *compat_robust_list; 299 #endif 300 struct list_head pi_state_list; 301 struct futex_pi_state *pi_state_cache; 302 #endif 303 #ifdef CONFIG_PERF_EVENTS 304 struct perf_event_context *perf_event_ctxp[perf_nr_task_contexts]; 305 struct mutex perf_event_mutex; 306 struct list_head perf_event_list; 307 #endif 308 #ifdef CONFIG_DEBUG_PREEMPT 309 unsigned long preempt_disable_ip; 310 #endif 311 #ifdef CONFIG_NUMA 312 struct mempolicy *mempolicy; /* Protected by alloc_lock */ 313 short il_next; 314 short pref_node_fork; 315 #endif 316 #ifdef CONFIG_NUMA_BALANCING 317 int numa_scan_seq; 318 unsigned int numa_scan_period; 319 unsigned int numa_scan_period_max; 320 int numa_preferred_nid; 321 unsigned long numa_migrate_retry; 322 u64 node_stamp; /* migration stamp */ 323 u64 last_task_numa_placement; 324 u64 last_sum_exec_runtime; 325 struct callback_head numa_work; 326 327 struct list_head numa_entry; 328 struct numa_group *numa_group; 329 330 /* 331 * numa_faults is an array split into four regions: 332 * faults_memory, faults_cpu, faults_memory_buffer, faults_cpu_buffer 333 * in this precise order. 334 * 335 * faults_memory: Exponential decaying average of faults on a per-node 336 * basis. Scheduling placement decisions are made based on these 337 * counts. The values remain static for the duration of a PTE scan. 338 * faults_cpu: Track the nodes the process was running on when a NUMA 339 * hinting fault was incurred. 340 * faults_memory_buffer and faults_cpu_buffer: Record faults per node 341 * during the current scan window. When the scan completes, the counts 342 * in faults_memory and faults_cpu decay and these values are copied. 343 */ 344 unsigned long *numa_faults; 345 unsigned long total_numa_faults; 346 347 /* 348 * numa_faults_locality tracks if faults recorded during the last 349 * scan window were remote/local or failed to migrate. The task scan 350 * period is adapted based on the locality of the faults with different 351 * weights depending on whether they were shared or private faults 352 */ 353 unsigned long numa_faults_locality[3]; 354 355 unsigned long numa_pages_migrated; 356 #endif /* CONFIG_NUMA_BALANCING */ 357 358 #ifdef CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH 359 struct tlbflush_unmap_batch tlb_ubc; 360 #endif 361 362 struct rcu_head rcu; 363 364 /* 365 * cache last used pipe for splice 366 */ 367 struct pipe_inode_info *splice_pipe; 368 369 struct page_frag task_frag; 370 371 #ifdef CONFIG_TASK_DELAY_ACCT 372 struct task_delay_info *delays; 373 #endif 374 #ifdef CONFIG_FAULT_INJECTION 375 int make_it_fail; 376 #endif 377 /* 378 * when (nr_dirtied >= nr_dirtied_pause), it's time to call 379 * balance_dirty_pages() for some dirty throttling pause 380 */ 381 int nr_dirtied; 382 int nr_dirtied_pause; 383 unsigned long dirty_paused_when; /* start of a write-and-pause period */ 384 385 #ifdef CONFIG_LATENCYTOP 386 int latency_record_count; 387 struct latency_record latency_record[LT_SAVECOUNT]; 388 #endif 389 /* 390 * time slack values; these are used to round up poll() and 391 * select() etc timeout values. These are in nanoseconds. 392 */ 393 unsigned long timer_slack_ns; 394 unsigned long default_timer_slack_ns; 395 396 #ifdef CONFIG_KASAN 397 unsigned int kasan_depth; 398 #endif 399 #ifdef CONFIG_FUNCTION_GRAPH_TRACER 400 /* Index of current stored address in ret_stack */ 401 int curr_ret_stack; 402 /* Stack of return addresses for return function tracing */ 403 struct ftrace_ret_stack *ret_stack; 404 /* time stamp for last schedule */ 405 unsigned long long ftrace_timestamp; 406 /* 407 * Number of functions that haven't been traced 408 * because of depth overrun. 409 */ 410 atomic_t trace_overrun; 411 /* Pause for the tracing */ 412 atomic_t tracing_graph_pause; 413 #endif 414 #ifdef CONFIG_TRACING 415 /* state flags for use by tracers */ 416 unsigned long trace; 417 /* bitmask and counter of trace recursion */ 418 unsigned long trace_recursion; 419 #endif /* CONFIG_TRACING */ 420 #ifdef CONFIG_MEMCG 421 struct mem_cgroup *memcg_in_oom; 422 gfp_t memcg_oom_gfp_mask; 423 int memcg_oom_order; 424 425 /* number of pages to reclaim on returning to userland */ 426 unsigned int memcg_nr_pages_over_high; 427 #endif 428 #ifdef CONFIG_UPROBES 429 struct uprobe_task *utask; 430 #endif 431 #if defined(CONFIG_BCACHE) || defined(CONFIG_BCACHE_MODULE) 432 unsigned int sequential_io; 433 unsigned int sequential_io_avg; 434 #endif 435 #ifdef CONFIG_DEBUG_ATOMIC_SLEEP 436 unsigned long task_state_change; 437 #endif 438 int pagefault_disabled; 439 /* CPU-specific state of this task */ 440 struct thread_struct thread; 441 /* 442 * WARNING: on x86, 'thread_struct' contains a variable-sized 443 * structure. It *MUST* be at the end of 'task_struct'. 444 * 445 * Do not put anything below here! 446 */ 447 };
并非所有进程都具有相同的重要性。除了大多数我们所熟悉的进程优先级之外，进程还有不同的关键度类别，以满足不同需求先进行比较粗糙的划分，进程可以分为 实时进程和 非实时进程 （普通进程）。实时进程优先级（0-99）都比普通 进程的优先级（100-139）高。当系统中有实时进程运行时，普通进程几乎无法分到赶时间片（只能分到5%的CPU时间）。
讨论fork和exec系列系统调用的实现。通常这些调用不是由应用程序直接发出的，而是通过一个中间层调用，即负责与内核通信的C标准库。从用户状态切换到核心态的方法，依不同的体系结构而各有不同。
1、进程复制
传统的UNIX中用于复制进程的系统调用是fork。但它并不是Linux为此实现的唯一调用，
实际上Linux实现了3个。
(1) fork是重量级调用，因为它建立了父进程的一个完整副本，然后作为子进程执行。为减少与该调用相关的工作量，Linux使用了写时复制（copy-on-write）技术。
(2) vfork类似于fork，但并不创建父进程数据的副本。相反，父子进程之间共享数据。这节省了大量CPU时间（如果一个进程操纵共享数据，则另一个会自动注意到）。
(3) clone产生线程，可以对父子进程之间的共享、复制进行精确控制。
五、进程系统调用【写时复制】
内核使用了写时复制（Copy-On-Write，COW）技术，以防止在fork执行时将父进程的所有数据复制到子进程。在调用fork时，内核通常对父进程的每个内存页，都为子进程创建一个相同的副本。
【执行系统调用】
fork、vfork和clone系统调用的入口点分别是sys_fork、sys_vfork和sys_clone函数。其定义依赖于
具体的体系结构，因为在用户空间和内核空间之间传递参数的方法因体系结构而异。
所有3个fork机制最终都调用kernel/fork.c中的do_fork（一个体系结构无关的函数），其代码流程
如图所示。
内核线程是直接由内核本身启动的进程。内核线程实际上是将内核函数委托给独立的
进程，与系统中其他进程“并行”执行（实际上，也并行于内核自身的执行）。内核线程经常
称之为（内核）守护进程。它们用于执行下列任务。
• 周期性地将修改的内存页与页来源块设备同步（例如，使用mmap的文件映射）。
• 如果内存页很少使用，则写入交换区。
• 管理延时动作（deferred action）。
• 实现文件系统的事务日志。
3、退出进程
进程必须用exit系统调用终止。这使得内核有机会将该进程使用的资源释放回系统。见kernel/exit.c——>do_exit。简而言之，该函数的实现就是将各个引用计数器减1，如果引用计数器归0而没有进程再使用对应的结构，那么将相应的内存区域返还给内存理模块。
exit () 系统调用一般由编译器在main函数结束位置添加
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