kernel/async.c

   1 /*
   2  * async.c: Asynchronous function calls for boot performance
   3  *
   4  * (C) Copyright 2009 Intel Corporation
   5  * Author: Arjan van de Ven <arjan@linux.intel.com>
   6  *
   7  * This program is free software; you can redistribute it and/or
   8  * modify it under the terms of the GNU General Public License
   9  * as published by the Free Software Foundation; version 2
  10  * of the License.
  11  */
  12
  13
  14 /*
  15
  16 Goals and Theory of Operation
  17
  18 The primary goal of this feature is to reduce the kernel boot time,
  19 by doing various independent hardware delays and discovery operations
  20 decoupled and not strictly serialized.
  21
  22 More specifically, the asynchronous function call concept allows
  23 certain operations (primarily during system boot) to happen
  24 asynchronously, out of order, while these operations still
  25 have their externally visible parts happen sequentially and in-order.
  26 (not unlike how out-of-order CPUs retire their instructions in order)
  27
  28 Key to the asynchronous function call implementation is the concept of
  29 a "sequence cookie" (which, although it has an abstracted type, can be
  30 thought of as a monotonically incrementing number).
  31
  32 The async core will assign each scheduled event such a sequence cookie and
  33 pass this to the called functions.
  34
  35 The asynchronously called function should before doing a globally visible
  36 operation, such as registering device numbers, call the
  37 async_synchronize_cookie() function and pass in its own cookie. The
  38 async_synchronize_cookie() function will make sure that all asynchronous
  39 operations that were scheduled prior to the operation corresponding with the
  40 cookie have completed.
  41
  42 Subsystem/driver initialization code that scheduled asynchronous probe
  43 functions, but which shares global resources with other drivers/subsystems
  44 that do not use the asynchronous call feature, need to do a full
  45 synchronization with the async_synchronize_full() function, before returning
  46 from their init function. This is to maintain strict ordering between the
  47 asynchronous and synchronous parts of the kernel.
  48
  49 */
  50
  51 #include <linux/async.h>
  52 #include <linux/bug.h>
  53 #include <linux/module.h>
  54 #include <linux/wait.h>
  55 #include <linux/sched.h>
  56 #include <linux/init.h>
  57 #include <linux/kthread.h>
  58 #include <linux/delay.h>
  59 #include <asm/atomic.h>
  60
  61 static async_cookie_t next_cookie = 1;
  62
  63 #define MAX_THREADS     256
  64 #define MAX_WORK        32768
  65
  66 static LIST_HEAD(async_pending);
  67 static LIST_HEAD(async_running);
  68 static DEFINE_SPINLOCK(async_lock);
  69
  70 static int async_enabled = 0;
  71
  72 struct async_entry {
  73         struct list_head list;
  74         async_cookie_t   cookie;
  75         async_func_ptr   *func;
  76         void             *data;
  77         struct list_head *running;
  78 };
  79
  80 static DECLARE_WAIT_QUEUE_HEAD(async_done);
  81 static DECLARE_WAIT_QUEUE_HEAD(async_new);
  82
  83 static atomic_t entry_count;
  84 static atomic_t thread_count;
  85
  86 extern int initcall_debug;
  87
  88
  89 /*
  90  * MUST be called with the lock held!
  91  */
  92 static async_cookie_t  __lowest_in_progress(struct list_head *running)
  93 {
  94         struct async_entry *entry;
  95
  96         if (!list_empty(running)) {
  97                 entry = list_first_entry(running,
  98                         struct async_entry, list);
  99                 return entry->cookie;
 100         }
 101
 102         list_for_each_entry(entry, &async_pending, list)
 103                 if (entry->running == running)
 104                         return entry->cookie;
 105
 106         return next_cookie;     /* "infinity" value */
 107 }
 108
 109 static async_cookie_t  lowest_in_progress(struct list_head *running)
 110 {
 111         unsigned long flags;
 112         async_cookie_t ret;
 113
 114         spin_lock_irqsave(&async_lock, flags);
 115         ret = __lowest_in_progress(running);
 116         spin_unlock_irqrestore(&async_lock, flags);
 117         return ret;
 118 }
 119 /*
 120  * pick the first pending entry and run it
 121  */
 122 static void run_one_entry(void)
 123 {
 124         unsigned long flags;
 125         struct async_entry *entry;
 126         ktime_t calltime, delta, rettime;
 127
 128         /* 1) pick one task from the pending queue */
 129
 130         spin_lock_irqsave(&async_lock, flags);
 131         if (list_empty(&async_pending))
 132                 goto out;
 133         entry = list_first_entry(&async_pending, struct async_entry, list);
 134
 135         /* 2) move it to the running queue */
 136         list_move_tail(&entry->list, entry->running);
 137         spin_unlock_irqrestore(&async_lock, flags);
 138
 139         /* 3) run it (and print duration)*/
 140         if (initcall_debug && system_state == SYSTEM_BOOTING) {
 141                 printk("calling  %lli_%pF @ %i\n", (long long)entry->cookie,
 142                         entry->func, task_pid_nr(current));
 143                 calltime = ktime_get();
 144         }
 145         entry->func(entry->data, entry->cookie);
 146         if (initcall_debug && system_state == SYSTEM_BOOTING) {
 147                 rettime = ktime_get();
 148                 delta = ktime_sub(rettime, calltime);
 149                 printk("initcall %lli_%pF returned 0 after %lld usecs\n",
 150                         (long long)entry->cookie,
 151                         entry->func,
 152                         (long long)ktime_to_ns(delta) >> 10);
 153         }
 154
 155         /* 4) remove it from the running queue */
 156         spin_lock_irqsave(&async_lock, flags);
 157         list_del(&entry->list);
 158
 159         /* 5) free the entry  */
 160         kfree(entry);
 161         atomic_dec(&entry_count);
 162
 163         spin_unlock_irqrestore(&async_lock, flags);
 164
 165         /* 6) wake up any waiters. */
 166         wake_up(&async_done);
 167         return;
 168
 169 out:
 170         spin_unlock_irqrestore(&async_lock, flags);
 171 }
 172
 173
 174 static async_cookie_t __async_schedule(async_func_ptr *ptr, void *data, struct list_head *running)
 175 {
 176         struct async_entry *entry;
 177         unsigned long flags;
 178         async_cookie_t newcookie;
 179
 180
 181         /* allow irq-off callers */
 182         entry = kzalloc(sizeof(struct async_entry), GFP_ATOMIC);
 183
 184         /*
 185          * If we're out of memory or if there's too much work
 186          * pending already, we execute synchronously.
 187          */
 188         if (!async_enabled || !entry || atomic_read(&entry_count) > MAX_WORK) {
 189                 kfree(entry);
 190                 spin_lock_irqsave(&async_lock, flags);
 191                 newcookie = next_cookie++;
 192                 spin_unlock_irqrestore(&async_lock, flags);
 193
 194                 /* low on memory.. run synchronously */
 195                 ptr(data, newcookie);
 196                 return newcookie;
 197         }
 198         entry->func = ptr;
 199         entry->data = data;
 200         entry->running = running;
 201
 202         spin_lock_irqsave(&async_lock, flags);
 203         newcookie = entry->cookie = next_cookie++;
 204         list_add_tail(&entry->list, &async_pending);
 205         atomic_inc(&entry_count);
 206         spin_unlock_irqrestore(&async_lock, flags);
 207         wake_up(&async_new);
 208         return newcookie;
 209 }
 210
 211 /**
 212  * async_schedule - schedule a function for asynchronous execution
 213  * @ptr: function to execute asynchronously
 214  * @data: data pointer to pass to the function
 215  *
 216  * Returns an async_cookie_t that may be used for checkpointing later.
 217  * Note: This function may be called from atomic or non-atomic contexts.
 218  */
 219 async_cookie_t async_schedule(async_func_ptr *ptr, void *data)
 220 {
 221         return __async_schedule(ptr, data, &async_running);
 222 }
 223 EXPORT_SYMBOL_GPL(async_schedule);
 224
 225 /**
 226  * async_schedule_domain - schedule a function for asynchronous execution within a certain domain
 227  * @ptr: function to execute asynchronously
 228  * @data: data pointer to pass to the function
 229  * @running: running list for the domain
 230  *
 231  * Returns an async_cookie_t that may be used for checkpointing later.
 232  * @running may be used in the async_synchronize_*_domain() functions
 233  * to wait within a certain synchronization domain rather than globally.
 234  * A synchronization domain is specified via the running queue @running to use.
 235  * Note: This function may be called from atomic or non-atomic contexts.
 236  */
 237 async_cookie_t async_schedule_domain(async_func_ptr *ptr, void *data,
 238                                      struct list_head *running)
 239 {
 240         return __async_schedule(ptr, data, running);
 241 }
 242 EXPORT_SYMBOL_GPL(async_schedule_domain);
 243
 244 /**
 245  * async_synchronize_full - synchronize all asynchronous function calls
 246  *
 247  * This function waits until all asynchronous function calls have been done.
 248  */
 249 void async_synchronize_full(void)
 250 {
 251         do {
 252                 async_synchronize_cookie(next_cookie);
 253         } while (!list_empty(&async_running) || !list_empty(&async_pending));
 254 }
 255 EXPORT_SYMBOL_GPL(async_synchronize_full);
 256
 257 /**
 258  * async_synchronize_full_domain - synchronize all asynchronous function within a certain domain
 259  * @list: running list to synchronize on
 260  *
 261  * This function waits until all asynchronous function calls for the
 262  * synchronization domain specified by the running list @list have been done.
 263  */
 264 void async_synchronize_full_domain(struct list_head *list)
 265 {
 266         async_synchronize_cookie_domain(next_cookie, list);
 267 }
 268 EXPORT_SYMBOL_GPL(async_synchronize_full_domain);
 269
 270 /**
 271  * async_synchronize_cookie_domain - synchronize asynchronous function calls within a certain domain with cookie checkpointing
 272  * @cookie: async_cookie_t to use as checkpoint
 273  * @running: running list to synchronize on
 274  *
 275  * This function waits until all asynchronous function calls for the
 276  * synchronization domain specified by the running list @list submitted
 277  * prior to @cookie have been done.
 278  */
 279 void async_synchronize_cookie_domain(async_cookie_t cookie,
 280                                      struct list_head *running)
 281 {
 282         ktime_t starttime, delta, endtime;
 283
 284         if (initcall_debug && system_state == SYSTEM_BOOTING) {
 285                 printk("async_waiting @ %i\n", task_pid_nr(current));
 286                 starttime = ktime_get();
 287         }
 288
 289         wait_event(async_done, lowest_in_progress(running) >= cookie);
 290
 291         if (initcall_debug && system_state == SYSTEM_BOOTING) {
 292                 endtime = ktime_get();
 293                 delta = ktime_sub(endtime, starttime);
 294
 295                 printk("async_continuing @ %i after %lli usec\n",
 296                         task_pid_nr(current),
 297                         (long long)ktime_to_ns(delta) >> 10);
 298         }
 299 }
 300 EXPORT_SYMBOL_GPL(async_synchronize_cookie_domain);
 301
 302 /**
 303  * async_synchronize_cookie - synchronize asynchronous function calls with cookie checkpointing
 304  * @cookie: async_cookie_t to use as checkpoint
 305  *
 306  * This function waits until all asynchronous function calls prior to @cookie
 307  * have been done.
 308  */
 309 void async_synchronize_cookie(async_cookie_t cookie)
 310 {
 311         async_synchronize_cookie_domain(cookie, &async_running);
 312 }
 313 EXPORT_SYMBOL_GPL(async_synchronize_cookie);
 314
 315
 316 static int async_thread(void *unused)
 317 {
 318         DECLARE_WAITQUEUE(wq, current);
 319         add_wait_queue(&async_new, &wq);
 320
 321         while (!kthread_should_stop()) {
 322                 int ret = HZ;
 323                 set_current_state(TASK_INTERRUPTIBLE);
 324                 /*
 325                  * check the list head without lock.. false positives
 326                  * are dealt with inside run_one_entry() while holding
 327                  * the lock.
 328                  */
 329                 rmb();
 330                 if (!list_empty(&async_pending))
 331                         run_one_entry();
 332                 else
 333                         ret = schedule_timeout(HZ);
 334
 335                 if (ret == 0) {
 336                         /*
 337                          * we timed out, this means we as thread are redundant.
 338                          * we sign off and die, but we to avoid any races there
 339                          * is a last-straw check to see if work snuck in.
 340                          */
 341                         atomic_dec(&thread_count);
 342                         wmb(); /* manager must see our departure first */
 343                         if (list_empty(&async_pending))
 344                                 break;
 345                         /*
 346                          * woops work came in between us timing out and us
 347                          * signing off; we need to stay alive and keep working.
 348                          */
 349                         atomic_inc(&thread_count);
 350                 }
 351         }
 352         remove_wait_queue(&async_new, &wq);
 353
 354         return 0;
 355 }
 356
 357 static int async_manager_thread(void *unused)
 358 {
 359         DECLARE_WAITQUEUE(wq, current);
 360         add_wait_queue(&async_new, &wq);
 361
 362         while (!kthread_should_stop()) {
 363                 int tc, ec;
 364
 365                 set_current_state(TASK_INTERRUPTIBLE);
 366
 367                 tc = atomic_read(&thread_count);
 368                 rmb();
 369                 ec = atomic_read(&entry_count);
 370
 371                 while (tc < ec && tc < MAX_THREADS) {
 372                         if (IS_ERR(kthread_run(async_thread, NULL, "async/%i",
 373                                                tc))) {
 374                                 msleep(100);
 375                                 continue;
 376                         }
 377                         atomic_inc(&thread_count);
 378                         tc++;
 379                 }
 380
 381                 schedule();
 382         }
 383         remove_wait_queue(&async_new, &wq);
 384
 385         return 0;
 386 }
 387
 388 static int __init async_init(void)
 389 {
 390         async_enabled =
 391                 !IS_ERR(kthread_run(async_manager_thread, NULL, "async/mgr"));
 392
 393         WARN_ON(!async_enabled);
 394         return 0;
 395 }
 396
 397 core_initcall(async_init);