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/module.h>
  53 #include <linux/wait.h>
  54 #include <linux/sched.h>
  55 #include <linux/init.h>
  56 #include <linux/kthread.h>
  57 #include <asm/atomic.h>
  58
  59 static async_cookie_t next_cookie = 1;
  60
  61 #define MAX_THREADS     256
  62 #define MAX_WORK        32768
  63
  64 static LIST_HEAD(async_pending);
  65 static LIST_HEAD(async_running);
  66 static DEFINE_SPINLOCK(async_lock);
  67
  68 static int async_enabled = 0;
  69
  70 struct async_entry {
  71         struct list_head list;
  72         async_cookie_t   cookie;
  73         async_func_ptr   *func;
  74         void             *data;
  75         struct list_head *running;
  76 };
  77
  78 static DECLARE_WAIT_QUEUE_HEAD(async_done);
  79 static DECLARE_WAIT_QUEUE_HEAD(async_new);
  80
  81 static atomic_t entry_count;
  82 static atomic_t thread_count;
  83
  84 extern int initcall_debug;
  85
  86
  87 /*
  88  * MUST be called with the lock held!
  89  */
  90 static async_cookie_t  __lowest_in_progress(struct list_head *running)
  91 {
  92         struct async_entry *entry;
  93         if (!list_empty(running)) {
  94                 entry = list_first_entry(running,
  95                         struct async_entry, list);
  96                 return entry->cookie;
  97         } else if (!list_empty(&async_pending)) {
  98                 entry = list_first_entry(&async_pending,
  99                         struct async_entry, list);
 100                 return entry->cookie;
 101         } else {
 102                 /* nothing in progress... next_cookie is "infinity" */
 103                 return next_cookie;
 104         }
 105
 106 }
 107
 108 static async_cookie_t  lowest_in_progress(struct list_head *running)
 109 {
 110         unsigned long flags;
 111         async_cookie_t ret;
 112
 113         spin_lock_irqsave(&async_lock, flags);
 114         ret = __lowest_in_progress(running);
 115         spin_unlock_irqrestore(&async_lock, flags);
 116         return ret;
 117 }
 118 /*
 119  * pick the first pending entry and run it
 120  */
 121 static void run_one_entry(void)
 122 {
 123         unsigned long flags;
 124         struct async_entry *entry;
 125         ktime_t calltime, delta, rettime;
 126
 127         /* 1) pick one task from the pending queue */
 128
 129         spin_lock_irqsave(&async_lock, flags);
 130         if (list_empty(&async_pending))
 131                 goto out;
 132         entry = list_first_entry(&async_pending, struct async_entry, list);
 133
 134         /* 2) move it to the running queue */
 135         list_del(&entry->list);
 136         list_add_tail(&entry->list, &async_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 async_cookie_t async_schedule(async_func_ptr *ptr, void *data)
 212 {
 213         return __async_schedule(ptr, data, &async_pending);
 214 }
 215 EXPORT_SYMBOL_GPL(async_schedule);
 216
 217 async_cookie_t async_schedule_special(async_func_ptr *ptr, void *data, struct list_head *running)
 218 {
 219         return __async_schedule(ptr, data, running);
 220 }
 221 EXPORT_SYMBOL_GPL(async_schedule_special);
 222
 223 void async_synchronize_full(void)
 224 {
 225         do {
 226                 async_synchronize_cookie(next_cookie);
 227         } while (!list_empty(&async_running) || !list_empty(&async_pending));
 228 }
 229 EXPORT_SYMBOL_GPL(async_synchronize_full);
 230
 231 void async_synchronize_full_special(struct list_head *list)
 232 {
 233         async_synchronize_cookie_special(next_cookie, list);
 234 }
 235 EXPORT_SYMBOL_GPL(async_synchronize_full_special);
 236
 237 void async_synchronize_cookie_special(async_cookie_t cookie, struct list_head *running)
 238 {
 239         ktime_t starttime, delta, endtime;
 240
 241         if (initcall_debug && system_state == SYSTEM_BOOTING) {
 242                 printk("async_waiting @ %i\n", task_pid_nr(current));
 243                 starttime = ktime_get();
 244         }
 245
 246         wait_event(async_done, lowest_in_progress(running) >= cookie);
 247
 248         if (initcall_debug && system_state == SYSTEM_BOOTING) {
 249                 endtime = ktime_get();
 250                 delta = ktime_sub(endtime, starttime);
 251
 252                 printk("async_continuing @ %i after %lli usec\n",
 253                         task_pid_nr(current),
 254                         (long long)ktime_to_ns(delta) >> 10);
 255         }
 256 }
 257 EXPORT_SYMBOL_GPL(async_synchronize_cookie_special);
 258
 259 void async_synchronize_cookie(async_cookie_t cookie)
 260 {
 261         async_synchronize_cookie_special(cookie, &async_running);
 262 }
 263 EXPORT_SYMBOL_GPL(async_synchronize_cookie);
 264
 265
 266 static int async_thread(void *unused)
 267 {
 268         DECLARE_WAITQUEUE(wq, current);
 269         add_wait_queue(&async_new, &wq);
 270
 271         while (!kthread_should_stop()) {
 272                 int ret = HZ;
 273                 set_current_state(TASK_INTERRUPTIBLE);
 274                 /*
 275                  * check the list head without lock.. false positives
 276                  * are dealt with inside run_one_entry() while holding
 277                  * the lock.
 278                  */
 279                 rmb();
 280                 if (!list_empty(&async_pending))
 281                         run_one_entry();
 282                 else
 283                         ret = schedule_timeout(HZ);
 284
 285                 if (ret == 0) {
 286                         /*
 287                          * we timed out, this means we as thread are redundant.
 288                          * we sign off and die, but we to avoid any races there
 289                          * is a last-straw check to see if work snuck in.
 290                          */
 291                         atomic_dec(&thread_count);
 292                         wmb(); /* manager must see our departure first */
 293                         if (list_empty(&async_pending))
 294                                 break;
 295                         /*
 296                          * woops work came in between us timing out and us
 297                          * signing off; we need to stay alive and keep working.
 298                          */
 299                         atomic_inc(&thread_count);
 300                 }
 301         }
 302         remove_wait_queue(&async_new, &wq);
 303
 304         return 0;
 305 }
 306
 307 static int async_manager_thread(void *unused)
 308 {
 309         DECLARE_WAITQUEUE(wq, current);
 310         add_wait_queue(&async_new, &wq);
 311
 312         while (!kthread_should_stop()) {
 313                 int tc, ec;
 314
 315                 set_current_state(TASK_INTERRUPTIBLE);
 316
 317                 tc = atomic_read(&thread_count);
 318                 rmb();
 319                 ec = atomic_read(&entry_count);
 320
 321                 while (tc < ec && tc < MAX_THREADS) {
 322                         kthread_run(async_thread, NULL, "async/%i", tc);
 323                         atomic_inc(&thread_count);
 324                         tc++;
 325                 }
 326
 327                 schedule();
 328         }
 329         remove_wait_queue(&async_new, &wq);
 330
 331         return 0;
 332 }
 333
 334 static int __init async_init(void)
 335 {
 336         if (async_enabled)
 337                 kthread_run(async_manager_thread, NULL, "async/mgr");
 338         return 0;
 339 }
 340
 341 static int __init setup_async(char *str)
 342 {
 343         async_enabled = 1;
 344         return 1;
 345 }
 346
 347 __setup("fastboot", setup_async);
 348
 349
 350 core_initcall(async_init);