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 struct async_entry {
  69         struct list_head list;
  70         async_cookie_t   cookie;
  71         async_func_ptr   *func;
  72         void             *data;
  73         struct list_head *running;
  74 };
  75
  76 static DECLARE_WAIT_QUEUE_HEAD(async_done);
  77 static DECLARE_WAIT_QUEUE_HEAD(async_new);
  78
  79 static atomic_t entry_count;
  80 static atomic_t thread_count;
  81
  82 extern int initcall_debug;
  83
  84
  85 /*
  86  * MUST be called with the lock held!
  87  */
  88 static async_cookie_t  __lowest_in_progress(struct list_head *running)
  89 {
  90         struct async_entry *entry;
  91         if (!list_empty(&async_pending)) {
  92                 entry = list_first_entry(&async_pending,
  93                         struct async_entry, list);
  94                 return entry->cookie;
  95         } else if (!list_empty(running)) {
  96                 entry = list_first_entry(running,
  97                         struct async_entry, list);
  98                 return entry->cookie;
  99         } else {
 100                 /* nothing in progress... next_cookie is "infinity" */
 101                 return next_cookie;
 102         }
 103
 104 }
 105 /*
 106  * pick the first pending entry and run it
 107  */
 108 static void run_one_entry(void)
 109 {
 110         unsigned long flags;
 111         struct async_entry *entry;
 112         ktime_t calltime, delta, rettime;
 113
 114         /* 1) pick one task from the pending queue */
 115
 116         spin_lock_irqsave(&async_lock, flags);
 117         if (list_empty(&async_pending))
 118                 goto out;
 119         entry = list_first_entry(&async_pending, struct async_entry, list);
 120
 121         /* 2) move it to the running queue */
 122         list_del(&entry->list);
 123         list_add_tail(&entry->list, &async_running);
 124         spin_unlock_irqrestore(&async_lock, flags);
 125
 126         /* 3) run it (and print duration)*/
 127         if (initcall_debug) {
 128                 printk("calling  %lli_%pF @ %i\n", entry->cookie, entry->func, task_pid_nr(current));
 129                 calltime = ktime_get();
 130         }
 131         entry->func(entry->data, entry->cookie);
 132         if (initcall_debug) {
 133                 rettime = ktime_get();
 134                 delta = ktime_sub(rettime, calltime);
 135                 printk("initcall %lli_%pF returned 0 after %lld usecs\n", entry->cookie,
 136                         entry->func, ktime_to_ns(delta) >> 10);
 137         }
 138
 139         /* 4) remove it from the running queue */
 140         spin_lock_irqsave(&async_lock, flags);
 141         list_del(&entry->list);
 142
 143         /* 5) free the entry  */
 144         kfree(entry);
 145         atomic_dec(&entry_count);
 146
 147         spin_unlock_irqrestore(&async_lock, flags);
 148
 149         /* 6) wake up any waiters. */
 150         wake_up(&async_done);
 151         return;
 152
 153 out:
 154         spin_unlock_irqrestore(&async_lock, flags);
 155 }
 156
 157
 158 static async_cookie_t __async_schedule(async_func_ptr *ptr, void *data, struct list_head *running)
 159 {
 160         struct async_entry *entry;
 161         unsigned long flags;
 162         async_cookie_t newcookie;
 163
 164
 165         /* allow irq-off callers */
 166         entry = kzalloc(sizeof(struct async_entry), GFP_ATOMIC);
 167
 168         /*
 169          * If we're out of memory or if there's too much work
 170          * pending already, we execute synchronously.
 171          */
 172         if (!entry || atomic_read(&entry_count) > MAX_WORK) {
 173                 kfree(entry);
 174                 spin_lock_irqsave(&async_lock, flags);
 175                 newcookie = next_cookie++;
 176                 spin_unlock_irqrestore(&async_lock, flags);
 177
 178                 /* low on memory.. run synchronously */
 179                 ptr(data, newcookie);
 180                 return newcookie;
 181         }
 182         entry->func = ptr;
 183         entry->data = data;
 184         entry->running = running;
 185
 186         spin_lock_irqsave(&async_lock, flags);
 187         newcookie = entry->cookie = next_cookie++;
 188         list_add_tail(&entry->list, &async_pending);
 189         atomic_inc(&entry_count);
 190         spin_unlock_irqrestore(&async_lock, flags);
 191         wake_up(&async_new);
 192         return newcookie;
 193 }
 194
 195 async_cookie_t async_schedule(async_func_ptr *ptr, void *data)
 196 {
 197         return __async_schedule(ptr, data, &async_pending);
 198 }
 199 EXPORT_SYMBOL_GPL(async_schedule);
 200
 201 async_cookie_t async_schedule_special(async_func_ptr *ptr, void *data, struct list_head *running)
 202 {
 203         return __async_schedule(ptr, data, running);
 204 }
 205 EXPORT_SYMBOL_GPL(async_schedule_special);
 206
 207 void async_synchronize_full(void)
 208 {
 209         async_synchronize_cookie(next_cookie);
 210 }
 211 EXPORT_SYMBOL_GPL(async_synchronize_full);
 212
 213 void async_synchronize_full_special(struct list_head *list)
 214 {
 215         async_synchronize_cookie_special(next_cookie, list);
 216 }
 217 EXPORT_SYMBOL_GPL(async_synchronize_full_special);
 218
 219 void async_synchronize_cookie_special(async_cookie_t cookie, struct list_head *running)
 220 {
 221         ktime_t starttime, delta, endtime;
 222
 223         if (initcall_debug) {
 224                 printk("async_waiting @ %i\n", task_pid_nr(current));
 225                 starttime = ktime_get();
 226         }
 227
 228         wait_event(async_done, __lowest_in_progress(running) >= cookie);
 229
 230         if (initcall_debug) {
 231                 endtime = ktime_get();
 232                 delta = ktime_sub(endtime, starttime);
 233
 234                 printk("async_continuing @ %i after %lli usec\n",
 235                         task_pid_nr(current), ktime_to_ns(delta) >> 10);
 236         }
 237 }
 238 EXPORT_SYMBOL_GPL(async_synchronize_cookie_special);
 239
 240 void async_synchronize_cookie(async_cookie_t cookie)
 241 {
 242         async_synchronize_cookie_special(cookie, &async_running);
 243 }
 244 EXPORT_SYMBOL_GPL(async_synchronize_cookie);
 245
 246
 247 static int async_thread(void *unused)
 248 {
 249         DECLARE_WAITQUEUE(wq, current);
 250         add_wait_queue(&async_new, &wq);
 251
 252         while (!kthread_should_stop()) {
 253                 int ret = HZ;
 254                 set_current_state(TASK_INTERRUPTIBLE);
 255                 /*
 256                  * check the list head without lock.. false positives
 257                  * are dealt with inside run_one_entry() while holding
 258                  * the lock.
 259                  */
 260                 rmb();
 261                 if (!list_empty(&async_pending))
 262                         run_one_entry();
 263                 else
 264                         ret = schedule_timeout(HZ);
 265
 266                 if (ret == 0) {
 267                         /*
 268                          * we timed out, this means we as thread are redundant.
 269                          * we sign off and die, but we to avoid any races there
 270                          * is a last-straw check to see if work snuck in.
 271                          */
 272                         atomic_dec(&thread_count);
 273                         wmb(); /* manager must see our departure first */
 274                         if (list_empty(&async_pending))
 275                                 break;
 276                         /*
 277                          * woops work came in between us timing out and us
 278                          * signing off; we need to stay alive and keep working.
 279                          */
 280                         atomic_inc(&thread_count);
 281                 }
 282         }
 283         remove_wait_queue(&async_new, &wq);
 284
 285         return 0;
 286 }
 287
 288 static int async_manager_thread(void *unused)
 289 {
 290         DECLARE_WAITQUEUE(wq, current);
 291         add_wait_queue(&async_new, &wq);
 292
 293         while (!kthread_should_stop()) {
 294                 int tc, ec;
 295
 296                 set_current_state(TASK_INTERRUPTIBLE);
 297
 298                 tc = atomic_read(&thread_count);
 299                 rmb();
 300                 ec = atomic_read(&entry_count);
 301
 302                 while (tc < ec && tc < MAX_THREADS) {
 303                         kthread_run(async_thread, NULL, "async/%i", tc);
 304                         atomic_inc(&thread_count);
 305                         tc++;
 306                 }
 307
 308                 schedule();
 309         }
 310         remove_wait_queue(&async_new, &wq);
 311
 312         return 0;
 313 }
 314
 315 static int __init async_init(void)
 316 {
 317         kthread_run(async_manager_thread, NULL, "async/mgr");
 318         return 0;
 319 }
 320
 321 core_initcall(async_init);