The Design and Implementation of the FreeBSD Operating System, Second Edition
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FreeBSD/Linux Kernel Cross Reference
sys/kernel/async.c

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    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/atomic.h>
   53 #include <linux/ktime.h>
   54 #include <linux/export.h>
   55 #include <linux/wait.h>
   56 #include <linux/sched.h>
   57 #include <linux/slab.h>
   58 #include <linux/workqueue.h>
   59 
   60 static async_cookie_t next_cookie = 1;
   61 
   62 #define MAX_WORK        32768
   63 
   64 static LIST_HEAD(async_pending);
   65 static ASYNC_DOMAIN(async_running);
   66 static LIST_HEAD(async_domains);
   67 static DEFINE_SPINLOCK(async_lock);
   68 static DEFINE_MUTEX(async_register_mutex);
   69 
   70 struct async_entry {
   71         struct list_head        list;
   72         struct work_struct      work;
   73         async_cookie_t          cookie;
   74         async_func_ptr          *func;
   75         void                    *data;
   76         struct async_domain     *running;
   77 };
   78 
   79 static DECLARE_WAIT_QUEUE_HEAD(async_done);
   80 
   81 static atomic_t entry_count;
   82 
   83 
   84 /*
   85  * MUST be called with the lock held!
   86  */
   87 static async_cookie_t  __lowest_in_progress(struct async_domain *running)
   88 {
   89         async_cookie_t first_running = next_cookie;     /* infinity value */
   90         async_cookie_t first_pending = next_cookie;     /* ditto */
   91         struct async_entry *entry;
   92 
   93         /*
   94          * Both running and pending lists are sorted but not disjoint.
   95          * Take the first cookies from both and return the min.
   96          */
   97         if (!list_empty(&running->domain)) {
   98                 entry = list_first_entry(&running->domain, typeof(*entry), list);
   99                 first_running = entry->cookie;
  100         }
  101 
  102         list_for_each_entry(entry, &async_pending, list) {
  103                 if (entry->running == running) {
  104                         first_pending = entry->cookie;
  105                         break;
  106                 }
  107         }
  108 
  109         return min(first_running, first_pending);
  110 }
  111 
  112 static async_cookie_t  lowest_in_progress(struct async_domain *running)
  113 {
  114         unsigned long flags;
  115         async_cookie_t ret;
  116 
  117         spin_lock_irqsave(&async_lock, flags);
  118         ret = __lowest_in_progress(running);
  119         spin_unlock_irqrestore(&async_lock, flags);
  120         return ret;
  121 }
  122 
  123 /*
  124  * pick the first pending entry and run it
  125  */
  126 static void async_run_entry_fn(struct work_struct *work)
  127 {
  128         struct async_entry *entry =
  129                 container_of(work, struct async_entry, work);
  130         struct async_entry *pos;
  131         unsigned long flags;
  132         ktime_t uninitialized_var(calltime), delta, rettime;
  133         struct async_domain *running = entry->running;
  134 
  135         /* 1) move self to the running queue, make sure it stays sorted */
  136         spin_lock_irqsave(&async_lock, flags);
  137         list_for_each_entry_reverse(pos, &running->domain, list)
  138                 if (entry->cookie < pos->cookie)
  139                         break;
  140         list_move_tail(&entry->list, &pos->list);
  141         spin_unlock_irqrestore(&async_lock, flags);
  142 
  143         /* 2) run (and print duration) */
  144         if (initcall_debug && system_state == SYSTEM_BOOTING) {
  145                 printk(KERN_DEBUG "calling  %lli_%pF @ %i\n",
  146                         (long long)entry->cookie,
  147                         entry->func, task_pid_nr(current));
  148                 calltime = ktime_get();
  149         }
  150         entry->func(entry->data, entry->cookie);
  151         if (initcall_debug && system_state == SYSTEM_BOOTING) {
  152                 rettime = ktime_get();
  153                 delta = ktime_sub(rettime, calltime);
  154                 printk(KERN_DEBUG "initcall %lli_%pF returned 0 after %lld usecs\n",
  155                         (long long)entry->cookie,
  156                         entry->func,
  157                         (long long)ktime_to_ns(delta) >> 10);
  158         }
  159 
  160         /* 3) remove self from the running queue */
  161         spin_lock_irqsave(&async_lock, flags);
  162         list_del(&entry->list);
  163         if (running->registered && --running->count == 0)
  164                 list_del_init(&running->node);
  165 
  166         /* 4) free the entry */
  167         kfree(entry);
  168         atomic_dec(&entry_count);
  169 
  170         spin_unlock_irqrestore(&async_lock, flags);
  171 
  172         /* 5) wake up any waiters */
  173         wake_up(&async_done);
  174 }
  175 
  176 static async_cookie_t __async_schedule(async_func_ptr *ptr, void *data, struct async_domain *running)
  177 {
  178         struct async_entry *entry;
  179         unsigned long flags;
  180         async_cookie_t newcookie;
  181 
  182         /* allow irq-off callers */
  183         entry = kzalloc(sizeof(struct async_entry), GFP_ATOMIC);
  184 
  185         /*
  186          * If we're out of memory or if there's too much work
  187          * pending already, we execute synchronously.
  188          */
  189         if (!entry || atomic_read(&entry_count) > MAX_WORK) {
  190                 kfree(entry);
  191                 spin_lock_irqsave(&async_lock, flags);
  192                 newcookie = next_cookie++;
  193                 spin_unlock_irqrestore(&async_lock, flags);
  194 
  195                 /* low on memory.. run synchronously */
  196                 ptr(data, newcookie);
  197                 return newcookie;
  198         }
  199         INIT_WORK(&entry->work, async_run_entry_fn);
  200         entry->func = ptr;
  201         entry->data = data;
  202         entry->running = running;
  203 
  204         spin_lock_irqsave(&async_lock, flags);
  205         newcookie = entry->cookie = next_cookie++;
  206         list_add_tail(&entry->list, &async_pending);
  207         if (running->registered && running->count++ == 0)
  208                 list_add_tail(&running->node, &async_domains);
  209         atomic_inc(&entry_count);
  210         spin_unlock_irqrestore(&async_lock, flags);
  211 
  212         /* mark that this task has queued an async job, used by module init */
  213         current->flags |= PF_USED_ASYNC;
  214 
  215         /* schedule for execution */
  216         queue_work(system_unbound_wq, &entry->work);
  217 
  218         return newcookie;
  219 }
  220 
  221 /**
  222  * async_schedule - schedule a function for asynchronous execution
  223  * @ptr: function to execute asynchronously
  224  * @data: data pointer to pass to the function
  225  *
  226  * Returns an async_cookie_t that may be used for checkpointing later.
  227  * Note: This function may be called from atomic or non-atomic contexts.
  228  */
  229 async_cookie_t async_schedule(async_func_ptr *ptr, void *data)
  230 {
  231         return __async_schedule(ptr, data, &async_running);
  232 }
  233 EXPORT_SYMBOL_GPL(async_schedule);
  234 
  235 /**
  236  * async_schedule_domain - schedule a function for asynchronous execution within a certain domain
  237  * @ptr: function to execute asynchronously
  238  * @data: data pointer to pass to the function
  239  * @running: running list for the domain
  240  *
  241  * Returns an async_cookie_t that may be used for checkpointing later.
  242  * @running may be used in the async_synchronize_*_domain() functions
  243  * to wait within a certain synchronization domain rather than globally.
  244  * A synchronization domain is specified via the running queue @running to use.
  245  * Note: This function may be called from atomic or non-atomic contexts.
  246  */
  247 async_cookie_t async_schedule_domain(async_func_ptr *ptr, void *data,
  248                                      struct async_domain *running)
  249 {
  250         return __async_schedule(ptr, data, running);
  251 }
  252 EXPORT_SYMBOL_GPL(async_schedule_domain);
  253 
  254 /**
  255  * async_synchronize_full - synchronize all asynchronous function calls
  256  *
  257  * This function waits until all asynchronous function calls have been done.
  258  */
  259 void async_synchronize_full(void)
  260 {
  261         mutex_lock(&async_register_mutex);
  262         do {
  263                 struct async_domain *domain = NULL;
  264 
  265                 spin_lock_irq(&async_lock);
  266                 if (!list_empty(&async_domains))
  267                         domain = list_first_entry(&async_domains, typeof(*domain), node);
  268                 spin_unlock_irq(&async_lock);
  269 
  270                 async_synchronize_cookie_domain(next_cookie, domain);
  271         } while (!list_empty(&async_domains));
  272         mutex_unlock(&async_register_mutex);
  273 }
  274 EXPORT_SYMBOL_GPL(async_synchronize_full);
  275 
  276 /**
  277  * async_unregister_domain - ensure no more anonymous waiters on this domain
  278  * @domain: idle domain to flush out of any async_synchronize_full instances
  279  *
  280  * async_synchronize_{cookie|full}_domain() are not flushed since callers
  281  * of these routines should know the lifetime of @domain
  282  *
  283  * Prefer ASYNC_DOMAIN_EXCLUSIVE() declarations over flushing
  284  */
  285 void async_unregister_domain(struct async_domain *domain)
  286 {
  287         mutex_lock(&async_register_mutex);
  288         spin_lock_irq(&async_lock);
  289         WARN_ON(!domain->registered || !list_empty(&domain->node) ||
  290                 !list_empty(&domain->domain));
  291         domain->registered = 0;
  292         spin_unlock_irq(&async_lock);
  293         mutex_unlock(&async_register_mutex);
  294 }
  295 EXPORT_SYMBOL_GPL(async_unregister_domain);
  296 
  297 /**
  298  * async_synchronize_full_domain - synchronize all asynchronous function within a certain domain
  299  * @domain: running list to synchronize on
  300  *
  301  * This function waits until all asynchronous function calls for the
  302  * synchronization domain specified by the running list @domain have been done.
  303  */
  304 void async_synchronize_full_domain(struct async_domain *domain)
  305 {
  306         async_synchronize_cookie_domain(next_cookie, domain);
  307 }
  308 EXPORT_SYMBOL_GPL(async_synchronize_full_domain);
  309 
  310 /**
  311  * async_synchronize_cookie_domain - synchronize asynchronous function calls within a certain domain with cookie checkpointing
  312  * @cookie: async_cookie_t to use as checkpoint
  313  * @running: running list to synchronize on
  314  *
  315  * This function waits until all asynchronous function calls for the
  316  * synchronization domain specified by running list @running submitted
  317  * prior to @cookie have been done.
  318  */
  319 void async_synchronize_cookie_domain(async_cookie_t cookie, struct async_domain *running)
  320 {
  321         ktime_t uninitialized_var(starttime), delta, endtime;
  322 
  323         if (!running)
  324                 return;
  325 
  326         if (initcall_debug && system_state == SYSTEM_BOOTING) {
  327                 printk(KERN_DEBUG "async_waiting @ %i\n", task_pid_nr(current));
  328                 starttime = ktime_get();
  329         }
  330 
  331         wait_event(async_done, lowest_in_progress(running) >= cookie);
  332 
  333         if (initcall_debug && system_state == SYSTEM_BOOTING) {
  334                 endtime = ktime_get();
  335                 delta = ktime_sub(endtime, starttime);
  336 
  337                 printk(KERN_DEBUG "async_continuing @ %i after %lli usec\n",
  338                         task_pid_nr(current),
  339                         (long long)ktime_to_ns(delta) >> 10);
  340         }
  341 }
  342 EXPORT_SYMBOL_GPL(async_synchronize_cookie_domain);
  343 
  344 /**
  345  * async_synchronize_cookie - synchronize asynchronous function calls with cookie checkpointing
  346  * @cookie: async_cookie_t to use as checkpoint
  347  *
  348  * This function waits until all asynchronous function calls prior to @cookie
  349  * have been done.
  350  */
  351 void async_synchronize_cookie(async_cookie_t cookie)
  352 {
  353         async_synchronize_cookie_domain(cookie, &async_running);
  354 }
  355 EXPORT_SYMBOL_GPL(async_synchronize_cookie);

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