/* Worker thread pool for slow items, such as filesystem lookups or mkdirs * * Copyright (C) 2008 Red Hat, Inc. All Rights Reserved. * Written by David Howells (dhowells@redhat.com) * * This program is free software; you can redistribute it and/or * modify it under the terms of the GNU General Public Licence * as published by the Free Software Foundation; either version * 2 of the Licence, or (at your option) any later version. * * See Documentation/slow-work.txt */ #include #include #include #include #include #define SLOW_WORK_CULL_TIMEOUT (5 * HZ) /* cull threads 5s after running out of * things to do */ #define SLOW_WORK_OOM_TIMEOUT (5 * HZ) /* can't start new threads for 5s after * OOM */ #define SLOW_WORK_THREAD_LIMIT 255 /* abs maximum number of slow-work threads */ static void slow_work_cull_timeout(unsigned long); static void slow_work_oom_timeout(unsigned long); #ifdef CONFIG_SYSCTL static int slow_work_min_threads_sysctl(struct ctl_table *, int, void __user *, size_t *, loff_t *); static int slow_work_max_threads_sysctl(struct ctl_table *, int , void __user *, size_t *, loff_t *); #endif /* * The pool of threads has at least min threads in it as long as someone is * using the facility, and may have as many as max. * * A portion of the pool may be processing very slow operations. */ static unsigned slow_work_min_threads = 2; static unsigned slow_work_max_threads = 4; static unsigned vslow_work_proportion = 50; /* % of threads that may process * very slow work */ #ifdef CONFIG_SYSCTL static const int slow_work_min_min_threads = 2; static int slow_work_max_max_threads = SLOW_WORK_THREAD_LIMIT; static const int slow_work_min_vslow = 1; static const int slow_work_max_vslow = 99; ctl_table slow_work_sysctls[] = { { .ctl_name = CTL_UNNUMBERED, .procname = "min-threads", .data = &slow_work_min_threads, .maxlen = sizeof(unsigned), .mode = 0644, .proc_handler = slow_work_min_threads_sysctl, .extra1 = (void *) &slow_work_min_min_threads, .extra2 = &slow_work_max_threads, }, { .ctl_name = CTL_UNNUMBERED, .procname = "max-threads", .data = &slow_work_max_threads, .maxlen = sizeof(unsigned), .mode = 0644, .proc_handler = slow_work_max_threads_sysctl, .extra1 = &slow_work_min_threads, .extra2 = (void *) &slow_work_max_max_threads, }, { .ctl_name = CTL_UNNUMBERED, .procname = "vslow-percentage", .data = &vslow_work_proportion, .maxlen = sizeof(unsigned), .mode = 0644, .proc_handler = &proc_dointvec_minmax, .extra1 = (void *) &slow_work_min_vslow, .extra2 = (void *) &slow_work_max_vslow, }, { .ctl_name = 0 } }; #endif /* * The active state of the thread pool */ static atomic_t slow_work_thread_count; static atomic_t vslow_work_executing_count; static bool slow_work_may_not_start_new_thread; static bool slow_work_cull; /* cull a thread due to lack of activity */ static DEFINE_TIMER(slow_work_cull_timer, slow_work_cull_timeout, 0, 0); static DEFINE_TIMER(slow_work_oom_timer, slow_work_oom_timeout, 0, 0); static struct slow_work slow_work_new_thread; /* new thread starter */ /* * slow work ID allocation (use slow_work_queue_lock) */ static DECLARE_BITMAP(slow_work_ids, SLOW_WORK_THREAD_LIMIT); /* * Unregistration tracking to prevent put_ref() from disappearing during module * unload */ #ifdef CONFIG_MODULES static struct module *slow_work_thread_processing[SLOW_WORK_THREAD_LIMIT]; static struct module *slow_work_unreg_module; static struct slow_work *slow_work_unreg_work_item; static DECLARE_WAIT_QUEUE_HEAD(slow_work_unreg_wq); static DEFINE_MUTEX(slow_work_unreg_sync_lock); #endif /* * The queues of work items and the lock governing access to them. These are * shared between all the CPUs. It doesn't make sense to have per-CPU queues * as the number of threads bears no relation to the number of CPUs. * * There are two queues of work items: one for slow work items, and one for * very slow work items. */ static LIST_HEAD(slow_work_queue); static LIST_HEAD(vslow_work_queue); static DEFINE_SPINLOCK(slow_work_queue_lock); /* * The thread controls. A variable used to signal to the threads that they * should exit when the queue is empty, a waitqueue used by the threads to wait * for signals, and a completion set by the last thread to exit. */ static bool slow_work_threads_should_exit; static DECLARE_WAIT_QUEUE_HEAD(slow_work_thread_wq); static DECLARE_COMPLETION(slow_work_last_thread_exited); /* * The number of users of the thread pool and its lock. Whilst this is zero we * have no threads hanging around, and when this reaches zero, we wait for all * active or queued work items to complete and kill all the threads we do have. */ static int slow_work_user_count; static DEFINE_MUTEX(slow_work_user_lock); static inline int slow_work_get_ref(struct slow_work *work) { if (work->ops->get_ref) return work->ops->get_ref(work); return 0; } static inline void slow_work_put_ref(struct slow_work *work) { if (work->ops->put_ref) work->ops->put_ref(work); } /* * Calculate the maximum number of active threads in the pool that are * permitted to process very slow work items. * * The answer is rounded up to at least 1, but may not equal or exceed the * maximum number of the threads in the pool. This means we always have at * least one thread that can process slow work items, and we always have at * least one thread that won't get tied up doing so. */ static unsigned slow_work_calc_vsmax(void) { unsigned vsmax; vsmax = atomic_read(&slow_work_thread_count) * vslow_work_proportion; vsmax /= 100; vsmax = max(vsmax, 1U); return min(vsmax, slow_work_max_threads - 1); } /* * Attempt to execute stuff queued on a slow thread. Return true if we managed * it, false if there was nothing to do. */ static bool slow_work_execute(int id) { #ifdef CONFIG_MODULES struct module *module; #endif struct slow_work *work = NULL; unsigned vsmax; bool very_slow; vsmax = slow_work_calc_vsmax(); /* see if we can schedule a new thread to be started if we're not * keeping up with the work */ if (!waitqueue_active(&slow_work_thread_wq) && (!list_empty(&slow_work_queue) || !list_empty(&vslow_work_queue)) && atomic_read(&slow_work_thread_count) < slow_work_max_threads && !slow_work_may_not_start_new_thread) slow_work_enqueue(&slow_work_new_thread); /* find something to execute */ spin_lock_irq(&slow_work_queue_lock); if (!list_empty(&vslow_work_queue) && atomic_read(&vslow_work_executing_count) < vsmax) { work = list_entry(vslow_work_queue.next, struct slow_work, link); if (test_and_set_bit_lock(SLOW_WORK_EXECUTING, &work->flags)) BUG(); list_del_init(&work->link); atomic_inc(&vslow_work_executing_count); very_slow = true; } else if (!list_empty(&slow_work_queue)) { work = list_entry(slow_work_queue.next, struct slow_work, link); if (test_and_set_bit_lock(SLOW_WORK_EXECUTING, &work->flags)) BUG(); list_del_init(&work->link); very_slow = false; } else { very_slow = false; /* avoid the compiler warning */ } #ifdef CONFIG_MODULES if (work) slow_work_thread_processing[id] = work->owner; #endif spin_unlock_irq(&slow_work_queue_lock); if (!work) return false; if (!test_and_clear_bit(SLOW_WORK_PENDING, &work->flags)) BUG(); /* don't execute if the work is in the process of being cancelled */ if (!test_bit(SLOW_WORK_CANCELLING, &work->flags)) work->ops->execute(work); if (very_slow) atomic_dec(&vslow_work_executing_count); clear_bit_unlock(SLOW_WORK_EXECUTING, &work->flags); /* wake up anyone waiting for this work to be complete */ wake_up_bit(&work->flags, SLOW_WORK_EXECUTING); /* if someone tried to enqueue the item whilst we were executing it, * then it'll be left unenqueued to avoid multiple threads trying to * execute it simultaneously * * there is, however, a race between us testing the pending flag and * getting the spinlock, and between the enqueuer setting the pending * flag and getting the spinlock, so we use a deferral bit to tell us * if the enqueuer got there first */ if (test_bit(SLOW_WORK_PENDING, &work->flags)) { spin_lock_irq(&slow_work_queue_lock); if (!test_bit(SLOW_WORK_EXECUTING, &work->flags) && test_and_clear_bit(SLOW_WORK_ENQ_DEFERRED, &work->flags)) goto auto_requeue; spin_unlock_irq(&slow_work_queue_lock); } /* sort out the race between module unloading and put_ref() */ slow_work_put_ref(work); #ifdef CONFIG_MODULES module = slow_work_thread_processing[id]; slow_work_thread_processing[id] = NULL; smp_mb(); if (slow_work_unreg_work_item == work || slow_work_unreg_module == module) wake_up_all(&slow_work_unreg_wq); #endif return true; auto_requeue: /* we must complete the enqueue operation * - we transfer our ref on the item back to the appropriate queue * - don't wake another thread up as we're awake already */ if (test_bit(SLOW_WORK_VERY_SLOW, &work->flags)) list_add_tail(&work->link, &vslow_work_queue); else list_add_tail(&work->link, &slow_work_queue); spin_unlock_irq(&slow_work_queue_lock); slow_work_thread_processing[id] = NULL; return true; } /** * slow_work_enqueue - Schedule a slow work item for processing * @work: The work item to queue * * Schedule a slow work item for processing. If the item is already undergoing * execution, this guarantees not to re-enter the execution routine until the * first execution finishes. * * The item is pinned by this function as it retains a reference to it, managed * through the item operations. The item is unpinned once it has been * executed. * * An item may hog the thread that is running it for a relatively large amount * of time, sufficient, for example, to perform several lookup, mkdir, create * and setxattr operations. It may sleep on I/O and may sleep to obtain locks. * * Conversely, if a number of items are awaiting processing, it may take some * time before any given item is given attention. The number of threads in the * pool may be increased to deal with demand, but only up to a limit. * * If SLOW_WORK_VERY_SLOW is set on the work item, then it will be placed in * the very slow queue, from which only a portion of the threads will be * allowed to pick items to execute. This ensures that very slow items won't * overly block ones that are just ordinarily slow. * * Returns 0 if successful, -EAGAIN if not (or -ECANCELED if cancelled work is * attempted queued) */ int slow_work_enqueue(struct slow_work *work) { unsigned long flags; int ret; if (test_bit(SLOW_WORK_CANCELLING, &work->flags)) return -ECANCELED; BUG_ON(slow_work_user_count <= 0); BUG_ON(!work); BUG_ON(!work->ops); /* when honouring an enqueue request, we only promise that we will run * the work function in the future; we do not promise to run it once * per enqueue request * * we use the PENDING bit to merge together repeat requests without * having to disable IRQs and take the spinlock, whilst still * maintaining our promise */ if (!test_and_set_bit_lock(SLOW_WORK_PENDING, &work->flags)) { spin_lock_irqsave(&slow_work_queue_lock, flags); if (unlikely(test_bit(SLOW_WORK_CANCELLING, &work->flags))) goto cancelled; /* we promise that we will not attempt to execute the work * function in more than one thread simultaneously * * this, however, leaves us with a problem if we're asked to * enqueue the work whilst someone is executing the work * function as simply queueing the work immediately means that * another thread may try executing it whilst it is already * under execution * * to deal with this, we set the ENQ_DEFERRED bit instead of * enqueueing, and the thread currently executing the work * function will enqueue the work item when the work function * returns and it has cleared the EXECUTING bit */ if (test_bit(SLOW_WORK_EXECUTING, &work->flags)) { set_bit(SLOW_WORK_ENQ_DEFERRED, &work->flags); } else { ret = slow_work_get_ref(work); if (ret < 0) goto failed; if (test_bit(SLOW_WORK_VERY_SLOW, &work->flags)) list_add_tail(&work->link, &vslow_work_queue); else list_add_tail(&work->link, &slow_work_queue); wake_up(&slow_work_thread_wq); } spin_unlock_irqrestore(&slow_work_queue_lock, flags); } return 0; cancelled: ret = -ECANCELED; failed: spin_unlock_irqrestore(&slow_work_queue_lock, flags); return ret; } EXPORT_SYMBOL(slow_work_enqueue); static int slow_work_wait(void *word) { schedule(); return 0; } /** * slow_work_cancel - Cancel a slow work item * @work: The work item to cancel * * This function will cancel a previously enqueued work item. If we cannot * cancel the work item, it is guarenteed to have run when this function * returns. */ void slow_work_cancel(struct slow_work *work) { bool wait = true, put = false; set_bit(SLOW_WORK_CANCELLING, &work->flags); spin_lock_irq(&slow_work_queue_lock); if (test_bit(SLOW_WORK_PENDING, &work->flags) && !list_empty(&work->link)) { /* the link in the pending queue holds a reference on the item * that we will need to release */ list_del_init(&work->link); wait = false; put = true; clear_bit(SLOW_WORK_PENDING, &work->flags); } else if (test_and_clear_bit(SLOW_WORK_ENQ_DEFERRED, &work->flags)) { /* the executor is holding our only reference on the item, so * we merely need to wait for it to finish executing */ clear_bit(SLOW_WORK_PENDING, &work->flags); } spin_unlock_irq(&slow_work_queue_lock); /* the EXECUTING flag is set by the executor whilst the spinlock is set * and before the item is dequeued - so assuming the above doesn't * actually dequeue it, simply waiting for the EXECUTING flag to be * released here should be sufficient */ if (wait) wait_on_bit(&work->flags, SLOW_WORK_EXECUTING, slow_work_wait, TASK_UNINTERRUPTIBLE); clear_bit(SLOW_WORK_CANCELLING, &work->flags); if (put) slow_work_put_ref(work); } EXPORT_SYMBOL(slow_work_cancel); /* * Schedule a cull of the thread pool at some time in the near future */ static void slow_work_schedule_cull(void) { mod_timer(&slow_work_cull_timer, round_jiffies(jiffies + SLOW_WORK_CULL_TIMEOUT)); } /* * Worker thread culling algorithm */ static bool slow_work_cull_thread(void) { unsigned long flags; bool do_cull = false; spin_lock_irqsave(&slow_work_queue_lock, flags); if (slow_work_cull) { slow_work_cull = false; if (list_empty(&slow_work_queue) && list_empty(&vslow_work_queue) && atomic_read(&slow_work_thread_count) > slow_work_min_threads) { slow_work_schedule_cull(); do_cull = true; } } spin_unlock_irqrestore(&slow_work_queue_lock, flags); return do_cull; } /* * Determine if there is slow work available for dispatch */ static inline bool slow_work_available(int vsmax) { return !list_empty(&slow_work_queue) || (!list_empty(&vslow_work_queue) && atomic_read(&vslow_work_executing_count) < vsmax); } /* * Worker thread dispatcher */ static int slow_work_thread(void *_data) { int vsmax, id; DEFINE_WAIT(wait); set_freezable(); set_user_nice(current, -5); /* allocate ourselves an ID */ spin_lock_irq(&slow_work_queue_lock); id = find_first_zero_bit(slow_work_ids, SLOW_WORK_THREAD_LIMIT); BUG_ON(id < 0 || id >= SLOW_WORK_THREAD_LIMIT); __set_bit(id, slow_work_ids); spin_unlock_irq(&slow_work_queue_lock); sprintf(current->comm, "kslowd%03u", id); for (;;) { vsmax = vslow_work_proportion; vsmax *= atomic_read(&slow_work_thread_count); vsmax /= 100; prepare_to_wait_exclusive(&slow_work_thread_wq, &wait, TASK_INTERRUPTIBLE); if (!freezing(current) && !slow_work_threads_should_exit && !slow_work_available(vsmax) && !slow_work_cull) schedule(); finish_wait(&slow_work_thread_wq, &wait); try_to_freeze(); vsmax = vslow_work_proportion; vsmax *= atomic_read(&slow_work_thread_count); vsmax /= 100; if (slow_work_available(vsmax) && slow_work_execute(id)) { cond_resched(); if (list_empty(&slow_work_queue) && list_empty(&vslow_work_queue) && atomic_read(&slow_work_thread_count) > slow_work_min_threads) slow_work_schedule_cull(); continue; } if (slow_work_threads_should_exit) break; if (slow_work_cull && slow_work_cull_thread()) break; } spin_lock_irq(&slow_work_queue_lock); __clear_bit(id, slow_work_ids); spin_unlock_irq(&slow_work_queue_lock); if (atomic_dec_and_test(&slow_work_thread_count)) complete_and_exit(&slow_work_last_thread_exited, 0); return 0; } /* * Handle thread cull timer expiration */ static void slow_work_cull_timeout(unsigned long data) { slow_work_cull = true; wake_up(&slow_work_thread_wq); } /* * Start a new slow work thread */ static void slow_work_new_thread_execute(struct slow_work *work) { struct task_struct *p; if (slow_work_threads_should_exit) return; if (atomic_read(&slow_work_thread_count) >= slow_work_max_threads) return; if (!mutex_trylock(&slow_work_user_lock)) return; slow_work_may_not_start_new_thread = true; atomic_inc(&slow_work_thread_count); p = kthread_run(slow_work_thread, NULL, "kslowd"); if (IS_ERR(p)) { printk(KERN_DEBUG "Slow work thread pool: OOM\n"); if (atomic_dec_and_test(&slow_work_thread_count)) BUG(); /* we're running on a slow work thread... */ mod_timer(&slow_work_oom_timer, round_jiffies(jiffies + SLOW_WORK_OOM_TIMEOUT)); } else { /* ratelimit the starting of new threads */ mod_timer(&slow_work_oom_timer, jiffies + 1); } mutex_unlock(&slow_work_user_lock); } static const struct slow_work_ops slow_work_new_thread_ops = { .owner = THIS_MODULE, .execute = slow_work_new_thread_execute, }; /* * post-OOM new thread start suppression expiration */ static void slow_work_oom_timeout(unsigned long data) { slow_work_may_not_start_new_thread = false; } #ifdef CONFIG_SYSCTL /* * Handle adjustment of the minimum number of threads */ static int slow_work_min_threads_sysctl(struct ctl_table *table, int write, void __user *buffer, size_t *lenp, loff_t *ppos) { int ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos); int n; if (ret == 0) { mutex_lock(&slow_work_user_lock); if (slow_work_user_count > 0) { /* see if we need to start or stop threads */ n = atomic_read(&slow_work_thread_count) - slow_work_min_threads; if (n < 0 && !slow_work_may_not_start_new_thread) slow_work_enqueue(&slow_work_new_thread); else if (n > 0) slow_work_schedule_cull(); } mutex_unlock(&slow_work_user_lock); } return ret; } /* * Handle adjustment of the maximum number of threads */ static int slow_work_max_threads_sysctl(struct ctl_table *table, int write, void __user *buffer, size_t *lenp, loff_t *ppos) { int ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos); int n; if (ret == 0) { mutex_lock(&slow_work_user_lock); if (slow_work_user_count > 0) { /* see if we need to stop threads */ n = slow_work_max_threads - atomic_read(&slow_work_thread_count); if (n < 0) slow_work_schedule_cull(); } mutex_unlock(&slow_work_user_lock); } return ret; } #endif /* CONFIG_SYSCTL */ /** * slow_work_register_user - Register a user of the facility * @module: The module about to make use of the facility * * Register a user of the facility, starting up the initial threads if there * aren't any other users at this point. This will return 0 if successful, or * an error if not. */ int slow_work_register_user(struct module *module) { struct task_struct *p; int loop; mutex_lock(&slow_work_user_lock); if (slow_work_user_count == 0) { printk(KERN_NOTICE "Slow work thread pool: Starting up\n"); init_completion(&slow_work_last_thread_exited); slow_work_threads_should_exit = false; slow_work_init(&slow_work_new_thread, &slow_work_new_thread_ops); slow_work_may_not_start_new_thread = false; slow_work_cull = false; /* start the minimum number of threads */ for (loop = 0; loop < slow_work_min_threads; loop++) { atomic_inc(&slow_work_thread_count); p = kthread_run(slow_work_thread, NULL, "kslowd"); if (IS_ERR(p)) goto error; } printk(KERN_NOTICE "Slow work thread pool: Ready\n"); } slow_work_user_count++; mutex_unlock(&slow_work_user_lock); return 0; error: if (atomic_dec_and_test(&slow_work_thread_count)) complete(&slow_work_last_thread_exited); if (loop > 0) { printk(KERN_ERR "Slow work thread pool:" " Aborting startup on ENOMEM\n"); slow_work_threads_should_exit = true; wake_up_all(&slow_work_thread_wq); wait_for_completion(&slow_work_last_thread_exited); printk(KERN_ERR "Slow work thread pool: Aborted\n"); } mutex_unlock(&slow_work_user_lock); return PTR_ERR(p); } EXPORT_SYMBOL(slow_work_register_user); /* * wait for all outstanding items from the calling module to complete * - note that more items may be queued whilst we're waiting */ static void slow_work_wait_for_items(struct module *module) { DECLARE_WAITQUEUE(myself, current); struct slow_work *work; int loop; mutex_lock(&slow_work_unreg_sync_lock); add_wait_queue(&slow_work_unreg_wq, &myself); for (;;) { spin_lock_irq(&slow_work_queue_lock); /* first of all, we wait for the last queued item in each list * to be processed */ list_for_each_entry_reverse(work, &vslow_work_queue, link) { if (work->owner == module) { set_current_state(TASK_UNINTERRUPTIBLE); slow_work_unreg_work_item = work; goto do_wait; } } list_for_each_entry_reverse(work, &slow_work_queue, link) { if (work->owner == module) { set_current_state(TASK_UNINTERRUPTIBLE); slow_work_unreg_work_item = work; goto do_wait; } } /* then we wait for the items being processed to finish */ slow_work_unreg_module = module; smp_mb(); for (loop = 0; loop < SLOW_WORK_THREAD_LIMIT; loop++) { if (slow_work_thread_processing[loop] == module) goto do_wait; } spin_unlock_irq(&slow_work_queue_lock); break; /* okay, we're done */ do_wait: spin_unlock_irq(&slow_work_queue_lock); schedule(); slow_work_unreg_work_item = NULL; slow_work_unreg_module = NULL; } remove_wait_queue(&slow_work_unreg_wq, &myself); mutex_unlock(&slow_work_unreg_sync_lock); } /** * slow_work_unregister_user - Unregister a user of the facility * @module: The module whose items should be cleared * * Unregister a user of the facility, killing all the threads if this was the * last one. * * This waits for all the work items belonging to the nominated module to go * away before proceeding. */ void slow_work_unregister_user(struct module *module) { /* first of all, wait for all outstanding items from the calling module * to complete */ if (module) slow_work_wait_for_items(module); /* then we can actually go about shutting down the facility if need * be */ mutex_lock(&slow_work_user_lock); BUG_ON(slow_work_user_count <= 0); slow_work_user_count--; if (slow_work_user_count == 0) { printk(KERN_NOTICE "Slow work thread pool: Shutting down\n"); slow_work_threads_should_exit = true; del_timer_sync(&slow_work_cull_timer); del_timer_sync(&slow_work_oom_timer); wake_up_all(&slow_work_thread_wq); wait_for_completion(&slow_work_last_thread_exited); printk(KERN_NOTICE "Slow work thread pool:" " Shut down complete\n"); } mutex_unlock(&slow_work_user_lock); } EXPORT_SYMBOL(slow_work_unregister_user); /* * Initialise the slow work facility */ static int __init init_slow_work(void) { unsigned nr_cpus = num_possible_cpus(); if (slow_work_max_threads < nr_cpus) slow_work_max_threads = nr_cpus; #ifdef CONFIG_SYSCTL if (slow_work_max_max_threads < nr_cpus * 2) slow_work_max_max_threads = nr_cpus * 2; #endif return 0; } subsys_initcall(init_slow_work);