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-rw-r--r--kernel/slow-work.c640
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1/* Worker thread pool for slow items, such as filesystem lookups or mkdirs
2 *
3 * Copyright (C) 2008 Red Hat, Inc. All Rights Reserved.
4 * Written by David Howells (dhowells@redhat.com)
5 *
6 * This program is free software; you can redistribute it and/or
7 * modify it under the terms of the GNU General Public Licence
8 * as published by the Free Software Foundation; either version
9 * 2 of the Licence, or (at your option) any later version.
10 *
11 * See Documentation/slow-work.txt
12 */
13
14#include <linux/module.h>
15#include <linux/slow-work.h>
16#include <linux/kthread.h>
17#include <linux/freezer.h>
18#include <linux/wait.h>
19
20#define SLOW_WORK_CULL_TIMEOUT (5 * HZ) /* cull threads 5s after running out of
21 * things to do */
22#define SLOW_WORK_OOM_TIMEOUT (5 * HZ) /* can't start new threads for 5s after
23 * OOM */
24
25static void slow_work_cull_timeout(unsigned long);
26static void slow_work_oom_timeout(unsigned long);
27
28#ifdef CONFIG_SYSCTL
29static int slow_work_min_threads_sysctl(struct ctl_table *, int, struct file *,
30 void __user *, size_t *, loff_t *);
31
32static int slow_work_max_threads_sysctl(struct ctl_table *, int , struct file *,
33 void __user *, size_t *, loff_t *);
34#endif
35
36/*
37 * The pool of threads has at least min threads in it as long as someone is
38 * using the facility, and may have as many as max.
39 *
40 * A portion of the pool may be processing very slow operations.
41 */
42static unsigned slow_work_min_threads = 2;
43static unsigned slow_work_max_threads = 4;
44static unsigned vslow_work_proportion = 50; /* % of threads that may process
45 * very slow work */
46
47#ifdef CONFIG_SYSCTL
48static const int slow_work_min_min_threads = 2;
49static int slow_work_max_max_threads = 255;
50static const int slow_work_min_vslow = 1;
51static const int slow_work_max_vslow = 99;
52
53ctl_table slow_work_sysctls[] = {
54 {
55 .ctl_name = CTL_UNNUMBERED,
56 .procname = "min-threads",
57 .data = &slow_work_min_threads,
58 .maxlen = sizeof(unsigned),
59 .mode = 0644,
60 .proc_handler = slow_work_min_threads_sysctl,
61 .extra1 = (void *) &slow_work_min_min_threads,
62 .extra2 = &slow_work_max_threads,
63 },
64 {
65 .ctl_name = CTL_UNNUMBERED,
66 .procname = "max-threads",
67 .data = &slow_work_max_threads,
68 .maxlen = sizeof(unsigned),
69 .mode = 0644,
70 .proc_handler = slow_work_max_threads_sysctl,
71 .extra1 = &slow_work_min_threads,
72 .extra2 = (void *) &slow_work_max_max_threads,
73 },
74 {
75 .ctl_name = CTL_UNNUMBERED,
76 .procname = "vslow-percentage",
77 .data = &vslow_work_proportion,
78 .maxlen = sizeof(unsigned),
79 .mode = 0644,
80 .proc_handler = &proc_dointvec_minmax,
81 .extra1 = (void *) &slow_work_min_vslow,
82 .extra2 = (void *) &slow_work_max_vslow,
83 },
84 { .ctl_name = 0 }
85};
86#endif
87
88/*
89 * The active state of the thread pool
90 */
91static atomic_t slow_work_thread_count;
92static atomic_t vslow_work_executing_count;
93
94static bool slow_work_may_not_start_new_thread;
95static bool slow_work_cull; /* cull a thread due to lack of activity */
96static DEFINE_TIMER(slow_work_cull_timer, slow_work_cull_timeout, 0, 0);
97static DEFINE_TIMER(slow_work_oom_timer, slow_work_oom_timeout, 0, 0);
98static struct slow_work slow_work_new_thread; /* new thread starter */
99
100/*
101 * The queues of work items and the lock governing access to them. These are
102 * shared between all the CPUs. It doesn't make sense to have per-CPU queues
103 * as the number of threads bears no relation to the number of CPUs.
104 *
105 * There are two queues of work items: one for slow work items, and one for
106 * very slow work items.
107 */
108static LIST_HEAD(slow_work_queue);
109static LIST_HEAD(vslow_work_queue);
110static DEFINE_SPINLOCK(slow_work_queue_lock);
111
112/*
113 * The thread controls. A variable used to signal to the threads that they
114 * should exit when the queue is empty, a waitqueue used by the threads to wait
115 * for signals, and a completion set by the last thread to exit.
116 */
117static bool slow_work_threads_should_exit;
118static DECLARE_WAIT_QUEUE_HEAD(slow_work_thread_wq);
119static DECLARE_COMPLETION(slow_work_last_thread_exited);
120
121/*
122 * The number of users of the thread pool and its lock. Whilst this is zero we
123 * have no threads hanging around, and when this reaches zero, we wait for all
124 * active or queued work items to complete and kill all the threads we do have.
125 */
126static int slow_work_user_count;
127static DEFINE_MUTEX(slow_work_user_lock);
128
129/*
130 * Calculate the maximum number of active threads in the pool that are
131 * permitted to process very slow work items.
132 *
133 * The answer is rounded up to at least 1, but may not equal or exceed the
134 * maximum number of the threads in the pool. This means we always have at
135 * least one thread that can process slow work items, and we always have at
136 * least one thread that won't get tied up doing so.
137 */
138static unsigned slow_work_calc_vsmax(void)
139{
140 unsigned vsmax;
141
142 vsmax = atomic_read(&slow_work_thread_count) * vslow_work_proportion;
143 vsmax /= 100;
144 vsmax = max(vsmax, 1U);
145 return min(vsmax, slow_work_max_threads - 1);
146}
147
148/*
149 * Attempt to execute stuff queued on a slow thread. Return true if we managed
150 * it, false if there was nothing to do.
151 */
152static bool slow_work_execute(void)
153{
154 struct slow_work *work = NULL;
155 unsigned vsmax;
156 bool very_slow;
157
158 vsmax = slow_work_calc_vsmax();
159
160 /* see if we can schedule a new thread to be started if we're not
161 * keeping up with the work */
162 if (!waitqueue_active(&slow_work_thread_wq) &&
163 (!list_empty(&slow_work_queue) || !list_empty(&vslow_work_queue)) &&
164 atomic_read(&slow_work_thread_count) < slow_work_max_threads &&
165 !slow_work_may_not_start_new_thread)
166 slow_work_enqueue(&slow_work_new_thread);
167
168 /* find something to execute */
169 spin_lock_irq(&slow_work_queue_lock);
170 if (!list_empty(&vslow_work_queue) &&
171 atomic_read(&vslow_work_executing_count) < vsmax) {
172 work = list_entry(vslow_work_queue.next,
173 struct slow_work, link);
174 if (test_and_set_bit_lock(SLOW_WORK_EXECUTING, &work->flags))
175 BUG();
176 list_del_init(&work->link);
177 atomic_inc(&vslow_work_executing_count);
178 very_slow = true;
179 } else if (!list_empty(&slow_work_queue)) {
180 work = list_entry(slow_work_queue.next,
181 struct slow_work, link);
182 if (test_and_set_bit_lock(SLOW_WORK_EXECUTING, &work->flags))
183 BUG();
184 list_del_init(&work->link);
185 very_slow = false;
186 } else {
187 very_slow = false; /* avoid the compiler warning */
188 }
189 spin_unlock_irq(&slow_work_queue_lock);
190
191 if (!work)
192 return false;
193
194 if (!test_and_clear_bit(SLOW_WORK_PENDING, &work->flags))
195 BUG();
196
197 work->ops->execute(work);
198
199 if (very_slow)
200 atomic_dec(&vslow_work_executing_count);
201 clear_bit_unlock(SLOW_WORK_EXECUTING, &work->flags);
202
203 /* if someone tried to enqueue the item whilst we were executing it,
204 * then it'll be left unenqueued to avoid multiple threads trying to
205 * execute it simultaneously
206 *
207 * there is, however, a race between us testing the pending flag and
208 * getting the spinlock, and between the enqueuer setting the pending
209 * flag and getting the spinlock, so we use a deferral bit to tell us
210 * if the enqueuer got there first
211 */
212 if (test_bit(SLOW_WORK_PENDING, &work->flags)) {
213 spin_lock_irq(&slow_work_queue_lock);
214
215 if (!test_bit(SLOW_WORK_EXECUTING, &work->flags) &&
216