1 /*
2 * mm/pdflush.c - worker threads for writing back filesystem data
3 *
4 * Copyright (C) 2002, Linus Torvalds.
5 *
6 * 09Apr2002 akpm@zip.com.au
7 * Initial version
8 * 29Feb2004 kaos@sgi.com
9 * Move worker thread creation to kthread to avoid chewing
10 * up stack space with nested calls to kernel_thread.
11 */
12
13 #include <linux/sched.h>
14 #include <linux/list.h>
15 #include <linux/signal.h>
16 #include <linux/spinlock.h>
17 #include <linux/gfp.h>
18 #include <linux/init.h>
19 #include <linux/module.h>
20 #include <linux/fs.h> // Needed by writeback.h
21 #include <linux/writeback.h> // Prototypes pdflush_operation()
22 #include <linux/kthread.h>
23 #include <linux/cpuset.h>
24 #include <linux/freezer.h>
25
26
27 /*
28 * Minimum and maximum number of pdflush instances
29 */
30 #define MIN_PDFLUSH_THREADS 2
31 #define MAX_PDFLUSH_THREADS 8
32
33 static void start_one_pdflush_thread(void);
34
35
36 /*
37 * The pdflush threads are worker threads for writing back dirty data.
38 * Ideally, we'd like one thread per active disk spindle. But the disk
39 * topology is very hard to divine at this level. Instead, we take
40 * care in various places to prevent more than one pdflush thread from
41 * performing writeback against a single filesystem. pdflush threads
42 * have the PF_FLUSHER flag set in current->flags to aid in this.
43 */
44
45 /*
46 * All the pdflush threads. Protected by pdflush_lock
47 */
48 static LIST_HEAD(pdflush_list);
49 static DEFINE_SPINLOCK(pdflush_lock);
50
51 /*
52 * The count of currently-running pdflush threads. Protected
53 * by pdflush_lock.
54 *
55 * Readable by sysctl, but not writable. Published to userspace at
56 * /proc/sys/vm/nr_pdflush_threads.
57 */
58 int nr_pdflush_threads = 0;
59
60 /*
61 * The time at which the pdflush thread pool last went empty
62 */
63 static unsigned long last_empty_jifs;
64
65 /*
66 * The pdflush thread.
67 *
68 * Thread pool management algorithm:
69 *
70 * - The minimum and maximum number of pdflush instances are bound
71 * by MIN_PDFLUSH_THREADS and MAX_PDFLUSH_THREADS.
72 *
73 * - If there have been no idle pdflush instances for 1 second, create
74 * a new one.
75 *
76 * - If the least-recently-went-to-sleep pdflush thread has been asleep
77 * for more than one second, terminate a thread.
78 */
79
80 /*
81 * A structure for passing work to a pdflush thread. Also for passing
82 * state information between pdflush threads. Protected by pdflush_lock.
83 */
84 struct pdflush_work {
85 struct task_struct *who; /* The thread */
86 void (*fn)(unsigned long); /* A callback function */
87 unsigned long arg0; /* An argument to the callback */
88 struct list_head list; /* On pdflush_list, when idle */
89 unsigned long when_i_went_to_sleep;
90 };
91
92 static int __pdflush(struct pdflush_work *my_work)
93 {
94 current->flags |= PF_FLUSHER | PF_SWAPWRITE;
95 set_freezable();
96 my_work->fn = NULL;
97 my_work->who = current;
98 INIT_LIST_HEAD(&my_work->list);
99
100 spin_lock_irq(&pdflush_lock);
101 nr_pdflush_threads++;
102 for ( ; ; ) {
103 struct pdflush_work *pdf;
104
105 set_current_state(TASK_INTERRUPTIBLE);
106 list_move(&my_work->list, &pdflush_list);
107 my_work->when_i_went_to_sleep = jiffies;
108 spin_unlock_irq(&pdflush_lock);
109 schedule();
110 try_to_freeze();
111 spin_lock_irq(&pdflush_lock);
112 if (!list_empty(&my_work->list)) {
113 /*
114 * Someone woke us up, but without removing our control
115 * structure from the global list. swsusp will do this
116 * in try_to_freeze()->refrigerator(). Handle it.
117 */
118 my_work->fn = NULL;
119 continue;
120 }
121 if (my_work->fn == NULL) {
122 printk("pdflush: bogus wakeup\n");
123 continue;
124 }
125 spin_unlock_irq(&pdflush_lock);
126
127 (*my_work->fn)(my_work->arg0);
128
129 /*
130 * Thread creation: For how long have there been zero
131 * available threads?
132 */
133 if (jiffies - last_empty_jifs > 1 * HZ) {
134 /* unlocked list_empty() test is OK here */
135 if (list_empty(&pdflush_list)) {
136 /* unlocked test is OK here */
137 if (nr_pdflush_threads < MAX_PDFLUSH_THREADS)
138 start_one_pdflush_thread();
139 }
140 }
141
142 spin_lock_irq(&pdflush_lock);
143 my_work->fn = NULL;
144
145 /*
146 * Thread destruction: For how long has the sleepiest
147 * thread slept?
148 */
149 if (list_empty(&pdflush_list))
150 continue;
151 if (nr_pdflush_threads <= MIN_PDFLUSH_THREADS)
152 continue;
153 pdf = list_entry(pdflush_list.prev, struct pdflush_work, list);
154 if (jiffies - pdf->when_i_went_to_sleep > 1 * HZ) {
155 /* Limit exit rate */
156 pdf->when_i_went_to_sleep = jiffies;
157 break; /* exeunt */
158 }
159 }
160 nr_pdflush_threads--;
161 spin_unlock_irq(&pdflush_lock);
162 return 0;
163 }
164
165 /*
166 * Of course, my_work wants to be just a local in __pdflush(). It is
167 * separated out in this manner to hopefully prevent the compiler from
168 * performing unfortunate optimisations against the auto variables. Because
169 * these are visible to other tasks and CPUs. (No problem has actually
170 * been observed. This is just paranoia).
171 */
172 static int pdflush(void *dummy)
173 {
174 struct pdflush_work my_work;
175 cpumask_t cpus_allowed;
176
177 /*
178 * pdflush can spend a lot of time doing encryption via dm-crypt. We
179 * don't want to do that at keventd's priority.
180 */
181 set_user_nice(current, 0);
182
183 /*
184 * Some configs put our parent kthread in a limited cpuset,
185 * which kthread() overrides, forcing cpus_allowed == CPU_MASK_ALL.
186 * Our needs are more modest - cut back to our cpusets cpus_allowed.
187 * This is needed as pdflush's are dynamically created and destroyed.
188 * The boottime pdflush's are easily placed w/o these 2 lines.
189 */
190 cpus_allowed = cpuset_cpus_allowed(current);
191 set_cpus_allowed(current, cpus_allowed);
192
193 return __pdflush(&my_work);
194 }
195
196 /*
197 * Attempt to wake up a pdflush thread, and get it to do some work for you.
198 * Returns zero if it indeed managed to find a worker thread, and passed your
199 * payload to it.
200 */
201 int pdflush_operation(void (*fn)(unsigned long), unsigned long arg0)
202 {
203 unsigned long flags;
204 int ret = 0;
205
206 BUG_ON(fn == NULL); /* Hard to diagnose if it's deferred */
207
208 spin_lock_irqsave(&pdflush_lock, flags);
209 if (list_empty(&pdflush_list)) {
210 spin_unlock_irqrestore(&pdflush_lock, flags);
211 ret = -1;
212 } else {
213 struct pdflush_work *pdf;
214
215 pdf = list_entry(pdflush_list.next, struct pdflush_work, list);
216 list_del_init(&pdf->list);
217 if (list_empty(&pdflush_list))
218 last_empty_jifs = jiffies;
219 pdf->fn = fn;
220 pdf->arg0 = arg0;
221 wake_up_process(pdf->who);
222 spin_unlock_irqrestore(&pdflush_lock, flags);
223 }
224 return ret;
225 }
226
227 static void start_one_pdflush_thread(void)
228 {
229 kthread_run(pdflush, NULL, "pdflush");
230 }
231
232 static int __init pdflush_init(void)
233 {
234 int i;
235
236 for (i = 0; i < MIN_PDFLUSH_THREADS; i++)
237 start_one_pdflush_thread();
238 return 0;
239 }
240
241 module_init(pdflush_init);
242
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