mm/pdflush.c

   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
  24
  25 /*
  26  * Minimum and maximum number of pdflush instances
  27  */
  28 #define MIN_PDFLUSH_THREADS     2
  29 #define MAX_PDFLUSH_THREADS     8
  30
  31 static void start_one_pdflush_thread(void);
  32
  33
  34 /*
  35  * The pdflush threads are worker threads for writing back dirty data.
  36  * Ideally, we'd like one thread per active disk spindle.  But the disk
  37  * topology is very hard to divine at this level.   Instead, we take
  38  * care in various places to prevent more than one pdflush thread from
  39  * performing writeback against a single filesystem.  pdflush threads
  40  * have the PF_FLUSHER flag set in current->flags to aid in this.
  41  */
  42
  43 /*
  44  * All the pdflush threads.  Protected by pdflush_lock
  45  */
  46 static LIST_HEAD(pdflush_list);
  47 static DEFINE_SPINLOCK(pdflush_lock);
  48
  49 /*
  50  * The count of currently-running pdflush threads.  Protected
  51  * by pdflush_lock.
  52  *
  53  * Readable by sysctl, but not writable.  Published to userspace at
  54  * /proc/sys/vm/nr_pdflush_threads.
  55  */
  56 int nr_pdflush_threads = 0;
  57
  58 /*
  59  * The time at which the pdflush thread pool last went empty
  60  */
  61 static unsigned long last_empty_jifs;
  62
  63 /*
  64  * The pdflush thread.
  65  *
  66  * Thread pool management algorithm:
  67  *
  68  * - The minimum and maximum number of pdflush instances are bound
  69  *   by MIN_PDFLUSH_THREADS and MAX_PDFLUSH_THREADS.
  70  *
  71  * - If there have been no idle pdflush instances for 1 second, create
  72  *   a new one.
  73  *
  74  * - If the least-recently-went-to-sleep pdflush thread has been asleep
  75  *   for more than one second, terminate a thread.
  76  */
  77
  78 /*
  79  * A structure for passing work to a pdflush thread.  Also for passing
  80  * state information between pdflush threads.  Protected by pdflush_lock.
  81  */
  82 struct pdflush_work {
  83         struct task_struct *who;        /* The thread */
  84         void (*fn)(unsigned long);      /* A callback function */
  85         unsigned long arg0;             /* An argument to the callback */
  86         struct list_head list;          /* On pdflush_list, when idle */
  87         unsigned long when_i_went_to_sleep;
  88 };
  89
  90 static int __pdflush(struct pdflush_work *my_work)
  91 {
  92         current->flags |= PF_FLUSHER;
  93         my_work->fn = NULL;
  94         my_work->who = current;
  95         INIT_LIST_HEAD(&my_work->list);
  96
  97         spin_lock_irq(&pdflush_lock);
  98         nr_pdflush_threads++;
  99         for ( ; ; ) {
 100                 struct pdflush_work *pdf;
 101
 102                 set_current_state(TASK_INTERRUPTIBLE);
 103                 list_move(&my_work->list, &pdflush_list);
 104                 my_work->when_i_went_to_sleep = jiffies;
 105                 spin_unlock_irq(&pdflush_lock);
 106
 107                 schedule();
 108                 if (try_to_freeze()) {
 109                         spin_lock_irq(&pdflush_lock);
 110                         continue;
 111                 }
 112
 113                 spin_lock_irq(&pdflush_lock);
 114                 if (!list_empty(&my_work->list)) {
 115                         printk("pdflush: bogus wakeup!\n");
 116                         my_work->fn = NULL;
 117                         continue;
 118                 }
 119                 if (my_work->fn == NULL) {
 120                         printk("pdflush: NULL work function\n");
 121                         continue;
 122                 }
 123                 spin_unlock_irq(&pdflush_lock);
 124
 125                 (*my_work->fn)(my_work->arg0);
 126
 127                 /*
 128                  * Thread creation: For how long have there been zero
 129                  * available threads?
 130                  */
 131                 if (jiffies - last_empty_jifs > 1 * HZ) {
 132                         /* unlocked list_empty() test is OK here */
 133                         if (list_empty(&pdflush_list)) {
 134                                 /* unlocked test is OK here */
 135                                 if (nr_pdflush_threads < MAX_PDFLUSH_THREADS)
 136                                         start_one_pdflush_thread();
 137                         }
 138                 }
 139
 140                 spin_lock_irq(&pdflush_lock);
 141                 my_work->fn = NULL;
 142
 143                 /*
 144                  * Thread destruction: For how long has the sleepiest
 145                  * thread slept?
 146                  */
 147                 if (list_empty(&pdflush_list))
 148                         continue;
 149                 if (nr_pdflush_threads <= MIN_PDFLUSH_THREADS)
 150                         continue;
 151                 pdf = list_entry(pdflush_list.prev, struct pdflush_work, list);
 152                 if (jiffies - pdf->when_i_went_to_sleep > 1 * HZ) {
 153                         /* Limit exit rate */
 154                         pdf->when_i_went_to_sleep = jiffies;
 155                         break;                                  /* exeunt */
 156                 }
 157         }
 158         nr_pdflush_threads--;
 159         spin_unlock_irq(&pdflush_lock);
 160         return 0;
 161 }
 162
 163 /*
 164  * Of course, my_work wants to be just a local in __pdflush().  It is
 165  * separated out in this manner to hopefully prevent the compiler from
 166  * performing unfortunate optimisations against the auto variables.  Because
 167  * these are visible to other tasks and CPUs.  (No problem has actually
 168  * been observed.  This is just paranoia).
 169  */
 170 static int pdflush(void *dummy)
 171 {
 172         struct pdflush_work my_work;
 173
 174         /*
 175          * pdflush can spend a lot of time doing encryption via dm-crypt.  We
 176          * don't want to do that at keventd's priority.
 177          */
 178         set_user_nice(current, 0);
 179         return __pdflush(&my_work);
 180 }
 181
 182 /*
 183  * Attempt to wake up a pdflush thread, and get it to do some work for you.
 184  * Returns zero if it indeed managed to find a worker thread, and passed your
 185  * payload to it.
 186  */
 187 int pdflush_operation(void (*fn)(unsigned long), unsigned long arg0)
 188 {
 189         unsigned long flags;
 190         int ret = 0;
 191
 192         if (fn == NULL)
 193                 BUG();          /* Hard to diagnose if it's deferred */
 194
 195         spin_lock_irqsave(&pdflush_lock, flags);
 196         if (list_empty(&pdflush_list)) {
 197                 spin_unlock_irqrestore(&pdflush_lock, flags);
 198                 ret = -1;
 199         } else {
 200                 struct pdflush_work *pdf;
 201
 202                 pdf = list_entry(pdflush_list.next, struct pdflush_work, list);
 203                 list_del_init(&pdf->list);
 204                 if (list_empty(&pdflush_list))
 205                         last_empty_jifs = jiffies;
 206                 pdf->fn = fn;
 207                 pdf->arg0 = arg0;
 208                 wake_up_process(pdf->who);
 209                 spin_unlock_irqrestore(&pdflush_lock, flags);
 210         }
 211         return ret;
 212 }
 213
 214 static void start_one_pdflush_thread(void)
 215 {
 216         kthread_run(pdflush, NULL, "pdflush");
 217 }
 218
 219 static int __init pdflush_init(void)
 220 {
 221         int i;
 222
 223         for (i = 0; i < MIN_PDFLUSH_THREADS; i++)
 224                 start_one_pdflush_thread();
 225         return 0;
 226 }
 227
 228 module_init(pdflush_init);