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