/*
* mm/pdflush.c - worker threads for writing back filesystem data
*
* Copyright (C) 2002, Linus Torvalds.
*
* 09Apr2002 akpm@zip.com.au
* Initial version
* 29Feb2004 kaos@sgi.com
* Move worker thread creation to kthread to avoid chewing
* up stack space with nested calls to kernel_thread.
*/
#include <linux/sched.h>
#include <linux/list.h>
#include <linux/signal.h>
#include <linux/spinlock.h>
#include <linux/gfp.h>
#include <linux/init.h>
#include <linux/module.h>
#include <linux/fs.h> // Needed by writeback.h
#include <linux/writeback.h> // Prototypes pdflush_operation()
#include <linux/kthread.h>
#include <linux/cpuset.h>
/*
* Minimum and maximum number of pdflush instances
*/
#define MIN_PDFLUSH_THREADS 2
#define MAX_PDFLUSH_THREADS 8
static void start_one_pdflush_thread(void);
/*
* The pdflush threads are worker threads for writing back dirty data.
* Ideally, we'd like one thread per active disk spindle. But the disk
* topology is very hard to divine at this level. Instead, we take
* care in various places to prevent more than one pdflush thread from
* performing writeback against a single filesystem. pdflush threads
* have the PF_FLUSHER flag set in current->flags to aid in this.
*/
/*
* All the pdflush threads. Protected by pdflush_lock
*/
static LIST_HEAD(pdflush_list);
static DEFINE_SPINLOCK(pdflush_lock);
/*
* The count of currently-running pdflush threads. Protected
* by pdflush_lock.
*
* Readable by sysctl, but not writable. Published to userspace at
* /proc/sys/vm/nr_pdflush_threads.
*/
int nr_pdflush_threads = 0;
/*
* The time at which the pdflush thread pool last went empty
*/
static unsigned long last_empty_jifs;
/*
* The pdflush thread.
*
* Thread pool management algorithm:
*
* - The minimum and maximum number of pdflush instances are bound
* by MIN_PDFLUSH_THREADS and MAX_PDFLUSH_THREADS.
*
* - If there have been no idle pdflush instances for 1 second, create
* a new one.
*
* - If the least-recently-went-to-sleep pdflush thread has been asleep
* for more than one second, terminate a thread.
*/
/*
* A structure for passing work to a pdflush thread. Also for passing
* state information between pdflush threads. Protected by pdflush_lock.
*/
struct pdflush_work {
struct task_struct *who; /* The thread */
void (*fn)(unsigned long); /* A callback function */
unsigned long arg0; /* An argument to the callback */
struct list_head list; /* On pdflush_list, when idle */
unsigned long when_i_went_to_sleep;
};
static int __pdflush(struct pdflush_work *my_work)
{
current->flags |= PF_FLUSHER;
my_work->fn = NULL;
my_work->who = current;
INIT_LIST_HEAD(&my_work->list);
spin_lock_irq(&pdflush_lock);
nr_pdflush_threads++;
for ( ; ; ) {
struct pdflush_work *pdf;
set_current_state(TASK_INTERRUPTIBLE);
list_move(&my_work->list, &pdflush_list);
my_work->when_i_went_to_sleep = jiffies;
spin_unlock_irq(&pdflush_lock);
schedule();
if (try_to_freeze()) {
spin_lock_irq(&pdflush_lock);
continue;
}
spin_lock_irq(&pdflush_lock);
if (!list_empty(&my_work->list)) {
printk("pdflush: bogus wakeup!\n");
my_work->fn = NULL;
continue;
}
if (my_work->fn == NULL) {
printk("pdflush: NULL work function\n");
continue;
}
spin_unlock_irq(&pdflush_lock);
(*my_work->fn)(my_work->arg0);
/*
* Thread creation: For how long have there been zero
* available threads?
*/
if (jiffies - last_empty_jifs > 1 * HZ) {
/* unlocked list_empty() test is OK here */
if (list_empty(&pdflush_list)) {
/* unlocked test is OK here */
if (nr_pdflush_threads < MAX_PDFLUSH_THREADS)
start_one_pdflush_thread();
}
}
spin_lock_irq(&pdflush_lock);
my_work->fn = NULL;
/*
* Thread destruction: For how long has the sleepiest
* thread slept?
*/
if (list_empty(&pdflush_list))
continue;
if (nr_pdflush_threads <= MIN_PDFLUSH_THREADS)
continue;
pdf = list_entry(pdflush_list.prev, struct pdflush_work, list);
if (jiffies - pdf->when_i_went_to_sleep > 1 * HZ) {
/* Limit exit rate */
pdf->when_i_went_to_sleep = jiffies;
break; /* exeunt */
}
}
nr_pdflush_threads--;
spin_unlock_irq(&pdflush_lock);
return 0;
}
/*
* Of course, my_work wants to be just a local in __pdflush(). It is
* separated out in this manner to hopefully prevent the compiler from
* performing unfortunate optimisations against the auto variables. Because
* these are visible to other tasks and CPUs. (No problem has actually
* been observed. This is just paranoia).
*/
static int pdflush(void *dummy)
{
struct pdflush_work my_work;
cpumask_t cpus_allowed;
/*
* pdflush can spend a lot of time doing encryption via dm-crypt. We
* don't want to do that at keventd's priority.
*/
set_user_nice(current, 0);
/*
* Some configs put our parent kthread in a limited cpuset,
* which kthread() overrides, forcing cpus_allowed == CPU_MASK_ALL.
* Our needs are more modest - cut back to our cpusets cpus_allowed.
* This is needed as pdflush's are dynamically created and destroyed.
* The boottime pdflush's are easily placed w/o these 2 lines.
*/
cpus_allowed = cpuset_cpus_allowed(current);
set_cpus_allowed(current, cpus_allowed);
return __pdflush(&my_work);
}
/*
* Attempt to wake up a pdflush thread, and get it to do some work for you.
* Returns zero if it indeed managed to find a worker thread, and passed your
* payload to it.
*/
int pdflush_operation(void (*fn)(unsigned long), unsigned long arg0)
{
unsigned long flags;
int ret = 0;
if (fn == NULL)
BUG(); /* Hard to diagnose if it's deferred */
spin_lock_irqsave(&pdflush_lock, flags);
if (list_empty(&pdflush_list)) {
spin_unlock_irqrestore(&pdflush_lock, flags);
ret = -1;
} else {
struct pdflush_work *pdf;
pdf = list_entry(pdflush_list.next, struct pdflush_work, list);
list_del_init(&pdf->list);
if (list_empty(&pdflush_list))
last_empty_jifs = jiffies;
pdf->fn = fn;
pdf->arg0 = arg0;
wake_up_process(pdf->who);
spin_unlock_irqrestore(&pdflush_lock, flags);
}
return ret;
}
static void start_one_pdflush_thread(void)
{
kthread_run(pdflush, NULL, "pdflush");
}
static int __init pdflush_init(void)
{
int i;
for (i = 0; i < MIN_PDFLUSH_THREADS; i++)
start_one_pdflush_thread();
return 0;
}
module_init(pdflush_init);
