linux/mm/pdflush.c
Peter W Morreale fafd688e4c mm: add /proc controls for pdflush threads
Add /proc entries to give the admin the ability to control the minimum and
maximum number of pdflush threads.  This allows finer control of pdflush
on both large and small machines.

The rationale is simply one size does not fit all.  Admins on large and/or
small systems may want to tune the min/max pdflush thread count to best
suit their needs.  Right now the min/max is hardcoded to 2/8.  While
probably a fair estimate for smaller machines, large machines with large
numbers of CPUs and large numbers of filesystems/block devices may benefit
from larger numbers of threads working on different block devices.

Even if the background flushing algorithm is radically changed, it is
still likely that multiple threads will be involved and admins would still
desire finer control on the min/max other than to have to recompile the
kernel.

The patch adds '/proc/sys/vm/nr_pdflush_threads_min' and
'/proc/sys/vm/nr_pdflush_threads_max' with r/w permissions.

The minimum value for nr_pdflush_threads_min is 1 and the maximum value is
the current value of nr_pdflush_threads_max.  This minimum is required
since additional thread creation is performed in a pdflush thread itself.

The minimum value for nr_pdflush_threads_max is the current value of
nr_pdflush_threads_min and the maximum value can be 1000.

Documentation/sysctl/vm.txt is also updated.

[akpm@linux-foundation.org: fix comment, fix whitespace, use __read_mostly]
Signed-off-by: Peter W Morreale <pmorreale@novell.com>
Reviewed-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2009-04-07 08:31:03 -07:00

277 lines
7.2 KiB
C

/*
* mm/pdflush.c - worker threads for writing back filesystem data
*
* Copyright (C) 2002, Linus Torvalds.
*
* 09Apr2002 Andrew Morton
* 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>
#include <linux/freezer.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 max/min number of pdflush threads. R/W by sysctl at
* /proc/sys/vm/nr_pdflush_threads_max/min
*/
int nr_pdflush_threads_max __read_mostly = MAX_PDFLUSH_THREADS;
int nr_pdflush_threads_min __read_mostly = MIN_PDFLUSH_THREADS;
/*
* 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 nr_pdflush_threads_min and nr_pdflush_threads_max.
*
* - 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 | PF_SWAPWRITE;
set_freezable();
my_work->fn = NULL;
my_work->who = current;
INIT_LIST_HEAD(&my_work->list);
spin_lock_irq(&pdflush_lock);
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();
try_to_freeze();
spin_lock_irq(&pdflush_lock);
if (!list_empty(&my_work->list)) {
/*
* Someone woke us up, but without removing our control
* structure from the global list. swsusp will do this
* in try_to_freeze()->refrigerator(). Handle it.
*/
my_work->fn = NULL;
continue;
}
if (my_work->fn == NULL) {
printk("pdflush: bogus wakeup\n");
continue;
}
spin_unlock_irq(&pdflush_lock);
(*my_work->fn)(my_work->arg0);
spin_lock_irq(&pdflush_lock);
/*
* Thread creation: For how long have there been zero
* available threads?
*
* To throttle creation, we reset last_empty_jifs.
*/
if (time_after(jiffies, last_empty_jifs + 1 * HZ)) {
if (list_empty(&pdflush_list) &&
nr_pdflush_threads < nr_pdflush_threads_max) {
last_empty_jifs = jiffies;
nr_pdflush_threads++;
spin_unlock_irq(&pdflush_lock);
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 <= nr_pdflush_threads_min)
continue;
pdf = list_entry(pdflush_list.prev, struct pdflush_work, list);
if (time_after(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_var_t cpus_allowed;
/*
* Since the caller doesn't even check kthread_run() worked, let's not
* freak out too much if this fails.
*/
if (!alloc_cpumask_var(&cpus_allowed, GFP_KERNEL)) {
printk(KERN_WARNING "pdflush failed to allocate cpumask\n");
return 0;
}
/*
* 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_all_mask.
* 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.
*/
cpuset_cpus_allowed(current, cpus_allowed);
set_cpus_allowed_ptr(current, cpus_allowed);
free_cpumask_var(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;
BUG_ON(fn == NULL); /* Hard to diagnose if it's deferred */
spin_lock_irqsave(&pdflush_lock, flags);
if (list_empty(&pdflush_list)) {
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)
{
struct task_struct *k;
k = kthread_run(pdflush, NULL, "pdflush");
if (unlikely(IS_ERR(k))) {
spin_lock_irq(&pdflush_lock);
nr_pdflush_threads--;
spin_unlock_irq(&pdflush_lock);
}
}
static int __init pdflush_init(void)
{
int i;
/*
* Pre-set nr_pdflush_threads... If we fail to create,
* the count will be decremented.
*/
nr_pdflush_threads = nr_pdflush_threads_min;
for (i = 0; i < nr_pdflush_threads_min; i++)
start_one_pdflush_thread();
return 0;
}
module_init(pdflush_init);