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>

1 files changed, 19 insertions, 12 deletions

diff --git a/mm/pdflush.c b/mm/pdflush.c
index 235ac440c44e..f2caf96993f8 100644
--- a/mm/pdflush.c
+++ b/mm/pdflush.c
@@ -58,6 +58,14 @@ static DEFINE_SPINLOCK(pdflush_lock);
 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;
@@ -68,7 +76,7 @@ static unsigned long last_empty_jifs;
  * Thread pool management algorithm:
  * 
  * - The minimum and maximum number of pdflush instances are bound
- *   by MIN_PDFLUSH_THREADS and MAX_PDFLUSH_THREADS.
+ *   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.
@@ -134,14 +142,13 @@ static int __pdflush(struct pdflush_work *my_work)
                  * To throttle creation, we reset last_empty_jifs.
                  */
                 if (time_after(jiffies, last_empty_jifs + 1 * HZ)) {
-                        if (list_empty(&pdflush_list)) {
-                                if (nr_pdflush_threads < MAX_PDFLUSH_THREADS) {
-                                        last_empty_jifs = jiffies;
-                                        nr_pdflush_threads++;
-                                        spin_unlock_irq(&pdflush_lock);
-                                        start_one_pdflush_thread();
-                                        spin_lock_irq(&pdflush_lock);
-                                }
+                        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);
                         }
                 }
 
@@ -153,7 +160,7 @@ static int __pdflush(struct pdflush_work *my_work)
                  */
                 if (list_empty(&pdflush_list))
                         continue;
-                if (nr_pdflush_threads <= MIN_PDFLUSH_THREADS)
+                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)) {
@@ -259,9 +266,9 @@ static int __init pdflush_init(void)
          * Pre-set nr_pdflush_threads...  If we fail to create,
          * the count will be decremented.
          */
-        nr_pdflush_threads = MIN_PDFLUSH_THREADS;
+        nr_pdflush_threads = nr_pdflush_threads_min;
 
-        for (i = 0; i < MIN_PDFLUSH_THREADS; i++)
+        for (i = 0; i < nr_pdflush_threads_min; i++)
                 start_one_pdflush_thread();
         return 0;
 }