commit 8311c29d40235062a843f4a8e8a70a44af6fe4c9
author NeilBrown <neilb@suse.de> Tue Mar 04 14:29:30 2008 -0800
committer Linus Torvalds <torvalds@woody.linux-foundation.org> Tue Mar 04 16:35:17 2008 -0800
tree c869143fae8f135646300d1bac6639af5bb30dd7
parent a35e63efa1fb18c6f20f38e3ddf3f8ffbcf0f6e7

md: reduce CPU wastage on idle md array with a write-intent bitmap

On an md array with a write-intent bitmap, a thread wakes up every few seconds
and scans the bitmap looking for work to do.  If the array is idle, there will
be no work to do, but a lot of scanning is done to discover this.

So cache the fact that the bitmap is completely clean, and avoid scanning the
whole bitmap when the cache is known to be clean.

Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/md/bitmap.c

diff --git a/drivers/md/bitmap.c b/drivers/md/bitmap.c
index 7aeceed..831aed9 100644
--- a/drivers/md/bitmap.c
+++ b/drivers/md/bitmap.c
@@ -1047,6 +1047,11 @@
 	if (time_before(jiffies, bitmap->daemon_lastrun + bitmap->daemon_sleep*HZ))
 		return;
 	bitmap->daemon_lastrun = jiffies;
+	if (bitmap->allclean) {
+		bitmap->mddev->thread->timeout = MAX_SCHEDULE_TIMEOUT;
+		return;
+	}
+	bitmap->allclean = 1;
 
 	for (j = 0; j < bitmap->chunks; j++) {
 		bitmap_counter_t *bmc;
@@ -1068,8 +1073,10 @@
 					clear_page_attr(bitmap, page, BITMAP_PAGE_NEEDWRITE);
 
 				spin_unlock_irqrestore(&bitmap->lock, flags);
-				if (need_write)
+				if (need_write) {
 					write_page(bitmap, page, 0);
+					bitmap->allclean = 0;
+				}
 				continue;
 			}
 
@@ -1098,6 +1105,9 @@
 /*
   if (j < 100) printk("bitmap: j=%lu, *bmc = 0x%x\n", j, *bmc);
 */
+			if (*bmc)
+				bitmap->allclean = 0;
+
 			if (*bmc == 2) {
 				*bmc=1; /* maybe clear the bit next time */
 				set_page_attr(bitmap, page, BITMAP_PAGE_CLEAN);
@@ -1132,6 +1142,8 @@
 		}
 	}
 
+	if (bitmap->allclean == 0)
+		bitmap->mddev->thread->timeout = bitmap->daemon_sleep * HZ;
 }
 
 static bitmap_counter_t *bitmap_get_counter(struct bitmap *bitmap,
@@ -1226,6 +1238,7 @@
 			sectors -= blocks;
 		else sectors = 0;
 	}
+	bitmap->allclean = 0;
 	return 0;
 }
 
@@ -1296,6 +1309,7 @@
 		}
 	}
 	spin_unlock_irq(&bitmap->lock);
+	bitmap->allclean = 0;
 	return rv;
 }
 
@@ -1332,6 +1346,7 @@
 	}
  unlock:
 	spin_unlock_irqrestore(&bitmap->lock, flags);
+	bitmap->allclean = 0;
 }
 
 void bitmap_close_sync(struct bitmap *bitmap)
@@ -1399,7 +1414,7 @@
 		set_page_attr(bitmap, page, BITMAP_PAGE_CLEAN);
 	}
 	spin_unlock_irq(&bitmap->lock);
-
+	bitmap->allclean = 0;
 }
 
 /* dirty the memory and file bits for bitmap chunks "s" to "e" */

include/linux/raid/bitmap.h

diff --git a/include/linux/raid/bitmap.h b/include/linux/raid/bitmap.h
index e51b531..47fbcba 100644
--- a/include/linux/raid/bitmap.h
+++ b/include/linux/raid/bitmap.h
@@ -235,6 +235,8 @@
 
 	unsigned long flags;
 
+	int allclean;
+
 	unsigned long max_write_behind; /* write-behind mode */
 	atomic_t behind_writes;