[RFC PATCH] v2 mm: balance_dirty_pages. reduce calls toglobal_page_state to reduce cache references

[Richard Kennedy]

Reducing the number of times balance_dirty_pages calls global_page_state
reduces the cache references and so improves write performance on a
variety of workloads.

'perf stats' of simple fio write tests shows the reduction in cache
access.
Where the test is fio 'write,mmap,600Mb,pre_read' on AMD AthlonX2 with
3Gb memory (dirty_threshold approx 600 Mb)
running each test 10 times, dropping the fasted & slowest values then
taking 
the average & standard deviation

		average (s.d.) in millions (10^6)
2.6.31-rc8	648.6 (14.6)
+patch		620.1 (16.5)

Achieving this reduction is by dropping clip_bdi_dirty_limit as it  
rereads the counters to apply the dirty_threshold and moving this check
up into balance_dirty_pages where it has already read the counters.

Also by rearrange the for loop to only contain one copy of the limit
tests allows the pdflush test after the loop to use the local copies of
the counters rather than rereading them.

In the common case with no throttling it now calls global_page_state 5
fewer times and bdi_stat 2 fewer.

This version includes the changes suggested by 
Wu Fengguang <fengguang.wu@xxxxxxxxx>

Signed-off-by: Richard Kennedy <richard@xxxxxxxxxxxxxxx>
----
Thanks to everybody for the feedback & suggestions.
This patch is against 2.6.31-rc8

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 81627eb..9581359 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -260,32 +260,6 @@ static void bdi_writeout_fraction(struct backing_dev_info *bdi,
 	}
 }
 
-/*
- * Clip the earned share of dirty pages to that which is actually available.
- * This avoids exceeding the total dirty_limit when the floating averages
- * fluctuate too quickly.
- */
-static void clip_bdi_dirty_limit(struct backing_dev_info *bdi,
-		unsigned long dirty, unsigned long *pbdi_dirty)
-{
-	unsigned long avail_dirty;
-
-	avail_dirty = global_page_state(NR_FILE_DIRTY) +
-		 global_page_state(NR_WRITEBACK) +
-		 global_page_state(NR_UNSTABLE_NFS) +
-		 global_page_state(NR_WRITEBACK_TEMP);
-
-	if (avail_dirty < dirty)
-		avail_dirty = dirty - avail_dirty;
-	else
-		avail_dirty = 0;
-
-	avail_dirty += bdi_stat(bdi, BDI_RECLAIMABLE) +
-		bdi_stat(bdi, BDI_WRITEBACK);
-
-	*pbdi_dirty = min(*pbdi_dirty, avail_dirty);
-}
-
 static inline void task_dirties_fraction(struct task_struct *tsk,
 		long *numerator, long *denominator)
 {
@@ -478,7 +452,6 @@ get_dirty_limits(unsigned long *pbackground, unsigned long *pdirty,
 			bdi_dirty = dirty * bdi->max_ratio / 100;
 
 		*pbdi_dirty = bdi_dirty;
-		clip_bdi_dirty_limit(bdi, dirty, pbdi_dirty);
 		task_dirty_limit(current, pbdi_dirty);
 	}
 }
@@ -499,7 +472,7 @@ static void balance_dirty_pages(struct address_space *mapping)
 	unsigned long bdi_thresh;
 	unsigned long pages_written = 0;
 	unsigned long write_chunk = sync_writeback_pages();
-
+	int dirty_exceeded;
 	struct backing_dev_info *bdi = mapping->backing_dev_info;
 
 	for (;;) {
@@ -512,16 +485,36 @@ static void balance_dirty_pages(struct address_space *mapping)
 		};
 
 		get_dirty_limits(&background_thresh, &dirty_thresh,
-				&bdi_thresh, bdi);
+				 &bdi_thresh, bdi);
 
 		nr_reclaimable = global_page_state(NR_FILE_DIRTY) +
-					global_page_state(NR_UNSTABLE_NFS);
-		nr_writeback = global_page_state(NR_WRITEBACK);
+			global_page_state(NR_UNSTABLE_NFS);
+		nr_writeback = global_page_state(NR_WRITEBACK) +
+			global_page_state(NR_WRITEBACK_TEMP);
 
-		bdi_nr_reclaimable = bdi_stat(bdi, BDI_RECLAIMABLE);
-		bdi_nr_writeback = bdi_stat(bdi, BDI_WRITEBACK);
+		/*
+		 * In order to avoid the stacked BDI deadlock we need
+		 * to ensure we accurately count the 'dirty' pages when
+		 * the threshold is low.
+		 *
+		 * Otherwise it would be possible to get thresh+n pages
+		 * reported dirty, even though there are thresh-m pages
+		 * actually dirty; with m+n sitting in the percpu
+		 * deltas.
+		 */
+		if (bdi_thresh < 2*bdi_stat_error(bdi)) {
+			bdi_nr_reclaimable = bdi_stat_sum(bdi, BDI_RECLAIMABLE);
+			bdi_nr_writeback = bdi_stat_sum(bdi, BDI_WRITEBACK);
+		} else if (bdi_nr_reclaimable) {
+			bdi_nr_reclaimable = bdi_stat(bdi, BDI_RECLAIMABLE);
+			bdi_nr_writeback = bdi_stat(bdi, BDI_WRITEBACK);
+		}
 
-		if (bdi_nr_reclaimable + bdi_nr_writeback <= bdi_thresh)
+		dirty_exceeded =
+			(bdi_nr_reclaimable + bdi_nr_writeback >= bdi_thresh)
+			|| (nr_reclaimable + nr_writeback >= dirty_thresh);
+
+		if (!dirty_exceeded)
 			break;
 
 		/*
@@ -530,7 +523,11 @@ static void balance_dirty_pages(struct address_space *mapping)
 		 * when the bdi limits are ramping up.
 		 */
 		if (nr_reclaimable + nr_writeback <
-				(background_thresh + dirty_thresh) / 2)
+		    (background_thresh + dirty_thresh) / 2)
+			break;
+
+		/* done enough? */
+		if (pages_written >= write_chunk)
 			break;
 
 		if (!bdi->dirty_exceeded)
@@ -548,38 +545,14 @@ static void balance_dirty_pages(struct address_space *mapping)
 		if (bdi_nr_reclaimable > bdi_thresh) {
 			writeback_inodes(&wbc);
 			pages_written += write_chunk - wbc.nr_to_write;
-			get_dirty_limits(&background_thresh, &dirty_thresh,
-				       &bdi_thresh, bdi);
-		}
-
-		/*
-		 * In order to avoid the stacked BDI deadlock we need
-		 * to ensure we accurately count the 'dirty' pages when
-		 * the threshold is low.
-		 *
-		 * Otherwise it would be possible to get thresh+n pages
-		 * reported dirty, even though there are thresh-m pages
-		 * actually dirty; with m+n sitting in the percpu
-		 * deltas.
-		 */
-		if (bdi_thresh < 2*bdi_stat_error(bdi)) {
-			bdi_nr_reclaimable = bdi_stat_sum(bdi, BDI_RECLAIMABLE);
-			bdi_nr_writeback = bdi_stat_sum(bdi, BDI_WRITEBACK);
-		} else if (bdi_nr_reclaimable) {
-			bdi_nr_reclaimable = bdi_stat(bdi, BDI_RECLAIMABLE);
-			bdi_nr_writeback = bdi_stat(bdi, BDI_WRITEBACK);
+			/* don't wait if we've done enough */
+			if (pages_written >= write_chunk)
+				continue;
 		}
-
-		if (bdi_nr_reclaimable + bdi_nr_writeback <= bdi_thresh)
-			break;
-		if (pages_written >= write_chunk)
-			break;		/* We've done our duty */
-
 		congestion_wait(BLK_RW_ASYNC, HZ/10);
 	}
 
-	if (bdi_nr_reclaimable + bdi_nr_writeback < bdi_thresh &&
-			bdi->dirty_exceeded)
+	if (!dirty_exceeded && bdi->dirty_exceeded)
 		bdi->dirty_exceeded = 0;
 
 	if (writeback_in_progress(bdi))
@@ -593,10 +566,8 @@ static void balance_dirty_pages(struct address_space *mapping)
 	 * In normal mode, we start background writeout at the lower
 	 * background_thresh, to keep the amount of dirty memory low.
 	 */
-	if ((laptop_mode && pages_written) ||
-			(!laptop_mode && (global_page_state(NR_FILE_DIRTY)
-					  + global_page_state(NR_UNSTABLE_NFS)
-					  > background_thresh)))
+	if ((laptop_mode && pages_written) || (!laptop_mode &&
+	     (nr_reclaimable > background_thresh)))
 		pdflush_operation(background_writeout, 0);
 }
 


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