[RFC PATCH v4 04/20] mm: use part per 1000000 for bdi ratios.

[Stefan Roesch <shr@devkernel.io>]

To get finer granularity for ratio calculations use part per million
instead of percentiles. This is especially important if we want to
automatically convert byte values to ratios. Otherwise the values that
are actually used can be quite different. This is also important for
machines with more main memory (1% of 256GB is already 2.5GB).

Signed-off-by: Stefan Roesch <shr@devkernel.io>
---
 include/linux/backing-dev.h |  3 +++
 mm/backing-dev.c            |  6 +++---
 mm/page-writeback.c         | 15 +++++++++------
 3 files changed, 15 insertions(+), 9 deletions(-)

diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
index 9c984ffc8a0a..1b50c028e5ad 100644
--- a/include/linux/backing-dev.h
+++ b/include/linux/backing-dev.h
@@ -102,6 +102,9 @@ static inline unsigned long wb_stat_error(void)
 #endif
 }
 
+/* BDI ratio is expressed as part per 1000000 for finer granularity. */
+#define BDI_RATIO_SCALE 10000
+
 int bdi_set_min_ratio(struct backing_dev_info *bdi, unsigned int min_ratio);
 int bdi_set_max_ratio(struct backing_dev_info *bdi, unsigned int max_ratio);
 int bdi_set_strict_limit(struct backing_dev_info *bdi, unsigned int strict_limit);
diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index a0899cce72ef..90fa517123dc 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -178,7 +178,7 @@ static ssize_t min_ratio_store(struct device *dev,
 
 	return ret;
 }
-BDI_SHOW(min_ratio, bdi->min_ratio)
+BDI_SHOW(min_ratio, bdi->min_ratio / BDI_RATIO_SCALE)
 
 static ssize_t max_ratio_store(struct device *dev,
 		struct device_attribute *attr, const char *buf, size_t count)
@@ -197,7 +197,7 @@ static ssize_t max_ratio_store(struct device *dev,
 
 	return ret;
 }
-BDI_SHOW(max_ratio, bdi->max_ratio)
+BDI_SHOW(max_ratio, bdi->max_ratio / BDI_RATIO_SCALE)
 
 static ssize_t stable_pages_required_show(struct device *dev,
 					  struct device_attribute *attr,
@@ -809,7 +809,7 @@ int bdi_init(struct backing_dev_info *bdi)
 
 	kref_init(&bdi->refcnt);
 	bdi->min_ratio = 0;
-	bdi->max_ratio = 100;
+	bdi->max_ratio = 100 * BDI_RATIO_SCALE;
 	bdi->max_prop_frac = FPROP_FRAC_BASE;
 	INIT_LIST_HEAD(&bdi->bdi_list);
 	INIT_LIST_HEAD(&bdi->wb_list);
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 3745b886722f..dd98b2654302 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -197,7 +197,7 @@ static void wb_min_max_ratio(struct bdi_writeback *wb,
 			min *= this_bw;
 			min = div64_ul(min, tot_bw);
 		}
-		if (max < 100) {
+		if (max < 100 * BDI_RATIO_SCALE) {
 			max *= this_bw;
 			max = div64_ul(max, tot_bw);
 		}
@@ -655,6 +655,8 @@ int bdi_set_min_ratio(struct backing_dev_info *bdi, unsigned int min_ratio)
 	unsigned int delta;
 	int ret = 0;
 
+	min_ratio *= BDI_RATIO_SCALE;
+
 	spin_lock_bh(&bdi_lock);
 	if (min_ratio > bdi->max_ratio) {
 		ret = -EINVAL;
@@ -665,7 +667,7 @@ int bdi_set_min_ratio(struct backing_dev_info *bdi, unsigned int min_ratio)
 			bdi->min_ratio = min_ratio;
 		} else {
 			delta = min_ratio - bdi->min_ratio;
-			if (bdi_min_ratio + delta < 100) {
+			if (bdi_min_ratio + delta < 100 * BDI_RATIO_SCALE) {
 				bdi_min_ratio += delta;
 				bdi->min_ratio = min_ratio;
 			} else {
@@ -684,6 +686,7 @@ int bdi_set_max_ratio(struct backing_dev_info *bdi, unsigned max_ratio)
 
 	if (max_ratio > 100)
 		return -EINVAL;
+	max_ratio *= BDI_RATIO_SCALE;
 
 	spin_lock_bh(&bdi_lock);
 	if (bdi->min_ratio > max_ratio) {
@@ -775,15 +778,15 @@ static unsigned long __wb_calc_thresh(struct dirty_throttle_control *dtc)
 	fprop_fraction_percpu(&dom->completions, dtc->wb_completions,
 			      &numerator, &denominator);
 
-	wb_thresh = (thresh * (100 - bdi_min_ratio)) / 100;
+	wb_thresh = (thresh * (100 * BDI_RATIO_SCALE - bdi_min_ratio)) / (100 * BDI_RATIO_SCALE);
 	wb_thresh *= numerator;
 	wb_thresh = div64_ul(wb_thresh, denominator);
 
 	wb_min_max_ratio(dtc->wb, &wb_min_ratio, &wb_max_ratio);
 
-	wb_thresh += (thresh * wb_min_ratio) / 100;
-	if (wb_thresh > (thresh * wb_max_ratio) / 100)
-		wb_thresh = thresh * wb_max_ratio / 100;
+	wb_thresh += (thresh * wb_min_ratio) / (100 * BDI_RATIO_SCALE);
+	if (wb_thresh > (thresh * wb_max_ratio) / (100 * BDI_RATIO_SCALE))
+		wb_thresh = thresh * wb_max_ratio / (100 * BDI_RATIO_SCALE);
 
 	return wb_thresh;
 }
-- 
2.30.2