block: Convert BDI proportion calculations to flexible proportions

Convert calculations of proportion of writeback each bdi does to new flexible
proportion code. That allows us to use aging period of fixed wallclock time
which gives better proportion estimates given the hugely varying throughput of
different devices.

Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>

1 files changed, 64 insertions, 39 deletions

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 93d8d2f7108..ec14419e53b 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -34,6 +34,7 @@
 #include <linux/syscalls.h>
 #include <linux/buffer_head.h> /* __set_page_dirty_buffers */
 #include <linux/pagevec.h>
+#include <linux/timer.h>
 #include <trace/events/writeback.h>
 
 /*
@@ -135,7 +136,20 @@ unsigned long global_dirty_limit;
  * measured in page writeback completions.
  *
  */
-static struct prop_descriptor vm_completions;
+static struct fprop_global writeout_completions;
+
+static void writeout_period(unsigned long t);
+/* Timer for aging of writeout_completions */
+static struct timer_list writeout_period_timer =
+		TIMER_DEFERRED_INITIALIZER(writeout_period, 0, 0);
+static unsigned long writeout_period_time = 0;
+
+/*
+ * Length of period for aging writeout fractions of bdis. This is an
+ * arbitrarily chosen number. The longer the period, the slower fractions will
+ * reflect changes in current writeout rate.
+ */
+#define VM_COMPLETIONS_PERIOD_LEN (3*HZ)
 
 /*
  * Work out the current dirty-memory clamping and background writeout
@@ -322,34 +336,6 @@ bool zone_dirty_ok(struct zone *zone)
 	       zone_page_state(zone, NR_WRITEBACK) <= limit;
 }
 
-/*
- * couple the period to the dirty_ratio:
- *
- *   period/2 ~ roundup_pow_of_two(dirty limit)
- */
-static int calc_period_shift(void)
-{
-	unsigned long dirty_total;
-
-	if (vm_dirty_bytes)
-		dirty_total = vm_dirty_bytes / PAGE_SIZE;
-	else
-		dirty_total = (vm_dirty_ratio * global_dirtyable_memory()) /
-				100;
-	return 2 + ilog2(dirty_total - 1);
-}
-
-/*
- * update the period when the dirty threshold changes.
- */
-static void update_completion_period(void)
-{
-	int shift = calc_period_shift();
-	prop_change_shift(&vm_completions, shift);
-
-	writeback_set_ratelimit();
-}
-
 int dirty_background_ratio_handler(struct ctl_table *table, int write,
 		void __user *buffer, size_t *lenp,
 		loff_t *ppos)
@@ -383,7 +369,7 @@ int dirty_ratio_handler(struct ctl_table *table, int write,
 
 	ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
 	if (ret == 0 && write && vm_dirty_ratio != old_ratio) {
-		update_completion_period();
+		writeback_set_ratelimit();
 		vm_dirty_bytes = 0;
 	}
 	return ret;
@@ -398,12 +384,21 @@ int dirty_bytes_handler(struct ctl_table *table, int write,
 
 	ret = proc_doulongvec_minmax(table, write, buffer, lenp, ppos);
 	if (ret == 0 && write && vm_dirty_bytes != old_bytes) {
-		update_completion_period();
+		writeback_set_ratelimit();
 		vm_dirty_ratio = 0;
 	}
 	return ret;
 }
 
+static unsigned long wp_next_time(unsigned long cur_time)
+{
+	cur_time += VM_COMPLETIONS_PERIOD_LEN;
+	/* 0 has a special meaning... */
+	if (!cur_time)
+		return 1;
+	return cur_time;
+}
+
 /*
  * Increment the BDI's writeout completion count and the global writeout
  * completion count. Called from test_clear_page_writeback().
@@ -411,8 +406,19 @@ int dirty_bytes_handler(struct ctl_table *table, int write,
 static inline void __bdi_writeout_inc(struct backing_dev_info *bdi)
 {
 	__inc_bdi_stat(bdi, BDI_WRITTEN);
-	__prop_inc_percpu_max(&vm_completions, &bdi->completions,
-			      bdi->max_prop_frac);
+	__fprop_inc_percpu_max(&writeout_completions, &bdi->completions,
+			       bdi->max_prop_frac);
+	/* First event after period switching was turned off? */
+	if (!unlikely(writeout_period_time)) {
+		/*
+		 * We can race with other __bdi_writeout_inc calls here but
+		 * it does not cause any harm since the resulting time when
+		 * timer will fire and what is in writeout_period_time will be
+		 * roughly the same.
+		 */
+		writeout_period_time = wp_next_time(jiffies);
+		mod_timer(&writeout_period_timer, writeout_period_time);
+	}
 }
 
 void bdi_writeout_inc(struct backing_dev_info *bdi)
@@ -431,11 +437,33 @@ EXPORT_SYMBOL_GPL(bdi_writeout_inc);
 static void bdi_writeout_fraction(struct backing_dev_info *bdi,
 		long *numerator, long *denominator)
 {
-	prop_fraction_percpu(&vm_completions, &bdi->completions,
+	fprop_fraction_percpu(&writeout_completions, &bdi->completions,
 				numerator, denominator);
 }
 
 /*
+ * On idle system, we can be called long after we scheduled because we use
+ * deferred timers so count with missed periods.
+ */
+static void writeout_period(unsigned long t)
+{
+	int miss_periods = (jiffies - writeout_period_time) /
+						 VM_COMPLETIONS_PERIOD_LEN;
+
+	if (fprop_new_period(&writeout_completions, miss_periods + 1)) {
+		writeout_period_time = wp_next_time(writeout_period_time +
+				miss_periods * VM_COMPLETIONS_PERIOD_LEN);
+		mod_timer(&writeout_period_timer, writeout_period_time);
+	} else {
+		/*
+		 * Aging has zeroed all fractions. Stop wasting CPU on period
+		 * updates.
+		 */
+		writeout_period_time = 0;
+	}
+}
+
+/*
  * bdi_min_ratio keeps the sum of the minimum dirty shares of all
  * registered backing devices, which, for obvious reasons, can not
  * exceed 100%.
@@ -475,7 +503,7 @@ int bdi_set_max_ratio(struct backing_dev_info *bdi, unsigned max_ratio)
 		ret = -EINVAL;
 	} else {
 		bdi->max_ratio = max_ratio;
-		bdi->max_prop_frac = (PROP_FRAC_BASE * max_ratio) / 100;
+		bdi->max_prop_frac = (FPROP_FRAC_BASE * max_ratio) / 100;
 	}
 	spin_unlock_bh(&bdi_lock);
 
@@ -1606,13 +1634,10 @@ static struct notifier_block __cpuinitdata ratelimit_nb = {
  */
 void __init page_writeback_init(void)
 {
-	int shift;
-
 	writeback_set_ratelimit();
 	register_cpu_notifier(&ratelimit_nb);
 
-	shift = calc_period_shift();
-	prop_descriptor_init(&vm_completions, shift);
+	fprop_global_init(&writeout_completions);
 }
 
 /**