writeback: reliably update bandwidth estimation

Currently we trigger writeback bandwidth estimation from
balance_dirty_pages() and from wb_writeback().  However neither of these
need to trigger when the system is relatively idle and writeback is
triggered e.g.  from fsync(2).  Make sure writeback estimates happen
reliably by triggering them from do_writepages().

Link: https://lkml.kernel.org/r/20210713104716.22868-2-jack@suse.cz
Signed-off-by: Jan Kara <jack@suse.cz>
Cc: Michael Stapelberg <stapelberg+linux@google.com>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 27 insertions, 12 deletions

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index e1aa1c9d8e36..e4a381b8944b 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -1332,7 +1332,6 @@ static void wb_update_dirty_ratelimit(struct dirty_throttle_control *dtc,
 
 static void __wb_update_bandwidth(struct dirty_throttle_control *gdtc,
 				  struct dirty_throttle_control *mdtc,
-				  unsigned long start_time,
 				  bool update_ratelimit)
 {
 	struct bdi_writeback *wb = gdtc->wb;
@@ -1352,13 +1351,6 @@ static void __wb_update_bandwidth(struct dirty_throttle_control *gdtc,
 	dirtied = percpu_counter_read(&wb->stat[WB_DIRTIED]);
 	written = percpu_counter_read(&wb->stat[WB_WRITTEN]);
 
-	/*
-	 * Skip quiet periods when disk bandwidth is under-utilized.
-	 * (at least 1s idle time between two flusher runs)
-	 */
-	if (elapsed > HZ && time_before(wb->bw_time_stamp, start_time))
-		goto snapshot;
-
 	if (update_ratelimit) {
 		domain_update_bandwidth(gdtc, now);
 		wb_update_dirty_ratelimit(gdtc, dirtied, elapsed);
@@ -1374,17 +1366,36 @@ static void __wb_update_bandwidth(struct dirty_throttle_control *gdtc,
 	}
 	wb_update_write_bandwidth(wb, elapsed, written);
 
-snapshot:
 	wb->dirtied_stamp = dirtied;
 	wb->written_stamp = written;
 	wb->bw_time_stamp = now;
 }
 
-void wb_update_bandwidth(struct bdi_writeback *wb, unsigned long start_time)
+static void wb_update_bandwidth(struct bdi_writeback *wb)
 {
 	struct dirty_throttle_control gdtc = { GDTC_INIT(wb) };
 
-	__wb_update_bandwidth(&gdtc, NULL, start_time, false);
+	spin_lock(&wb->list_lock);
+	__wb_update_bandwidth(&gdtc, NULL, false);
+	spin_unlock(&wb->list_lock);
+}
+
+/* Interval after which we consider wb idle and don't estimate bandwidth */
+#define WB_BANDWIDTH_IDLE_JIF (HZ)
+
+static void wb_bandwidth_estimate_start(struct bdi_writeback *wb)
+{
+	unsigned long now = jiffies;
+	unsigned long elapsed = now - READ_ONCE(wb->bw_time_stamp);
+
+	if (elapsed > WB_BANDWIDTH_IDLE_JIF &&
+	    !atomic_read(&wb->writeback_inodes)) {
+		spin_lock(&wb->list_lock);
+		wb->dirtied_stamp = wb_stat(wb, WB_DIRTIED);
+		wb->written_stamp = wb_stat(wb, WB_WRITTEN);
+		wb->bw_time_stamp = now;
+		spin_unlock(&wb->list_lock);
+	}
 }
 
 /*
@@ -1713,7 +1724,7 @@ free_running:
 		if (time_is_before_jiffies(wb->bw_time_stamp +
 					   BANDWIDTH_INTERVAL)) {
 			spin_lock(&wb->list_lock);
-			__wb_update_bandwidth(gdtc, mdtc, start_time, true);
+			__wb_update_bandwidth(gdtc, mdtc, true);
 			spin_unlock(&wb->list_lock);
 		}
 
@@ -2347,9 +2358,12 @@ EXPORT_SYMBOL(generic_writepages);
 int do_writepages(struct address_space *mapping, struct writeback_control *wbc)
 {
 	int ret;
+	struct bdi_writeback *wb;
 
 	if (wbc->nr_to_write <= 0)
 		return 0;
+	wb = inode_to_wb_wbc(mapping->host, wbc);
+	wb_bandwidth_estimate_start(wb);
 	while (1) {
 		if (mapping->a_ops->writepages)
 			ret = mapping->a_ops->writepages(mapping, wbc);
@@ -2360,6 +2374,7 @@ int do_writepages(struct address_space *mapping, struct writeback_control *wbc)
 		cond_resched();
 		congestion_wait(BLK_RW_ASYNC, HZ/50);
 	}
+	wb_update_bandwidth(wb);
 	return ret;
 }