[PATCH] mm/page-writeback: Fix performance when BDI's share of ratio is 0.

[Chi Wu <wuchi.zero@gmail.com>]

Fix performance when BDI's share of ratio is 0.

The issue is similar to commit 74d369443325 ("writeback: Fix
performance regression in wb_over_bg_thresh()").

Balance_dirty_pages and the writeback worker will also disagree on
whether writeback when a BDI uses BDI_CAP_STRICTLIMIT and BDI's share
of the thresh ratio is zero.

For example, A thread on cpu0 writes 32 pages and then
balance_dirty_pages, it will wake up background writeback and pauses
because wb_dirty > wb->wb_thresh = 0 (share of thresh ratio is zero).
A thread may runs on cpu0 again because scheduler prefers pre_cpu.
Then writeback worker may runs on other cpus(1,2..) which causes the
value of wb_stat(wb, WB_RECLAIMABLE) in wb_over_bg_thresh is 0 and does
not writeback and returns.

Thus, balance_dirty_pages keeps looping, sleeping and then waking up the
worker who will do nothing. It remains stuck in this state until the
writeback worker hit the right dirty cpu or the dirty pages expire.

The fix that we should get the wb_stat_sum radically when thresh is low.

Signed-off-by: Chi Wu <wuchi.zero@gmail.com>
---
 mm/page-writeback.c | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 0062d5c57d41..bd7052295246 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -1945,6 +1945,8 @@ bool wb_over_bg_thresh(struct bdi_writeback *wb)
 	struct dirty_throttle_control * const gdtc = &gdtc_stor;
 	struct dirty_throttle_control * const mdtc = mdtc_valid(&mdtc_stor) ?
 						     &mdtc_stor : NULL;
+	unsigned long reclaimable;
+	unsigned long thresh;
 
 	/*
 	 * Similar to balance_dirty_pages() but ignores pages being written
@@ -1957,8 +1959,13 @@ bool wb_over_bg_thresh(struct bdi_writeback *wb)
 	if (gdtc->dirty > gdtc->bg_thresh)
 		return true;
 
-	if (wb_stat(wb, WB_RECLAIMABLE) >
-	    wb_calc_thresh(gdtc->wb, gdtc->bg_thresh))
+	thresh = wb_calc_thresh(gdtc->wb, gdtc->bg_thresh);
+	if (thresh < 2 * wb_stat_error())
+		reclaimable = wb_stat_sum(wb, WB_RECLAIMABLE);
+	else
+		reclaimable = wb_stat(wb, WB_RECLAIMABLE);
+
+	if (reclaimable > thresh)
 		return true;
 
 	if (mdtc) {
@@ -1972,8 +1979,13 @@ bool wb_over_bg_thresh(struct bdi_writeback *wb)
 		if (mdtc->dirty > mdtc->bg_thresh)
 			return true;
 
-		if (wb_stat(wb, WB_RECLAIMABLE) >
-		    wb_calc_thresh(mdtc->wb, mdtc->bg_thresh))
+		thresh = wb_calc_thresh(mdtc->wb, mdtc->bg_thresh);
+		if (thresh < 2 * wb_stat_error())
+			reclaimable = wb_stat_sum(wb, WB_RECLAIMABLE);
+		else
+			reclaimable = wb_stat(wb, WB_RECLAIMABLE);
+
+		if (reclaimable > thresh)
 			return true;
 	}
 
-- 
2.17.1