[PATCH v1 1/1] mm: start background writeback based on per-wb threshold for strictlimit BDIs

[Joanne Koong <joannelkoong@gmail.com>]

The proactive nr_dirty > gdtc->bg_thresh check in balance_dirty_pages()
only checks the global dirty threshold to start background writeback
while the writer is still free-running, but for strictlimit BDIs (eg
fuse), the per-wb dirty count can exceed the per-wb background threshold
while the global threshold is not yet exceeded, so background writeback
for this case never gets proactively started.

Add a per-wb threshold check for strictlimit BDIs so that background
writeback is started when wb_dirty exceeds wb_bg_thresh, which drains
dirty pages before the writer hits the throttle wall, matching the
proactive behavior that the global check provides for non-strictlimit
BDIs.

fio runs on fuse show about a 3-4% improvement in perf for buffered
writes:
fio --name=writeback_test --ioengine=psync --rw=write --bs=128k \
    --size=2G --numjobs=4 --ramp_time=10 --runtime=20 \
    --time_based --group_reporting=1 --direct=0

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
---
 mm/page-writeback.c | 16 ++++++----------
 1 file changed, 6 insertions(+), 10 deletions(-)

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index c1a4b32af1a7..30f3d5a6270f 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -1835,7 +1835,9 @@ static int balance_dirty_pages(struct bdi_writeback *wb,
 			balance_domain_limits(mdtc, strictlimit);
 		}
 
-		if (nr_dirty > gdtc->bg_thresh && !writeback_in_progress(wb))
+		if (!writeback_in_progress(wb) &&
+		    (nr_dirty > gdtc->bg_thresh ||
+		     (strictlimit && gdtc->wb_dirty > gdtc->wb_bg_thresh)))
 			wb_start_background_writeback(wb);
 
 		/*
@@ -1862,15 +1864,9 @@ static int balance_dirty_pages(struct bdi_writeback *wb,
 		 * Unconditionally start background writeback if it's not
 		 * already in progress. We need to do this because the global
 		 * dirty threshold check above (nr_dirty > gdtc->bg_thresh)
-		 * doesn't account for these cases:
-		 *
-		 * a) strictlimit BDIs: throttling is calculated using per-wb
-		 * thresholds. The per-wb threshold can be exceeded even when
-		 * nr_dirty < gdtc->bg_thresh
-		 *
-		 * b) memcg-based throttling: memcg uses its own dirty count and
-		 * thresholds and can trigger throttling even when global
-		 * nr_dirty < gdtc->bg_thresh
+		 * doesn't account for the memcg-based throttling case. memcg
+		 * uses its own dirty count and thresholds and can trigger
+		 * throttling even when global nr_dirty < gdtc->bg_thresh
 		 *
 		 * Writeback needs to be started else the writer stalls in the
 		 * throttle loop waiting for dirty pages to be written back
-- 
2.52.0