[PATCH v2 1/1] mm: reinstate unconditional writeback start in balance_dirty_pages()

[Joanne Koong <joannelkoong@gmail.com>]

Commit 64dd89ae01f2 ("mm/block/fs: remove laptop_mode") removed this
unconditional writeback start from balance_dirty_pages():

       if (unlikely(!writeback_in_progress(wb)))
	       wb_start_background_writeback(wb);

This logic needs to be reinstated to prevent performance regressions for
strictlimited BDIs and memcg setups. The problem occurs because:

a) For strictlimited BDIs, throttling is calculated using per-wb
thresholds. The per-wb threshold can be exceeded even when the global
dirty threshold was not exceeded (nr_dirty < gdtc->bg_thresh)

b) For memcg-based throttling, memcg uses its own dirty count /
thresholds and can trigger throttling even when the global threshold
isn't exceeded

Without the unconditional writeback start, IO is throttled as it waits
for dirty pages to be written back but there is no writeback running.
This leads to severe stalls. On fuse, buffered write performance
dropped from 1400 MiB/s to 2000 KiB/s.

Reinstate the unconditional writeback start so that writeback is
guaranteed to be running whenever IO needs to be throttled.

Fixes: 64dd89ae01f2 ("mm/block/fs: remove laptop_mode")
Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
---
 mm/page-writeback.c | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 601a5e048d12..c1a4b32af1a7 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -1858,6 +1858,27 @@ static int balance_dirty_pages(struct bdi_writeback *wb,
 			break;
 		}
 
+		/*
+		 * 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
+		 *
+		 * Writeback needs to be started else the writer stalls in the
+		 * throttle loop waiting for dirty pages to be written back
+		 * while no writeback is running.
+		 */
+		if (unlikely(!writeback_in_progress(wb)))
+			wb_start_background_writeback(wb);
+
 		mem_cgroup_flush_foreign(wb);
 
 		/*
-- 
2.52.0