mm: start background writeback based on per-wb threshold for strictlimit BDIs

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

Link: https://lore.kernel.org/20260326234629.840938-2-joannelkoong@gmail.com
Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Miklos Szeredi <miklos@szeredi.hu>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 88cd53d4ba0929e3d4e74d7d72d36b3ceca51189..833f743f309f9b254142fc979c7727f3c9a9968b 100644 (file)
--- 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 @@ free_running:
                 * 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