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 --- 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