[PATCH v1 0/1] mm: start background writeback based on per-wb threshold for strictlimit BDIs

[PATCH v1 0/1] mm: start background writeback based on per-wb threshold for strictlimit BDIs

[Joanne Koong]

This patch has a dependency on the writeback regression fix in [1]. This is
submitted as a separate patchset from [1] because [1] is a fix targeting the
7.0 rc cycle while this patch is an optimization that targets the next merge
window.

Thanks,
Joanne

[1] https://lore.kernel.org/linux-fsdevel/20260326215127.3857682-1-joannelkoong@gmail.com/

Joanne Koong (1):
  mm: start background writeback based on per-wb threshold for
    strictlimit BDIs

 mm/page-writeback.c | 16 ++++++----------
 1 file changed, 6 insertions(+), 10 deletions(-)

-- 
2.52.0

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

[Joanne Koong]

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

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

[Andrew Morton]

On Thu, 26 Mar 2026 16:46:29 -0700 Joanne Koong <joannelkoong@gmail.com> wrote:

> 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
> 

Thanks.

Your v1 patch's changelog had useful information showing the
performance impact on fuse.  I've pasted one sentence from that
changelog into my copy of this changelog, but please don't be sparing
in describing the importance of a fix.

Also, we lost the Fixes tag.  I've added

Fixes: 64dd89ae01f2 ("mm/block/fs: remove laptop_mode") 
Cc: <stable@vger.kernel.org>

to my copy of this v2 patch.  Please confirm that this is appropriate.

> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -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

My copy of balance_dirty_pages() doesn't have this comment block.  I
assume you're patching a different kernel version.  I dropped this
hunk.  Please check current Linus mainline and see if there's somewhere
else where you'd like to add this information.

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

[Andrew Morton]

On Thu, 26 Mar 2026 17:42:37 -0700 Andrew Morton <akpm@linux-foundation.org> wrote:

> > 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
> > 
> 
> Thanks.
>
> [bunch of nonsense]
>

argh, sorry, I didn't read your [0/N].  Ignore.

btw, a [0/N] isn't really needed for a singleton patch - best to just
put everything into the patch's changelog.

Transient info such as "this depends on my other patch" can be placed
below the singleton patch's "---" separator.