[PATCH v2 3/3] writeback: Fix calculations in trace_balance_dirty_pages() for cgwb

[Tang Yizhou <yizhou.tang@shopee.com>]

From: Tang Yizhou <yizhou.tang@shopee.com>

In the commit dcc25ae76eb7 ("writeback: move global_dirty_limit into
wb_domain") of the cgroup writeback backpressure propagation patchset,
Tejun made some adaptations to trace_balance_dirty_pages() for cgroup
writeback. However, this adaptation was incomplete and Tejun missed
further adaptation in the subsequent patches.

In the cgroup writeback scenario, if sdtc in balance_dirty_pages() is
assigned to mdtc, then upon entering trace_balance_dirty_pages(),
__entry->limit should be assigned based on the dirty_limit of the
corresponding memcg's wb_domain, rather than global_wb_domain.

To address this issue and simplify the implementation, introduce a 'limit'
field in struct dirty_throttle_control to store the hard_limit value
computed in wb_position_ratio() by calling hard_dirty_limit(). This field
will then be used in trace_balance_dirty_pages() to assign the value to
__entry->limit.

Fixes: dcc25ae76eb7 ("writeback: move global_dirty_limit into wb_domain")
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Tang Yizhou <yizhou.tang@shopee.com>
---
 include/linux/writeback.h        | 1 +
 include/trace/events/writeback.h | 5 ++---
 mm/page-writeback.c              | 2 +-
 3 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index 32095928365c..58bda3347914 100644
--- a/include/linux/writeback.h
+++ b/include/linux/writeback.h
@@ -326,6 +326,7 @@ struct dirty_throttle_control {
 	unsigned long		dirty;		/* file_dirty + write + nfs */
 	unsigned long		thresh;		/* dirty threshold */
 	unsigned long		bg_thresh;	/* dirty background threshold */
+	unsigned long		limit;		/* hard dirty limit */
 
 	unsigned long		wb_dirty;	/* per-wb counterparts */
 	unsigned long		wb_thresh;
diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h
index 3046ca6b08ea..0ff388131fc9 100644
--- a/include/trace/events/writeback.h
+++ b/include/trace/events/writeback.h
@@ -663,9 +663,8 @@ TRACE_EVENT(balance_dirty_pages,
 		unsigned long freerun = (dtc->thresh + dtc->bg_thresh) / 2;
 		strscpy_pad(__entry->bdi, bdi_dev_name(wb->bdi), 32);
 
-		__entry->limit		= global_wb_domain.dirty_limit;
-		__entry->setpoint	= (global_wb_domain.dirty_limit +
-						freerun) / 2;
+		__entry->limit		= dtc->limit;
+		__entry->setpoint	= (dtc->limit + freerun) / 2;
 		__entry->dirty		= dtc->dirty;
 		__entry->wb_setpoint	= __entry->setpoint *
 						dtc->wb_thresh / (dtc->thresh + 1);
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index e980b2aec352..3147119a9a04 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -1072,7 +1072,7 @@ static void wb_position_ratio(struct dirty_throttle_control *dtc)
 	struct bdi_writeback *wb = dtc->wb;
 	unsigned long write_bw = READ_ONCE(wb->avg_write_bandwidth);
 	unsigned long freerun = dirty_freerun_ceiling(dtc->thresh, dtc->bg_thresh);
-	unsigned long limit = hard_dirty_limit(dtc_dom(dtc), dtc->thresh);
+	unsigned long limit = dtc->limit = hard_dirty_limit(dtc_dom(dtc), dtc->thresh);
 	unsigned long wb_thresh = dtc->wb_thresh;
 	unsigned long x_intercept;
 	unsigned long setpoint;		/* dirty pages' target balance point */
-- 
2.25.1