writeback: let trace_balance_dirty_pages() take struct dtc as parameter

Patch series "Fix calculations in trace_balance_dirty_pages() for cgwb", v2.

In my experiment, I found that the output of trace_balance_dirty_pages()
in the cgroup writeback scenario was strange because
trace_balance_dirty_pages() always uses global_wb_domain.dirty_limit for
related calculations instead of the dirty_limit of the corresponding
memcg's wb_domain.

The basic idea of the fix is to store the hard dirty limit value computed
in wb_position_ratio() into struct dirty_throttle_control and use it for
calculations in trace_balance_dirty_pages().


This patch (of 3):

Currently, trace_balance_dirty_pages() already has 12 parameters.  In the
patch #3, I initially attempted to introduce an additional parameter. 
However, in include/linux/trace_events.h, bpf_trace_run12() only supports
up to 12 parameters and bpf_trace_run13() does not exist.

To reduce the number of parameters in trace_balance_dirty_pages(), we can
make it accept a pointer to struct dirty_throttle_control as a parameter. 
To achieve this, we need to move the definition of struct
dirty_throttle_control from mm/page-writeback.c to
include/linux/writeback.h.

Link: https://lkml.kernel.org/r/20250304110318.159567-1-yizhou.tang@shopee.com
Link: https://lkml.kernel.org/r/20250304110318.159567-2-yizhou.tang@shopee.com
Signed-off-by: Tang Yizhou <yizhou.tang@shopee.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Jan Kara <jack@suse.cz>
Cc: "Masami Hiramatsu (Google)" <mhiramat@kernel.org>
Cc: Matthew Wilcow (Oracle) <willy@infradead.org>
Cc: Tang Yizhou <yizhou.tang@shopee.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: Tang Yizhou

include/linux/writeback.h

@@ -313,6 +313,29 @@ static inline void cgroup_writeback_umount(struct super_block *sb)
 /*
  * mm/page-writeback.c
  */
+/* consolidated parameters for balance_dirty_pages() and its subroutines */
+struct dirty_throttle_control {
+#ifdef CONFIG_CGROUP_WRITEBACK
+	struct wb_domain	*dom;
+	struct dirty_throttle_control *gdtc;	/* only set in memcg dtc's */
+#endif
+	struct bdi_writeback	*wb;
+	struct fprop_local_percpu *wb_completions;
+
+	unsigned long		avail;		/* dirtyable */
+	unsigned long		dirty;		/* file_dirty + write + nfs */
+	unsigned long		thresh;		/* dirty threshold */
+	unsigned long		bg_thresh;	/* dirty background threshold */
+
+	unsigned long		wb_dirty;	/* per-wb counterparts */
+	unsigned long		wb_thresh;
+	unsigned long		wb_bg_thresh;
+
+	unsigned long		pos_ratio;
+	bool			freerun;
+	bool			dirty_exceeded;
+};
+
 void laptop_io_completion(struct backing_dev_info *info);
 void laptop_sync_completion(void);
 void laptop_mode_timer_fn(struct timer_list *t);

include/trace/events/writeback.h

@@ -629,19 +629,15 @@ TRACE_EVENT(bdi_dirty_ratelimit,
 TRACE_EVENT(balance_dirty_pages,
 
 	TP_PROTO(struct bdi_writeback *wb,
-		 unsigned long thresh,
-		 unsigned long bg_thresh,
-		 unsigned long dirty,
-		 unsigned long bdi_thresh,
-		 unsigned long bdi_dirty,
+		 struct dirty_throttle_control *dtc,
 		 unsigned long dirty_ratelimit,
 		 unsigned long task_ratelimit,
 		 unsigned long dirtied,
 		 unsigned long period,
 		 long pause,
 		 unsigned long start_time),
 
-	TP_ARGS(wb, thresh, bg_thresh, dirty, bdi_thresh, bdi_dirty,
+	TP_ARGS(wb, dtc,
 		dirty_ratelimit, task_ratelimit,
 		dirtied, period, pause, start_time),
 
@@ -664,16 +660,16 @@ TRACE_EVENT(balance_dirty_pages,
 	),
 
 	TP_fast_assign(
-		unsigned long freerun = (thresh + bg_thresh) / 2;
+		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->dirty		= dirty;
+		__entry->dirty		= dtc->dirty;
 		__entry->bdi_setpoint	= __entry->setpoint *
-						bdi_thresh / (thresh + 1);
-		__entry->bdi_dirty	= bdi_dirty;
+						dtc->wb_thresh / (dtc->thresh + 1);
+		__entry->bdi_dirty	= dtc->wb_dirty;
 		__entry->dirty_ratelimit = KBps(dirty_ratelimit);
 		__entry->task_ratelimit	= KBps(task_ratelimit);
 		__entry->dirtied	= dirtied;

mm/page-writeback.c

@@ -120,29 +120,6 @@ EXPORT_SYMBOL(laptop_mode);
 
 struct wb_domain global_wb_domain;
 
-/* consolidated parameters for balance_dirty_pages() and its subroutines */
-struct dirty_throttle_control {
-#ifdef CONFIG_CGROUP_WRITEBACK
-	struct wb_domain	*dom;
-	struct dirty_throttle_control *gdtc;	/* only set in memcg dtc's */
-#endif
-	struct bdi_writeback	*wb;
-	struct fprop_local_percpu *wb_completions;
-
-	unsigned long		avail;		/* dirtyable */
-	unsigned long		dirty;		/* file_dirty + write + nfs */
-	unsigned long		thresh;		/* dirty threshold */
-	unsigned long		bg_thresh;	/* dirty background threshold */
-
-	unsigned long		wb_dirty;	/* per-wb counterparts */
-	unsigned long		wb_thresh;
-	unsigned long		wb_bg_thresh;
-
-	unsigned long		pos_ratio;
-	bool			freerun;
-	bool			dirty_exceeded;
-};
-
 /*
  * Length of period for aging writeout fractions of bdis. This is an
  * arbitrarily chosen number. The longer the period, the slower fractions will
@@ -1962,11 +1939,7 @@ static int balance_dirty_pages(struct bdi_writeback *wb,
 		 */
 		if (pause < min_pause) {
 			trace_balance_dirty_pages(wb,
-						  sdtc->thresh,
-						  sdtc->bg_thresh,
-						  sdtc->dirty,
-						  sdtc->wb_thresh,
-						  sdtc->wb_dirty,
+						  sdtc,
 						  dirty_ratelimit,
 						  task_ratelimit,
 						  pages_dirtied,
@@ -1991,11 +1964,7 @@ static int balance_dirty_pages(struct bdi_writeback *wb,
 
 pause:
 		trace_balance_dirty_pages(wb,
-					  sdtc->thresh,
-					  sdtc->bg_thresh,
-					  sdtc->dirty,
-					  sdtc->wb_thresh,
-					  sdtc->wb_dirty,
+					  sdtc,
 					  dirty_ratelimit,
 					  task_ratelimit,
 					  pages_dirtied,