[PATCH RESEND 0/2] Fix calculations in trace_balance_dirty_pages() for cgwb

[PATCH RESEND 0/2] Fix calculations in trace_balance_dirty_pages() for cgwb

[Tang Yizhou]

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

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().

Tang Yizhou (2):
  writeback: Let trace_balance_dirty_pages() take struct dtc as
    parameter
  writeback: Fix calculations in trace_balance_dirty_pages() for cgwb

 include/linux/writeback.h        | 24 +++++++++++++++++++++
 include/trace/events/writeback.h | 33 ++++++++++++----------------
 mm/page-writeback.c              | 37 +++-----------------------------
 3 files changed, 41 insertions(+), 53 deletions(-)

-- 
2.25.1

[PATCH RESEND 1/2] writeback: Let trace_balance_dirty_pages() take struct dtc as parameter

[Tang Yizhou]

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

Currently, trace_balance_dirty_pages() already has 12 parameters. In the
next patch, 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.

By the way, rename bdi_setpoint and bdi_dirty in the tracepoint to
wb_setpoint and wb_dirty, respectively. These changes were omitted by
Tejun in the cgroup writeback patchset.

Signed-off-by: Tang Yizhou <yizhou.tang@shopee.com>
---
 include/linux/writeback.h        | 23 +++++++++++++++++++++
 include/trace/events/writeback.h | 28 +++++++++++--------------
 mm/page-writeback.c              | 35 ++------------------------------
 3 files changed, 37 insertions(+), 49 deletions(-)

diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index d11b903c2edb..32095928365c 100644
--- a/include/linux/writeback.h
+++ b/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);
diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h
index a261e86e61fa..3046ca6b08ea 100644
--- a/include/trace/events/writeback.h
+++ b/include/trace/events/writeback.h
@@ -629,11 +629,7 @@ 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,
@@ -641,7 +637,7 @@ TRACE_EVENT(balance_dirty_pages,
 		 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),
 
@@ -650,8 +646,8 @@ TRACE_EVENT(balance_dirty_pages,
 		__field(unsigned long,	limit)
 		__field(unsigned long,	setpoint)
 		__field(unsigned long,	dirty)
-		__field(unsigned long,	bdi_setpoint)
-		__field(unsigned long,	bdi_dirty)
+		__field(unsigned long,	wb_setpoint)
+		__field(unsigned long,	wb_dirty)
 		__field(unsigned long,	dirty_ratelimit)
 		__field(unsigned long,	task_ratelimit)
 		__field(unsigned int,	dirtied)
@@ -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->bdi_setpoint	= __entry->setpoint *
-						bdi_thresh / (thresh + 1);
-		__entry->bdi_dirty	= bdi_dirty;
+		__entry->dirty		= dtc->dirty;
+		__entry->wb_setpoint	= __entry->setpoint *
+						dtc->wb_thresh / (dtc->thresh + 1);
+		__entry->wb_dirty	= dtc->wb_dirty;
 		__entry->dirty_ratelimit = KBps(dirty_ratelimit);
 		__entry->task_ratelimit	= KBps(task_ratelimit);
 		__entry->dirtied	= dirtied;
@@ -689,7 +685,7 @@ TRACE_EVENT(balance_dirty_pages,
 
 	TP_printk("bdi %s: "
 		  "limit=%lu setpoint=%lu dirty=%lu "
-		  "bdi_setpoint=%lu bdi_dirty=%lu "
+		  "wb_setpoint=%lu wb_dirty=%lu "
 		  "dirty_ratelimit=%lu task_ratelimit=%lu "
 		  "dirtied=%u dirtied_pause=%u "
 		  "paused=%lu pause=%ld period=%lu think=%ld cgroup_ino=%lu",
@@ -697,8 +693,8 @@ TRACE_EVENT(balance_dirty_pages,
 		  __entry->limit,
 		  __entry->setpoint,
 		  __entry->dirty,
-		  __entry->bdi_setpoint,
-		  __entry->bdi_dirty,
+		  __entry->wb_setpoint,
+		  __entry->wb_dirty,
 		  __entry->dirty_ratelimit,
 		  __entry->task_ratelimit,
 		  __entry->dirtied,
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index eb55ece39c56..e980b2aec352 100644
--- a/mm/page-writeback.c
+++ b/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,
-- 
2.25.1

Re: [PATCH RESEND 1/2] writeback: Let trace_balance_dirty_pages() take struct dtc as parameter

[Jan Kara]

On Mon 03-03-25 18:06:16, Tang Yizhou wrote:
> From: Tang Yizhou <yizhou.tang@shopee.com>
> 
> Currently, trace_balance_dirty_pages() already has 12 parameters. In the
> next patch, 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.
> 
> By the way, rename bdi_setpoint and bdi_dirty in the tracepoint to
> wb_setpoint and wb_dirty, respectively. These changes were omitted by
> Tejun in the cgroup writeback patchset.
> 
> Signed-off-by: Tang Yizhou <yizhou.tang@shopee.com>

Looks good. Feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

								Honza

> ---
>  include/linux/writeback.h        | 23 +++++++++++++++++++++
>  include/trace/events/writeback.h | 28 +++++++++++--------------
>  mm/page-writeback.c              | 35 ++------------------------------
>  3 files changed, 37 insertions(+), 49 deletions(-)
> 
> diff --git a/include/linux/writeback.h b/include/linux/writeback.h
> index d11b903c2edb..32095928365c 100644
> --- a/include/linux/writeback.h
> +++ b/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);
> diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h
> index a261e86e61fa..3046ca6b08ea 100644
> --- a/include/trace/events/writeback.h
> +++ b/include/trace/events/writeback.h
> @@ -629,11 +629,7 @@ 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,
> @@ -641,7 +637,7 @@ TRACE_EVENT(balance_dirty_pages,
>  		 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),
>  
> @@ -650,8 +646,8 @@ TRACE_EVENT(balance_dirty_pages,
>  		__field(unsigned long,	limit)
>  		__field(unsigned long,	setpoint)
>  		__field(unsigned long,	dirty)
> -		__field(unsigned long,	bdi_setpoint)
> -		__field(unsigned long,	bdi_dirty)
> +		__field(unsigned long,	wb_setpoint)
> +		__field(unsigned long,	wb_dirty)
>  		__field(unsigned long,	dirty_ratelimit)
>  		__field(unsigned long,	task_ratelimit)
>  		__field(unsigned int,	dirtied)
> @@ -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->bdi_setpoint	= __entry->setpoint *
> -						bdi_thresh / (thresh + 1);
> -		__entry->bdi_dirty	= bdi_dirty;
> +		__entry->dirty		= dtc->dirty;
> +		__entry->wb_setpoint	= __entry->setpoint *
> +						dtc->wb_thresh / (dtc->thresh + 1);
> +		__entry->wb_dirty	= dtc->wb_dirty;
>  		__entry->dirty_ratelimit = KBps(dirty_ratelimit);
>  		__entry->task_ratelimit	= KBps(task_ratelimit);
>  		__entry->dirtied	= dirtied;
> @@ -689,7 +685,7 @@ TRACE_EVENT(balance_dirty_pages,
>  
>  	TP_printk("bdi %s: "
>  		  "limit=%lu setpoint=%lu dirty=%lu "
> -		  "bdi_setpoint=%lu bdi_dirty=%lu "
> +		  "wb_setpoint=%lu wb_dirty=%lu "
>  		  "dirty_ratelimit=%lu task_ratelimit=%lu "
>  		  "dirtied=%u dirtied_pause=%u "
>  		  "paused=%lu pause=%ld period=%lu think=%ld cgroup_ino=%lu",
> @@ -697,8 +693,8 @@ TRACE_EVENT(balance_dirty_pages,
>  		  __entry->limit,
>  		  __entry->setpoint,
>  		  __entry->dirty,
> -		  __entry->bdi_setpoint,
> -		  __entry->bdi_dirty,
> +		  __entry->wb_setpoint,
> +		  __entry->wb_dirty,
>  		  __entry->dirty_ratelimit,
>  		  __entry->task_ratelimit,
>  		  __entry->dirtied,
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index eb55ece39c56..e980b2aec352 100644
> --- a/mm/page-writeback.c
> +++ b/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,
> -- 
> 2.25.1
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

[PATCH RESEND 2/2] writeback: Fix calculations in trace_balance_dirty_pages() for cgwb

[Tang Yizhou]

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")
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

Re: [PATCH RESEND 2/2] writeback: Fix calculations in trace_balance_dirty_pages() for cgwb

[Jan Kara]

On Mon 03-03-25 18:06:17, Tang Yizhou wrote:
> 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")
> Signed-off-by: Tang Yizhou <yizhou.tang@shopee.com>

In principle this looks fine but one nit below:

> 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 */
				^^^ I'd call this dirty_limit to not invent
a new name for the same thing. I've noticed the tracepoint has 'limit' as
well but that is the outlier that should be modified if anything. Also I'd
modify the comment to /* smoothed dirty limit */ to better explain what
this is about.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR