[PATCH 21/35] writeback: trace balance_dirty_pages()

[Wu Fengguang <fengguang.wu@intel.com>]

[-- Attachment #1: writeback-trace-balance_dirty_pages.patch --]
[-- Type: text/plain, Size: 7519 bytes --]

It would be useful for analyzing the dynamics of the throttling
algorithms, and helpful for debugging user reported problems.

Here is an interesting test to verify the theory with balance_dirty_pages()
tracing. On a partition that can do ~60MB/s, a sparse file is created and
4 rsync tasks with different write bandwidth started:

	dd if=/dev/zero of=/mnt/1T bs=1M count=1 seek=1024000
	echo 1 > /debug/tracing/events/writeback/balance_dirty_pages/enable

	rsync localhost:/mnt/1T /mnt/a --bwlimit 10000&
	rsync localhost:/mnt/1T /mnt/A --bwlimit 10000&
	rsync localhost:/mnt/1T /mnt/b --bwlimit 20000&
	rsync localhost:/mnt/1T /mnt/c --bwlimit 30000&

Trace outputs within 0.1 second, grouped by tasks:

rsync-3824  [004] 15002.076447: balance_dirty_pages: bdi=btrfs-2 weight=15% limit=130876 gap=5340 dirtied=192 pause=20

rsync-3822  [003] 15002.091701: balance_dirty_pages: bdi=btrfs-2 weight=15% limit=130777 gap=5113 dirtied=192 pause=20

rsync-3821  [006] 15002.004667: balance_dirty_pages: bdi=btrfs-2 weight=30% limit=129570 gap=3714 dirtied=64 pause=8
rsync-3821  [006] 15002.012654: balance_dirty_pages: bdi=btrfs-2 weight=30% limit=129589 gap=3733 dirtied=64 pause=8
rsync-3821  [006] 15002.021838: balance_dirty_pages: bdi=btrfs-2 weight=30% limit=129604 gap=3748 dirtied=64 pause=8
rsync-3821  [004] 15002.091193: balance_dirty_pages: bdi=btrfs-2 weight=29% limit=129583 gap=3983 dirtied=64 pause=8
rsync-3821  [004] 15002.102729: balance_dirty_pages: bdi=btrfs-2 weight=29% limit=129594 gap=3802 dirtied=64 pause=8
rsync-3821  [000] 15002.109252: balance_dirty_pages: bdi=btrfs-2 weight=29% limit=129619 gap=3827 dirtied=64 pause=8

rsync-3823  [002] 15002.009029: balance_dirty_pages: bdi=btrfs-2 weight=39% limit=128762 gap=2842 dirtied=64 pause=12
rsync-3823  [002] 15002.021598: balance_dirty_pages: bdi=btrfs-2 weight=39% limit=128813 gap=3021 dirtied=64 pause=12
rsync-3823  [003] 15002.032973: balance_dirty_pages: bdi=btrfs-2 weight=39% limit=128805 gap=2885 dirtied=64 pause=12
rsync-3823  [003] 15002.048800: balance_dirty_pages: bdi=btrfs-2 weight=39% limit=128823 gap=2967 dirtied=64 pause=12
rsync-3823  [003] 15002.060728: balance_dirty_pages: bdi=btrfs-2 weight=39% limit=128821 gap=3221 dirtied=64 pause=12
rsync-3823  [000] 15002.073152: balance_dirty_pages: bdi=btrfs-2 weight=39% limit=128825 gap=3225 dirtied=64 pause=12
rsync-3823  [005] 15002.090111: balance_dirty_pages: bdi=btrfs-2 weight=39% limit=128782 gap=3214 dirtied=64 pause=12
rsync-3823  [004] 15002.102520: balance_dirty_pages: bdi=btrfs-2 weight=39% limit=128764 gap=3036 dirtied=64 pause=12

The data vividly show that

- the heaviest writer is throttled a bit (weight=39%)

- the lighter writers run at full speed (weight=15%,15%,30%)

Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
 include/trace/events/writeback.h |   87 ++++++++++++++++++++++++++++-
 mm/page-writeback.c              |   20 ++++++
 2 files changed, 104 insertions(+), 3 deletions(-)

--- linux-next.orig/include/trace/events/writeback.h	2010-12-13 21:46:09.000000000 +0800
+++ linux-next/include/trace/events/writeback.h	2010-12-13 21:46:18.000000000 +0800
@@ -147,11 +147,92 @@ DEFINE_EVENT(wbc_class, name, \
 DEFINE_WBC_EVENT(wbc_writeback_start);
 DEFINE_WBC_EVENT(wbc_writeback_written);
 DEFINE_WBC_EVENT(wbc_writeback_wait);
-DEFINE_WBC_EVENT(wbc_balance_dirty_start);
-DEFINE_WBC_EVENT(wbc_balance_dirty_written);
-DEFINE_WBC_EVENT(wbc_balance_dirty_wait);
 DEFINE_WBC_EVENT(wbc_writepage);
 
+#define KBps(x)			((x) << (PAGE_SHIFT - 10))
+#define BDP_PERCENT(a, b, c)	(((__entry->a) - (__entry->b)) * 100 * (c) + \
+				  __entry->bdi_limit/2) / (__entry->bdi_limit|1)
+
+TRACE_EVENT(balance_dirty_pages,
+
+	TP_PROTO(struct backing_dev_info *bdi,
+		 long bdi_dirty,
+		 long avg_dirty,
+		 long bdi_limit,
+		 long task_limit,
+		 long dirtied,
+		 long task_bw,
+		 long period,
+		 long pause),
+
+	TP_ARGS(bdi, bdi_dirty, avg_dirty, bdi_limit, task_limit,
+		dirtied, task_bw, period, pause),
+
+	TP_STRUCT__entry(
+		__array(char,	bdi, 32)
+		__field(long,	bdi_dirty)
+		__field(long,	avg_dirty)
+		__field(long,	bdi_limit)
+		__field(long,	task_limit)
+		__field(long,	dirtied)
+		__field(long,	bdi_bw)
+		__field(long,	base_bw)
+		__field(long,	task_bw)
+		__field(long,	period)
+		__field(long,	think)
+		__field(long,	pause)
+	),
+
+	TP_fast_assign(
+		strlcpy(__entry->bdi, dev_name(bdi->dev), 32);
+		__entry->bdi_dirty	= bdi_dirty;
+		__entry->avg_dirty	= avg_dirty;
+		__entry->bdi_limit	= bdi_limit;
+		__entry->task_limit	= task_limit;
+		__entry->dirtied	= dirtied;
+		__entry->bdi_bw		= KBps(bdi->write_bandwidth);
+		__entry->base_bw	= KBps(bdi->throttle_bandwidth);
+		__entry->task_bw	= KBps(task_bw);
+		__entry->think		= current->paused_when == 0 ? 0 :
+			 (long)(jiffies - current->paused_when) * 1000 / HZ;
+		__entry->period		= period * 1000 / HZ;
+		__entry->pause		= pause * 1000 / HZ;
+	),
+
+
+	/*
+	 *            [..............soft throttling range............]
+	 *            ^               |<=========== bdi_gap =========>|
+	 * (background+dirty)/2       |<== task_gap ==>|
+	 * -------------------|-------+----------------|--------------|
+	 *   (bdi_limit * 7/8)^       ^bdi_dirty       ^task_limit    ^bdi_limit
+	 *
+	 * Reasonable large gaps help produce smooth pause times.
+	 */
+	TP_printk("bdi %s: "
+		  "bdi_limit=%lu task_limit=%lu bdi_dirty=%lu avg_dirty=%lu "
+		  "bdi_gap=%ld%% task_gap=%ld%% task_weight=%ld%% "
+		  "bdi_bw=%lu base_bw=%lu task_bw=%lu "
+		  "dirtied=%lu period=%lu think=%ld pause=%ld",
+		  __entry->bdi,
+		  __entry->bdi_limit,
+		  __entry->task_limit,
+		  __entry->bdi_dirty,
+		  __entry->avg_dirty,
+		  BDP_PERCENT(bdi_limit, bdi_dirty, BDI_SOFT_DIRTY_LIMIT),
+		  BDP_PERCENT(task_limit, avg_dirty, TASK_SOFT_DIRTY_LIMIT),
+		  /* task weight: proportion of recent dirtied pages */
+		  BDP_PERCENT(bdi_limit, task_limit, TASK_SOFT_DIRTY_LIMIT),
+		  __entry->bdi_bw,	/* bdi write bandwidth */
+		  __entry->base_bw,	/* bdi base throttle bandwidth */
+		  __entry->task_bw,	/* task throttle bandwidth */
+		  __entry->dirtied,
+		  __entry->period,	/* ms */
+		  __entry->think,	/* ms */
+		  __entry->pause	/* ms */
+		  )
+);
+
 DECLARE_EVENT_CLASS(writeback_congest_waited_template,
 
 	TP_PROTO(unsigned int usec_timeout, unsigned int usec_delayed),
--- linux-next.orig/mm/page-writeback.c	2010-12-13 21:46:17.000000000 +0800
+++ linux-next/mm/page-writeback.c	2010-12-13 21:46:18.000000000 +0800
@@ -778,6 +778,8 @@ static void balance_dirty_pages(struct a
 		pause_max = max_pause(bdi_thresh);
 
 		if (avg_dirty >= task_thresh || nr_dirty > dirty_thresh) {
+			bw = 0;
+			period = 0;
 			pause = pause_max;
 			goto pause;
 		}
@@ -805,6 +807,15 @@ static void balance_dirty_pages(struct a
 		 * it may be a light dirtier.
 		 */
 		if (unlikely(-pause < HZ*10)) {
+			trace_balance_dirty_pages(bdi,
+						  bdi_dirty,
+						  avg_dirty,
+						  bdi_thresh,
+						  task_thresh,
+						  pages_dirtied,
+						  bw,
+						  period,
+						  pause);
 			if (-pause <= HZ/10)
 				current->paused_when += period;
 			else
@@ -815,6 +826,15 @@ static void balance_dirty_pages(struct a
 		pause = clamp_val(pause, 1, pause_max);
 
 pause:
+		trace_balance_dirty_pages(bdi,
+					  bdi_dirty,
+					  avg_dirty,
+					  bdi_thresh,
+					  task_thresh,
+					  pages_dirtied,
+					  bw,
+					  period,
+					  pause);
 		current->paused_when = jiffies;
 		__set_current_state(TASK_UNINTERRUPTIBLE);
 		io_schedule_timeout(pause);