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

Wu Fengguang
2010-12-13T15:15:57Z

[-- 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);