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Subject[PATCH 12/13] writeback: add trace event for balance_dirty_pages()
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%)
rsync is smart enough to compensate for the in-kernel pause time

Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
include/trace/events/writeback.h | 61 +++++++++++++++++++++++++++--
mm/page-writeback.c | 6 ++
2 files changed, 64 insertions(+), 3 deletions(-)
--- linux-next.orig/include/trace/events/writeback.h 2010-11-15 18:59:00.000000000 +0800
+++ linux-next/include/trace/events/writeback.h 2010-11-15 21:31:46.000000000 +0800
@@ -147,11 +147,66 @@ 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 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 bdi_limit,
+ long task_limit,
+ long pages_dirtied,
+ long pause),
+
+ TP_ARGS(bdi, bdi_dirty, bdi_limit, task_limit,
+ pages_dirtied, pause),
+
+ TP_STRUCT__entry(
+ __array(char, bdi, 32)
+ __field(long, bdi_dirty)
+ __field(long, bdi_limit)
+ __field(long, task_limit)
+ __field(long, pages_dirtied)
+ __field(long, pause)
+ ),
+
+ TP_fast_assign(
+ strlcpy(__entry->bdi, dev_name(bdi->dev), 32);
+ __entry->bdi_dirty = bdi_dirty;
+ __entry->bdi_limit = bdi_limit;
+ __entry->task_limit = task_limit;
+ __entry->pages_dirtied = pages_dirtied;
+ __entry->pause = pause * 1000 / HZ;
+ ),
+
+
+ /*
+ * [..................soft throttling range.........]
+ * ^ |<=========== bdi_gap =========>|
+ * background_thresh |<== 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_dirty=%lu bdi_limit=%lu task_limit=%lu "
+ "task_weight=%ld%% task_gap=%ld%% bdi_gap=%ld%% "
+ "pages_dirtied=%lu pause=%lu",
+ __entry->bdi,
+ __entry->bdi_dirty,
+ __entry->bdi_limit,
+ __entry->task_limit,
+ /* task weight: proportion of recent dirtied pages */
+ BDP_PERCENT(bdi_limit, task_limit, TASK_SOFT_DIRTY_LIMIT),
+ BDP_PERCENT(task_limit, bdi_dirty, TASK_SOFT_DIRTY_LIMIT),
+ BDP_PERCENT(bdi_limit, bdi_dirty, BDI_SOFT_DIRTY_LIMIT),
+ __entry->pages_dirtied,
+ __entry->pause
+ )
+);
+
DECLARE_EVENT_CLASS(writeback_congest_waited_template,

TP_PROTO(unsigned int usec_timeout, unsigned int usec_delayed),
--- linux-next.orig/mm/page-writeback.c 2010-11-15 21:30:45.000000000 +0800
+++ linux-next/mm/page-writeback.c 2010-11-15 21:31:46.000000000 +0800
@@ -629,6 +629,12 @@ static void balance_dirty_pages(struct a
pause = clamp_val(pause, 1, HZ/10);

pause:
+ trace_balance_dirty_pages(bdi,
+ bdi_dirty,
+ bdi_thresh,
+ task_thresh,
+ pages_dirtied,
+ pause);
bdi_update_write_bandwidth(bdi, &bw_time, &bw_written);
__set_current_state(TASK_INTERRUPTIBLE);
io_schedule_timeout(pause);



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