writeback: add wb_monitor.py script to monitor writeback info on bdi

Add wb_monitor.py script to monitor writeback information on backing dev
which makes it easier and more convenient to observe wri

writeback: add wb_monitor.py script to monitor writeback info on bdi

Add wb_monitor.py script to monitor writeback information on backing dev
which makes it easier and more convenient to observe writeback behaviors
of running system.

The wb_monitor.py script is written based on wq_monitor.py.

Following domain hierarchy is tested:
global domain (320G)
/ \
cgroup domain1(10G) cgroup domain2(10G)
| |
bdi wb1 wb2

The wb_monitor.py script output is as following:
./wb_monitor.py 252:16 -c
writeback reclaimable dirtied written avg_bw
252:16_1 0 0 0 0 102400
252:16_4284 672 820064 9230368 8410304 685612
252:16_4325 896 819840 10491264 9671648 652348
252:16 1568 1639904 19721632 18081952 1440360

writeback reclaimable dirtied written avg_bw
252:16_1 0 0 0 0 102400
252:16_4284 672 820064 9230368 8410304 685612
252:16_4325 896 819840 10491264 9671648 652348
252:16 1568 1639904 19721632 18081952 1440360
...

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Kemeng Shi <[email protected]>
Suggested-by: Tejun Heo <[email protected]>
Cc: Brian Foster <[email protected]>
Cc: David Howells <[email protected]>
Cc: David Sterba <[email protected]>
Cc: Jan Kara <[email protected]>
Cc: Mateusz Guzik <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: SeongJae Park <[email protected]>
Cc: Stephen Rothwell <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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