writeback: Tracing support V2

Hi Jens,

This series is an update of the writeback tracing that I'd previously posted against mainline. This version is based on the block/for-2.6.36 branch.

The tracing provides visibility into the behaviour of the writeback path and is sufficient to understand questions such as what process is writing data, what process is blocking on congestion, etc. It also allows view the differences in behaviour for different sync modes and calling contexts.

This series is based on the block/for-2.6.36 branch.

Output (slighty trimmed) for a background flush of a clean bdi is as follows:

flush-8:0-[...]: wbc_writeback_start: bdi 8:0: towrt=1024 skip=0 mode=0 kupd=1 bgrd=0 reclm=0 cyclic=1 more=0 older=0xffff5d8e start=0x0 end=0x0 flush-8:0-[...]: wbc_writeback_written: bdi 8:0: towrt=1024 skip=0 mode=0 kupd=1 bgrd=0 reclm=0 cyclic=1 more=0 older=0xffff5d8e start=0x0 end=0x0 flush-8:0-[...]: writeback_pages_written: 0

Here we see that we are doing writeback for kupdate in cyclic mode with inodes that are older than 0xffff5d8e. The filesystem is clean, so there is nothing to write...

Output (slighty trimmed) for a sync(1) based flush of a clean bdi is as follows:

     sync-[...]: wb_sched_queue: bdi 8:0: sb_dev 0:0 nr_pages=0 sync_mode=0 kupdate=0 range_cyclic=0 background=0
     sync-[...]: wb_sched_task: bdi 8:0: sb_dev 0:0 nr_pages=0 sync_mode=0 kupdate=0 range_cyclic=0 background=0
flush-8:0-[...]: wb_sched_exec: bdi 8:0: sb_dev 0:0 nr_pages=0 sync_mode=0 kupdate=0 range_cyclic=0 background=0
flush-8:0-[...]: writeback_pages_written: 0
     sync-[...]: wb_sched_queue: bdi 8:0: sb_dev 8:1 nr_pages=7712 sync_mode=0 kupdate=0 range_cyclic=0 background=0
     sync-[...]: wb_sched_task: bdi 8:0: sb_dev 8:1 nr_pages=7712 sync_mode=0 kupdate=0 range_cyclic=0 background=0

flush-8:0-[...]: wb_sched_exec: bdi 8:0: sb_dev 8:1 nr_pages=7712 sync_mode=0 kupdate=0 range_cyclic=0 background=0 flush-8:0-[...]: wbc_writeback_start: bdi 8:0: towrt=1024 skip=0 mode=0 kupd=0 bgrd=0 reclm=0 cyclic=0 more=0 older=0x0 start=0x0 end=0x7fffffffffffffff flush-8:0-[...]: wbc_writeback_written: bdi 8:0: towrt=1024 skip=0 mode=0 kupd=0 bgrd=0 reclm=0 cyclic=0 more=0 older=0x0 start=0x0 end=0x7fffffffffffffff flush-8:0-[...]: writeback_pages_written: 0

     sync-[...]: wb_sched_queue: bdi 8:0: sb_dev 8:1 nr_pages=9223372036854775807 sync_mode=1 kupdate=0 range_cyclic=0 background=0
     sync-[...]: wb_sched_task: bdi 8:0: sb_dev 8:1 nr_pages=9223372036854775807 sync_mode=1 kupdate=0 range_cyclic=0 background=0

flush-8:0-[...]: wb_sched_exec: bdi 8:0: sb_dev 8:1 nr_pages=9223372036854775807 sync_mode=1 kupdate=0 range_cyclic=0 background=0 flush-8:0-[...]: wbc_writeback_start: bdi 8:0: wbc=ffff88011cb4bd20 towrt=1024 skip=0 mode=1 kupd=0 bgrd=0 reclm=0 cyclic=0 more=0 older=0x0 start=0x0 end=0x7fffffffffffffff flush-8:0-[...]: wbc_writeback_written: bdi 8:0: wbc=ffff88011cb4bd20 towrt=1024 skip=0 mode=1 kupd=0 bgrd=0 reclm=0 cyclic=0 more=0 older=0x0 start=0x0 end=0x7fffffffffffffff flush-8:0-[...]: writeback_pages_written: 0

What we can see here is that:

- sync queued an async flush to the entire bdi
- the bdi flusher thread wrote no pages (i.e. clean)
- sync has issued an async flush of superblock 8:1 for 7712 pages
- the bdi flush ran writeback on the superblock, wrote no pages
- sync queued a synchronous flush of the superblock
- the bdi flush ran writeback on the superblock, wrote no pages

---
Version 2:
- rewrite of initial patch from previous 2.6.34 based code to match new bdi scheduling code as part of rebasing onto the block/for-2.6.36 tree.
- removed work structure address from output as it was mostly redundant and took up lots of space in the output.
- removed writeback control structure address from output as it was mostly redundant and took up lots of space in the output.