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.
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-[...]: writeback_queue: bdi 8:0: sb_dev 0:0 nr_pages=0 sync_mode=0 kupdate=0 range_cyclic=0 background=0
flush-8:0-[...]: writeback_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-[...]: writeback_queue: bdi 8:0: sb_dev 8:1 nr_pages=7712 sync_mode=0 kupdate=0 range_cyclic=0 background=0
flush-8:0-[...]: writeback_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-[...]: writeback_queue: bdi 8:0: sb_dev 8:1 nr_pages=9223372036854775807 sync_mode=1 kupdate=0 range_cyclic=0 background=0
flush-8:0-[...]: writeback_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 3:
- renamed "wb_sched" tracepoints to "writeback"
- renamed trace class definitions to be more use specific
- made work structure mandatory for writeback work events
- removed unnecessary task/no_task wb_sched trace points.
- converted no_work wb_sched tracepoint to use the basic writeback tracepoint
class
- removed check for null bdi in writeback control trace events
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.
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