ext3/4: enhance fsync performance when using CFQ

Hi,

Running iozone with the fsync flag, or fs_mark, the performance of CFQ is
far worse than that of deadline for enterprise class storage when dealing
with file sizes of 8MB or less.  I used the following command line as a
representative test case:

  fs_mark -S 1 -D 10000 -N 100000 -d /mnt/test/fs_mark -s 65536 -t 1 -w 4096 -F

When run using the deadline I/O scheduler, an average of the first 5 numbers
will give you 448.4 files / second.  CFQ will yield only 106.7.  With
this patch series applied (and the two patches I sent yesterday), CFQ now
achieves 462.5 files / second.

This patch set is still an RFC.  I'd like to make it perform better when
there is a competing sequential reader present.  For now, I've addressed
the concerns voiced about the previous posting.

Review and testing would be greatly appreciated.

Thanks!
Jeff

---

New from the last round:

- removed the think time calculation I added for the sync-noidle service tree
- replaced above with a suggestion from Vivek to only guard against currently
  active sequential readers when determining if we can preempt the sync-noidle
  service tree.
- bug fixes

Over all, I think it's simpler now thanks to the suggestions from Jens and
Vivek.

[PATCH 1/3] block: Implement a blk_yield function to voluntarily give up the I/O scheduler.
[PATCH 2/3] jbd: yield the device queue when waiting for commits
[PATCH 3/3] jbd2: yield the device queue when waiting for journal commits
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