2.6.23-rc6: hanging ext3 dbench tests

[Andy Whitcroft <apw@shadowen.org>]

I have a couple of failed test runs against 2.6.23-rc6 where the
job timed out while running dbench over ext3.  Both on powerpc,
though both significantly different hardware setups.  A failed
run like this implies that the machine was still responsive to
other processes but the dbench was making no progress.  There is
no console diagnostics during the failure.

beavis was lost during a plain ext3 dbench run, having just
successfully run a complete ext2 run.  elm3b19 was lost during an
ext3 "data=writeback" dbench run, having already completed an plain
ext2, and ext3 runs.

A quick poke at the dbench logs on the second machine shows this
for the working ext3 dbench run:

  4 clients started
  4     35288  814.49 MB/sec
  0     62477  822.99 MB/sec
  Throughput 822.954 MB/sec 4 procs

Whereas the hanging run shows the following continuing until the
machine is reset, which confirms that the machine as a whole was
still with us:

  4 clients started
  4     36479  824.92 MB/sec
  1     46857  519.98 MB/sec
  1     46857  346.65 MB/sec
  1     46857  259.99 MB/sec
  1     46857  207.99 MB/sec
  1     46857  173.32 MB/sec
  1     46857  148.56 MB/sec
  1     46857  129.99 MB/sec
  1     46857  115.55 MB/sec
  1     46857  103.99 MB/sec
  1     46857  94.54 MB/sec
  1     46857  86.66 MB/sec
  1     46857  80.00 MB/sec
  [...]

The first machine is very similar:

  4 clients started
  4     18468  445.29 MB/sec
  4     41945  469.36 MB/sec
  1     46857  346.68 MB/sec
  1     46857  260.00 MB/sec
  1     46857  208.00 MB/sec
  [...]

Not sure if there is any significance to the 46857.  Though it feels
like we may be at the end of the run when it fails.

I will try and reproduce this on one of the machines and see if I
can get any further info.

-apw