[RFC] few hacks to optimize write-back

[Artem Bityutskiy <Artem.Bityutskiy@nokia.com>]

Hi,

I've spotted that the periodic write-back threads wakes up every
5 seconds, even though there are no dirty data. And it still wakes
up and wake up and disturbs the CPU which is resting in a low
power consumption state.

I've tried to optimize this a bit and make the periodic
write-back stop when there are no dirty data.

Here are 2 patches which kind of help. They are just quick
hacks. I did not want to spend much time on this and I'd
like to get some comments/directions first:
 * I'm not sure I went the right path;
 * there are new per-BDI patches from Jens, and I probably
   should work on top of them.

AFAIU, the periodic write-back may be stopped if there are
no dirty inodes and no dirty super-blocks. And it should
start an inode or superblock is marked as dirty.

For inodes, I've hooked to '__mark_inode_dirty()'.

For superblocks. Well, probably a new VFS helper like
'mark_sb_dirty()' should be introduced. Also, FSes
currently may mark SB as clean directly, but probably
they should use a 'mark_sb_clean()' helper. FSes
should not touch sb->s_dirt directly. Anyway, I have
not implemented this part.

-- 
Best Regards,
Artem Bityutskiy (Артём Битюцкий)
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