Re: aio: bump i_count instead of using igrab

[Chris Mason <chris.mason@oracle.com>]

On Mon, Aug 23, 2010 at 10:50:31AM -0400, Christoph Hellwig wrote:
> On Mon, Aug 23, 2010 at 10:47:55AM -0400, Chris Mason wrote:
> > The aio batching code is using igrab to get an extra reference on the
> > inode so it can safely batch.  igrab will go ahead and take the global
> > inode spinlock, which can be a bottleneck on large machines doing lots
> > of AIO.
> > 
> > In this case, igrab isn't required because we already have a reference
> > on the file handle.  It is safe to just bump the i_count directly
> > on the inode.
> > 
> > Benchmarking shows this patch brings IOP/s on tons of flash up by about
> > 2.5X.
> 
> There's some places in XFS where we do the same, and it showed up as a
> bottle neck before.  Instead of open coding the increment we have
> a wrapper that includes and assert that the numbers is always positive.
> 
> I think we really want a proper helper for general use instead of
> completly opencoding it.
> 

Nick, this is about a 1 liner to fs/aio.c replacing igrab with
atomic_inc directly on the inode reference count.

I know your scalability tree gets rid of the global, but in this case I
think it still makes sense to avoid the locking completely when the
caller knows it is safe.  Do you already have something similar hiding
in the scalability tree?

-chris