Re: [PATCH 02/11] inode: add IOP_NOTHASHED to avoid inode hash lock in evict

[Dave Chinner <david@fromorbit.com>]

On Thu, Aug 01, 2013 at 01:12:35AM -0700, Christoph Hellwig wrote:
> On Wed, Jul 31, 2013 at 02:15:41PM +1000, Dave Chinner wrote:
> > From: Dave Chinner <dchinner@redhat.com>
> > 
> > Some filesystems don't use the VFS inode hash and fake the fact they
> > are hashed so that all the writeback code works correctly. However,
> > this means the evict() path still tries to remove the inode from the
> > hash, meaning that the inode_hash_lock() needs to be taken
> > unnecessarily. Hence under certain workloads the inode_hash_lock can
> > be contended even if the inode is never actually hashed.
> > 
> > To avoid this, add an inode opflag to allow inode_hash_remove() to
> > avoid taking the hash lock on inodes have never actually been
> > hashed.
> 
> Good idea, but I don't like how it's implemented.
> 
> First a formality: i_opflags really is for flags showing that inode
> operations exist, not for addional bits.  Just use i_flags for it.
> 
> Second this is a hack hacking around a hack.  We just mark the inode
> hashed so that writeback doesn't ignore it, and not we need to work
> around the fact that we don't want an inode marked hashed from the
> hashlist.
> 
> As the most simple version I'd suggest to just add an I_NEEDS_WRITEBACK
> flag which gets set by __insert_inode_hash, and all the current users
> of hlist_add_fake on i_hash, as well as the block devices that currently
> have another special case in the writeback code.

But that doesn't fix the problem of taking the hash lock in evict()
when it is not necessary. If everything sets I_NEEDS_WRITEBACK, and
we still fake hashing the inode, how are do we know that we don't
need to unhash it in evict()?

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com