Re: [PATCH] nfsd: convert nfs4_file searches to use RCU

[Christoph Hellwig <hch@infradead.org>]

On Fri, Oct 17, 2014 at 06:21:15AM -0400, Jeff Layton wrote:
> The global state_lock protects the file_hashtbl, and that has the
> potential to be a scalability bottleneck.
> 
> Address this by making the file_hashtbl use RCU. Add a rcu_head to the
> nfs4_file and use that when freeing ones that have been hashed.
> 
> Convert find_file to use a lockless lookup. Convert find_or_add_file to
> attempt a lockless lookup first, and then fall back to doing the
> "normal" locked search and insert if that fails to find anything.
> 
> Signed-off-by: Jeff Layton <jlayton@primarydata.com>
> ---
>  fs/nfsd/nfs4state.c | 36 +++++++++++++++++++++++++++---------
>  fs/nfsd/state.h     |  1 +
>  2 files changed, 28 insertions(+), 9 deletions(-)
> 
> diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
> index e9c3afe4b5d3..9bd3bcfee3c2 100644
> --- a/fs/nfsd/nfs4state.c
> +++ b/fs/nfsd/nfs4state.c
> @@ -280,15 +280,22 @@ static void nfsd4_free_file(struct nfs4_file *f)
>  	kmem_cache_free(file_slab, f);
>  }
>  
> +static void nfsd4_free_file_rcu(struct rcu_head *rcu)
> +{
> +	struct nfs4_file *fp = container_of(rcu, struct nfs4_file, fi_rcu);
> +
> +	nfsd4_free_file(fp);

You might as well kill the pointless nfsd4_free_file wrapper while
you're at it.

> @@ -3313,12 +3320,19 @@ find_file_locked(struct knfsd_fh *fh)
>  static struct nfs4_file *
>  find_file(struct knfsd_fh *fh)
>  {
> -	struct nfs4_file *fp;
> +	struct nfs4_file *fp, *ret = NULL;
> +	unsigned int hashval = file_hashval(fh);
>  
> -	spin_lock(&state_lock);
> -	fp = find_file_locked(fh);
> -	spin_unlock(&state_lock);
> -	return fp;
> +	rcu_read_lock();
> +	hlist_for_each_entry_rcu(fp, &file_hashtbl[hashval], fi_hash) {
> +		if (nfsd_fh_match(&fp->fi_fhandle, fh)) {
> +			if (atomic_inc_not_zero(&fp->fi_ref))
> +				ret = fp;
> +			break;
> +		}
> +	}
> +	rcu_read_unlock();
> +	return ret;

I think it would be better to just switch find_file_locked ti use
hlist_for_each_entry_rcu instead of duplicating it.

> diff --git a/fs/nfsd/state.h b/fs/nfsd/state.h
> index 8e85e07efce6..530470a35ecd 100644
> --- a/fs/nfsd/state.h
> +++ b/fs/nfsd/state.h
> @@ -490,6 +490,7 @@ struct nfs4_file {
>  	atomic_t		fi_access[2];
>  	u32			fi_share_deny;
>  	struct file		*fi_deleg_file;
> +	struct rcu_head		fi_rcu;

Can we union this over a field that's guaranteed to be unused on
a file that has been unhashed?


Also a slightly related question:  Is the small fixes size hash table
still fine for the workloads where the RCU access matters?  It seems
like we should aim for a more scalable data structure to look up the
files.  It also irks me a bit how this duplicates the inode cache,
which for some filesystems (e.g. XFS) already is very scalable.