Re: [PATCH v7 14/14] NFSD: Use rhashtable for managing nfs4_file objects

[Jeff Layton <jlayton@redhat.com>]

On Fri, 2022-10-28 at 10:48 -0400, Chuck Lever wrote:
> fh_match() is costly, especially when filehandles are large (as is
> the case for NFSv4). It needs to be used sparingly when searching
> data structures. Unfortunately, with common workloads, I see
> multiple thousands of objects stored in file_hashtbl[], which has
> just 256 buckets, making its bucket hash chains quite lengthy.
> 
> Walking long hash chains with the state_lock held blocks other
> activity that needs that lock. Sizable hash chains are a common
> occurrance once the server has handed out some delegations, for
> example -- IIUC, each delegated file is held open on the server by
> an nfs4_file object.
> 
> To help mitigate the cost of searching with fh_match(), replace the
> nfs4_file hash table with an rhashtable, which can dynamically
> resize its bucket array to minimize hash chain length.
> 
> The result of this modification is an improvement in the latency of
> NFSv4 operations, and the reduction of nfsd CPU utilization due to
> eliminating the cost of multiple calls to fh_match() and reducing
> the CPU cache misses incurred while walking long hash chains in the
> nfs4_file hash table.
> 
> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
> Reviewed-by: NeilBrown <neilb@suse.de>
> ---
>  fs/nfsd/nfs4state.c |   77 ++++++++++++++++++++++++++-------------------------
>  fs/nfsd/state.h     |    4 ---
>  2 files changed, 40 insertions(+), 41 deletions(-)
> 
> diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
> index c2ef2db9c84c..c78b66e678dd 100644
> --- a/fs/nfsd/nfs4state.c
> +++ b/fs/nfsd/nfs4state.c
> @@ -591,11 +591,8 @@ static void nfsd4_free_file_rcu(struct rcu_head *rcu)
>  void
>  put_nfs4_file(struct nfs4_file *fi)
>  {
> -	might_lock(&state_lock);
> -
> -	if (refcount_dec_and_lock(&fi->fi_ref, &state_lock)) {
> +	if (refcount_dec_and_test(&fi->fi_ref)) {
>  		nfsd4_file_hash_remove(fi);
> -		spin_unlock(&state_lock);
>  		WARN_ON_ONCE(!list_empty(&fi->fi_clnt_odstate));
>  		WARN_ON_ONCE(!list_empty(&fi->fi_delegations));
>  		call_rcu(&fi->fi_rcu, nfsd4_free_file_rcu);
> @@ -709,20 +706,6 @@ static unsigned int ownerstr_hashval(struct xdr_netobj *ownername)
>  	return ret & OWNER_HASH_MASK;
>  }
>  
> -/* hash table for nfs4_file */
> -#define FILE_HASH_BITS                   8
> -#define FILE_HASH_SIZE                  (1 << FILE_HASH_BITS)
> -
> -static unsigned int file_hashval(const struct svc_fh *fh)
> -{
> -	struct inode *inode = d_inode(fh->fh_dentry);
> -
> -	/* XXX: why not (here & in file cache) use inode? */
> -	return (unsigned int)hash_long(inode->i_ino, FILE_HASH_BITS);
> -}
> -
> -static struct hlist_head file_hashtbl[FILE_HASH_SIZE];
> -
>  static struct rhltable nfs4_file_rhltable ____cacheline_aligned_in_smp;
>  
>  static const struct rhashtable_params nfs4_file_rhash_params = {
> @@ -4687,12 +4670,14 @@ move_to_close_lru(struct nfs4_ol_stateid *s, struct net *net)
>  static noinline_for_stack struct nfs4_file *
>  nfsd4_file_hash_lookup(const struct svc_fh *fhp)
>  {
> -	unsigned int hashval = file_hashval(fhp);
> +	struct inode *inode = d_inode(fhp->fh_dentry);
> +	struct rhlist_head *tmp, *list;
>  	struct nfs4_file *fi;
>  
>  	rcu_read_lock();
> -	hlist_for_each_entry_rcu(fi, &file_hashtbl[hashval], fi_hash,
> -				 lockdep_is_held(&state_lock)) {
> +	list = rhltable_lookup(&nfs4_file_rhltable, &inode,
> +			       nfs4_file_rhash_params);
> +	rhl_for_each_entry_rcu(fi, tmp, list, fi_rlist) {
>  		if (fh_match(&fi->fi_fhandle, &fhp->fh_handle)) {
>  			if (refcount_inc_not_zero(&fi->fi_ref)) {
>  				rcu_read_unlock();
> @@ -4706,40 +4691,56 @@ nfsd4_file_hash_lookup(const struct svc_fh *fhp)
>  
>  /*
>   * On hash insertion, identify entries with the same inode but
> - * distinct filehandles. They will all be in the same hash bucket
> - * because nfs4_file's are hashed by the address in the fi_inode
> - * field.
> + * distinct filehandles. They will all be on the list returned
> + * by rhltable_lookup().
> + *
> + * inode->i_lock prevents racing insertions from adding an entry
> + * for the same inode/fhp pair twice.
>   */
>  static noinline_for_stack struct nfs4_file *
>  nfsd4_file_hash_insert(struct nfs4_file *new, const struct svc_fh *fhp)
>  {
> -	unsigned int hashval = file_hashval(fhp);
> +	struct inode *inode = d_inode(fhp->fh_dentry);
> +	struct rhlist_head *tmp, *list;
>  	struct nfs4_file *ret = NULL;
>  	bool alias_found = false;
>  	struct nfs4_file *fi;
> +	int err;
>  
> -	spin_lock(&state_lock);
> -	hlist_for_each_entry_rcu(fi, &file_hashtbl[hashval], fi_hash,
> -				 lockdep_is_held(&state_lock)) {
> +	rcu_read_lock();
> +	spin_lock(&inode->i_lock);
> +
> +	list = rhltable_lookup(&nfs4_file_rhltable, &inode,
> +			       nfs4_file_rhash_params);
> +	rhl_for_each_entry_rcu(fi, tmp, list, fi_rlist) {
>  		if (fh_match(&fi->fi_fhandle, &fhp->fh_handle)) {
>  			if (refcount_inc_not_zero(&fi->fi_ref))
>  				ret = fi;
> -		} else if (d_inode(fhp->fh_dentry) == fi->fi_inode)
> +		} else
>  			fi->fi_aliased = alias_found = true;
>  	}
> -	if (likely(ret == NULL)) {
> -		nfsd4_file_init(fhp, new);
> -		hlist_add_head_rcu(&new->fi_hash, &file_hashtbl[hashval]);
> -		new->fi_aliased = alias_found;
> -		ret = new;
> -	}
> -	spin_unlock(&state_lock);
> +	if (ret)
> +		goto out_unlock;
> +
> +	nfsd4_file_init(fhp, new);
> +	err = rhltable_insert(&nfs4_file_rhltable, &new->fi_rlist,
> +			      nfs4_file_rhash_params);
> +	if (err)
> +		goto out_unlock;
> +
> +	new->fi_aliased = alias_found;
> +	ret = new;
> +
> +out_unlock:
> +	spin_unlock(&inode->i_lock);
> +	rcu_read_unlock();
>  	return ret;
>  }
>  
>  static noinline_for_stack void nfsd4_file_hash_remove(struct nfs4_file *fi)
>  {
> -	hlist_del_rcu(&fi->fi_hash);
> +	rhltable_remove(&nfs4_file_rhltable, &fi->fi_rlist,
> +			nfs4_file_rhash_params);
>  }
>  
>  /*
> @@ -5629,6 +5630,8 @@ nfsd4_process_open2(struct svc_rqst *rqstp, struct svc_fh *current_fh, struct nf
>  	 * If not found, create the nfs4_file struct
>  	 */
>  	fp = nfsd4_file_hash_insert(open->op_file, current_fh);
> +	if (unlikely(!fp))
> +		return nfserr_jukebox;
>  	if (fp != open->op_file) {
>  		status = nfs4_check_deleg(cl, open, &dp);
>  		if (status)
> diff --git a/fs/nfsd/state.h b/fs/nfsd/state.h
> index 190fc7e418a4..eadd7f465bf5 100644
> --- a/fs/nfsd/state.h
> +++ b/fs/nfsd/state.h
> @@ -536,16 +536,12 @@ struct nfs4_clnt_odstate {
>   * inode can have multiple filehandles associated with it, so there is
>   * (potentially) a many to one relationship between this struct and struct
>   * inode.
> - *
> - * These are hashed by filehandle in the file_hashtbl, which is protected by
> - * the global state_lock spinlock.
>   */
>  struct nfs4_file {
>  	refcount_t		fi_ref;
>  	struct inode *		fi_inode;
>  	bool			fi_aliased;
>  	spinlock_t		fi_lock;
> -	struct hlist_node       fi_hash;	/* hash on fi_fhandle */
>  	struct rhlist_head	fi_rlist;
>  	struct list_head        fi_stateids;
>  	union {
> 
> 

You can add this to 13 and 14:

Reviewed-by: Jeff Layton <jlayton@redhat.com>

That said, I'd probably prefer to see the two squashed together, since
you're adding unused infrastructure in 13.