nfsd: convert nfs4_file searches to use RCU

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 <[email protected]>

---
 fs/nfsd/nfs4state.c | 36 +++++++++++++++++++++++++++---------
 fs/nfsd/state.h     |  1 +
 2 files changed, 28 insertions(+), 9 deletions(-)

fs/nfsd/nfs4state.c +0 -0

index e9c3afe4b5d3..9bd3bcfee3c2 100644

fs/nfsd/nfs4state.c +2 -5

@@ -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;

Quoted 24 lines · Jeff Layton

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 <[email protected]>

---
 fs/nfsd/nfs4state.c | 36 +++++++++++++++++++++++++++---------
 fs/nfsd/state.h     |  1 +
 2 files changed, 28 insertions(+), 9 deletions(-)

fs/nfsd/nfs4state.c +0 -0

index e9c3afe4b5d3..9bd3bcfee3c2 100644
@@ -280,15 +280,22 @@ static void nfsd4_free_file(struct nfs4_file *f)
 	kmem_cache_free(file_slab, f);
 }
 

fs/nfsd/nfs4state.c static void nfsd4_free_file(struct nfs4_file *f) · line 287

Quoted 5 lines

fs/nfsd/nfs4state.c +5 -0

+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.

Quoted 17 lines

fs/nfsd/nfs4state.c +7 -5

@@ -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;

fs/nfsd/nfs4state.c find_file_locked(struct knfsd_fh *fh) · line 3335

Quoted 5 lines

fs/nfsd/nfs4state.c +5 -0

+			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.

Quoted 4 lines

fs/nfsd/state.h +0 -0

index 8e85e07efce6..530470a35ecd 100644

fs/nfsd/state.h struct nfs4_file { · line 493

Quoted 5 lines

fs/nfsd/state.h +1 -0

@@ -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.

Yeah, that's probably fine. Suggestions on what to union it with?

struct callback_head is two pointers, so maybe we can use one of the list_heads (fi_delegations maybe?).

If we do want to change to a different type of structure I'd be fine with that, but would prefer that it be RCU-friendly. What sort of structure did you have in mind?

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. In order to conserve space, we union the fi_rcu field with the fi_delegations list_head which must be clear by the time the last reference to the file is dropped.

Convert find_file_locked to use RCU lookup primitives and not to require that the state_lock be held, and convert find_file to do a lockless lookup. Convert find_or_add_file to attempt a lockless lookup first, and then fall back to doing a locked search and insert if that fails to find anything.

Also, minimize the number of times we need to calculate the hash value by passing it in as an argument to the search and insert functions, and optimize the order of arguments in nfsd4_init_file.

Cc Christoph Hellwig <[email protected]>

Signed-off-by Jeff Layton <[email protected]>

---
 fs/nfsd/nfs4state.c | 51 +++++++++++++++++++++++++++++----------------------
 fs/nfsd/state.h     |  5 ++++-
 2 files changed, 33 insertions(+), 23 deletions(-)

fs/nfsd/nfs4state.c +29 -22

index 1afd7d4420bd..1379d86f7b4f 100644
@@ -275,9 +275,11 @@ opaque_hashval(const void *ptr, int nbytes)
 	return x;
 }
 
-static void nfsd4_free_file(struct nfs4_file *f)
+static void nfsd4_free_file_rcu(struct rcu_head *rcu)
 {
-	kmem_cache_free(file_slab, f);
+	struct nfs4_file *fp = container_of(rcu, struct nfs4_file, fi_rcu);
+
+	kmem_cache_free(file_slab, fp);
 }
 
 static inline void

@@ -286,9 +288,10 @@ put_nfs4_file(struct nfs4_file *fi)
 	might_lock(&state_lock);
 
 	if (atomic_dec_and_lock(&fi->fi_ref, &state_lock)) {
-		hlist_del(&fi->fi_hash);
+		hlist_del_rcu(&fi->fi_hash);
 		spin_unlock(&state_lock);
-		nfsd4_free_file(fi);
+		WARN_ON_ONCE(!list_empty(&fi->fi_delegations));
+		call_rcu(&fi->fi_rcu, nfsd4_free_file_rcu);
 	}
 }
 

@@ -3057,10 +3060,9 @@ static struct nfs4_file *nfsd4_alloc_file(void)
 }
 
 /* OPEN Share state helper functions */
-static void nfsd4_init_file(struct nfs4_file *fp, struct knfsd_fh *fh)
+static void nfsd4_init_file(struct knfsd_fh *fh, unsigned int hashval,
+				struct nfs4_file *fp)
 {
-	unsigned int hashval = file_hashval(fh);
-
 	lockdep_assert_held(&state_lock);
 
 	atomic_set(&fp->fi_ref, 1);

@@ -3073,7 +3075,7 @@ static void nfsd4_init_file(struct nfs4_file *fp, struct knfsd_fh *fh)
 	fp->fi_share_deny = 0;
 	memset(fp->fi_fds, 0, sizeof(fp->fi_fds));
 	memset(fp->fi_access, 0, sizeof(fp->fi_access));
-	hlist_add_head(&fp->fi_hash, &file_hashtbl[hashval]);
+	hlist_add_head_rcu(&fp->fi_hash, &file_hashtbl[hashval]);
 }
 
 void

@@ -3294,17 +3296,14 @@ move_to_close_lru(struct nfs4_ol_stateid *s, struct net *net)
 
 /* search file_hashtbl[] for file */
 static struct nfs4_file *
-find_file_locked(struct knfsd_fh *fh)
+find_file_locked(struct knfsd_fh *fh, unsigned int hashval)
 {
-	unsigned int hashval = file_hashval(fh);
 	struct nfs4_file *fp;
 
-	lockdep_assert_held(&state_lock);
-
-	hlist_for_each_entry(fp, &file_hashtbl[hashval], fi_hash) {
+	hlist_for_each_entry_rcu(fp, &file_hashtbl[hashval], fi_hash) {
 		if (nfsd_fh_match(&fp->fi_fhandle, fh)) {
-			get_nfs4_file(fp);
-			return fp;
+			if (atomic_inc_not_zero(&fp->fi_ref))
+				return fp;
 		}
 	}
 	return NULL;

@@ -3314,10 +3313,11 @@ static struct nfs4_file *
 find_file(struct knfsd_fh *fh)
 {
 	struct nfs4_file *fp;
+	unsigned int hashval = file_hashval(fh);
 
-	spin_lock(&state_lock);
-	fp = find_file_locked(fh);
-	spin_unlock(&state_lock);
+	rcu_read_lock();
+	fp = find_file_locked(fh, hashval);
+	rcu_read_unlock();
 	return fp;
 }
 

@@ -3325,11 +3325,18 @@ static struct nfs4_file *
 find_or_add_file(struct nfs4_file *new, struct knfsd_fh *fh)
 {
 	struct nfs4_file *fp;
+	unsigned int hashval = file_hashval(fh);
+
+	rcu_read_lock();
+	fp = find_file_locked(fh, hashval);
+	rcu_read_unlock();
+	if (fp)
+		return fp;
 
 	spin_lock(&state_lock);
-	fp = find_file_locked(fh);
-	if (fp == NULL) {
-		nfsd4_init_file(new, fh);
+	fp = find_file_locked(fh, hashval);
+	if (likely(fp == NULL)) {
+		nfsd4_init_file(fh, hashval, new);
 		fp = new;
 	}
 	spin_unlock(&state_lock);

@@ -4127,7 +4134,7 @@ void nfsd4_cleanup_open_state(struct nfsd4_compound_state *cstate,
 		nfs4_put_stateowner(so);
 	}
 	if (open->op_file)
-		nfsd4_free_file(open->op_file);
+		kmem_cache_free(file_slab, open->op_file);
 	if (open->op_stp)
 		nfs4_put_stid(&open->op_stp->st_stid);
 }

fs/nfsd/state.h +4 -1

index 8e85e07efce6..9d3be371240a 100644
@@ -477,7 +477,10 @@ struct nfs4_file {
 	spinlock_t		fi_lock;
 	struct hlist_node       fi_hash;	/* hash on fi_fhandle */
 	struct list_head        fi_stateids;
-	struct list_head	fi_delegations;
+	union {
+		struct list_head	fi_delegations;
+		struct rcu_head		fi_rcu;
+	};
 	/* One each for O_RDONLY, O_WRONLY, O_RDWR: */
 	struct file *		fi_fds[3];
 	/*

Signature · 1 line

1.9.3