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From:   Nagendra Tomar <Nagendra.Tomar@microsoft.com>
To:     "linux-nfs@vger.kernel.org" <linux-nfs@vger.kernel.org>
CC:     Trond Myklebust <trond.myklebust@hammerspace.com>
Subject: [RFC] nconnect xprt stickiness for a file
Thread-Topic: [RFC] nconnect xprt stickiness for a file
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Date:   Thu, 18 Mar 2021 01:56:23 +0000
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We have a clustered NFS server behind a L4 load-balancer with the following
Characteristics (relevant to this discussion):

1. RPC requests for the same file issued to different cluster nodes are not=
 efficient.
    One file one cluster node is efficient. This is particularly true for W=
RITEs.
2. Multiple nconnect xprts land on different cluster nodes due to the sourc=
e=20
    port being different for all.

Due to this, the default nconnect roundrobin policy does not work very well=
 as
it results in RPCs targeted to the same file to be serviced by different cl=
uster nodes.

To solve this, we tweaked the nfs multipath code to always choose the same =
xprt=20
for the same file. We do that by adding a new integer field to rpc_message,
rpc_xprt_hint, which is set by NFS layer and used by RPC layer to pick a xp=
rt.
NFS layer sets it to the hash of the target file's filehandle, thus ensurin=
g same file
requests always use the same xprt. This works well.

I am interested in knowing your thoughts on this, has anyone else also come=
 across
similar issue, is there any other way of solving this, etc.

Following  patch is just to demonstrate the idea. It works but covers only =
what I
needed for the experiment. Based on the feedback, I can follow it up with a=
 formal=20
patch if needed.

Thanks,
Tomar

---
diff --git a/fs/nfs/nfs3proc.c b/fs/nfs/nfs3proc.c
index 5c4e23abc..8f1cf03dc 100644
--- a/fs/nfs/nfs3proc.c
+++ b/fs/nfs/nfs3proc.c
@@ -108,6 +108,7 @@ nfs3_proc_getattr(struct nfs_server *server, struct nfs=
_fh *fhandle,
 		.rpc_proc	=3D &nfs3_procedures[NFS3PROC_GETATTR],
 		.rpc_argp	=3D fhandle,
 		.rpc_resp	=3D fattr,
+		.rpc_xprt_hint	=3D nfs_fh_hash(fhandle),
 	};
 	int	status;
 	unsigned short task_flags =3D 0;
@@ -136,6 +137,7 @@ nfs3_proc_setattr(struct dentry *dentry, struct nfs_fat=
tr *fattr,
 		.rpc_proc	=3D &nfs3_procedures[NFS3PROC_SETATTR],
 		.rpc_argp	=3D &arg,
 		.rpc_resp	=3D fattr,
+		.rpc_xprt_hint	=3D nfs_fh_hash(arg.fh),
 	};
 	int	status;
=20
@@ -171,6 +173,7 @@ __nfs3_proc_lookup(struct inode *dir, const char *name,=
 size_t len,
 		.rpc_proc	=3D &nfs3_procedures[NFS3PROC_LOOKUP],
 		.rpc_argp	=3D &arg,
 		.rpc_resp	=3D &res,
+		.rpc_xprt_hint	=3D nfs_fh_hash(arg.fh),
 	};
 	int			status;
=20
@@ -235,6 +238,7 @@ static int nfs3_proc_access(struct inode *inode, struct=
 nfs_access_entry *entry)
 		.rpc_argp	=3D &arg,
 		.rpc_resp	=3D &res,
 		.rpc_cred	=3D entry->cred,
+		.rpc_xprt_hint	=3D nfs_fh_hash(arg.fh),
 	};
 	int status =3D -ENOMEM;
=20
@@ -266,6 +270,7 @@ static int nfs3_proc_readlink(struct inode *inode, stru=
ct page *page,
 	struct rpc_message msg =3D {
 		.rpc_proc	=3D &nfs3_procedures[NFS3PROC_READLINK],
 		.rpc_argp	=3D &args,
+		.rpc_xprt_hint	=3D nfs_fh_hash(args.fh),
 	};
 	int status =3D -ENOMEM;
=20
@@ -355,6 +360,7 @@ nfs3_proc_create(struct inode *dir, struct dentry *dent=
ry, struct iattr *sattr,
 	data->arg.create.name =3D dentry->d_name.name;
 	data->arg.create.len =3D dentry->d_name.len;
 	data->arg.create.sattr =3D sattr;
+	data->msg.rpc_xprt_hint =3D nfs_fh_hash(data->arg.create.fh);
=20
 	data->arg.create.createmode =3D NFS3_CREATE_UNCHECKED;
 	if (flags & O_EXCL) {
@@ -442,6 +448,7 @@ nfs3_proc_remove(struct inode *dir, struct dentry *dent=
ry)
 		.rpc_proc =3D &nfs3_procedures[NFS3PROC_REMOVE],
 		.rpc_argp =3D &arg,
 		.rpc_resp =3D &res,
+		.rpc_xprt_hint =3D nfs_fh_hash(arg.fh),
 	};
 	int status =3D -ENOMEM;
=20
@@ -524,6 +531,7 @@ nfs3_proc_link(struct inode *inode, struct inode *dir, =
const struct qstr *name)
 		.rpc_proc	=3D &nfs3_procedures[NFS3PROC_LINK],
 		.rpc_argp	=3D &arg,
 		.rpc_resp	=3D &res,
+		.rpc_xprt_hint	=3D nfs_fh_hash(arg.tofh),
 	};
 	int status =3D -ENOMEM;
=20
@@ -566,6 +574,7 @@ nfs3_proc_symlink(struct inode *dir, struct dentry *den=
try, struct page *page,
 	data->arg.symlink.pages =3D &page;
 	data->arg.symlink.pathlen =3D len;
 	data->arg.symlink.sattr =3D sattr;
+	data->msg.rpc_xprt_hint =3D nfs_fh_hash(data->arg.symlink.fromfh);
=20
 	d_alias =3D nfs3_do_create(dir, dentry, data);
 	status =3D PTR_ERR_OR_ZERO(d_alias);
@@ -602,6 +611,7 @@ nfs3_proc_mkdir(struct inode *dir, struct dentry *dentr=
y, struct iattr *sattr)
 	data->arg.mkdir.name =3D dentry->d_name.name;
 	data->arg.mkdir.len =3D dentry->d_name.len;
 	data->arg.mkdir.sattr =3D sattr;
+	data->msg.rpc_xprt_hint =3D nfs_fh_hash(data->arg.mkdir.fh);
=20
 	d_alias =3D nfs3_do_create(dir, dentry, data);
 	status =3D PTR_ERR_OR_ZERO(d_alias);
@@ -636,6 +646,7 @@ nfs3_proc_rmdir(struct inode *dir, const struct qstr *n=
ame)
 	struct rpc_message msg =3D {
 		.rpc_proc	=3D &nfs3_procedures[NFS3PROC_RMDIR],
 		.rpc_argp	=3D &arg,
+		.rpc_xprt_hint	=3D nfs_fh_hash(arg.fh),
 	};
 	int status =3D -ENOMEM;
=20
@@ -682,6 +693,7 @@ static int nfs3_proc_readdir(struct nfs_readdir_arg *nr=
_arg,
 		.rpc_argp	=3D &arg,
 		.rpc_resp	=3D &res,
 		.rpc_cred	=3D nr_arg->cred,
+		.rpc_xprt_hint	=3D nfs_fh_hash(arg.fh),
 	};
 	int status =3D -ENOMEM;
=20
@@ -735,6 +747,7 @@ nfs3_proc_mknod(struct inode *dir, struct dentry *dentr=
y, struct iattr *sattr,
 	data->arg.mknod.len =3D dentry->d_name.len;
 	data->arg.mknod.sattr =3D sattr;
 	data->arg.mknod.rdev =3D rdev;
+	data->msg.rpc_xprt_hint =3D nfs_fh_hash(data->arg.mknod.fh);
=20
 	switch (sattr->ia_mode & S_IFMT) {
 	case S_IFBLK:
@@ -782,6 +795,7 @@ nfs3_proc_statfs(struct nfs_server *server, struct nfs_=
fh *fhandle,
 		.rpc_proc	=3D &nfs3_procedures[NFS3PROC_FSSTAT],
 		.rpc_argp	=3D fhandle,
 		.rpc_resp	=3D stat,
+		.rpc_xprt_hint	=3D nfs_fh_hash(fhandle),
 	};
 	int	status;
=20
@@ -800,6 +814,7 @@ do_proc_fsinfo(struct rpc_clnt *client, struct nfs_fh *=
fhandle,
 		.rpc_proc	=3D &nfs3_procedures[NFS3PROC_FSINFO],
 		.rpc_argp	=3D fhandle,
 		.rpc_resp	=3D info,
+		.rpc_xprt_hint	=3D nfs_fh_hash(fhandle),
 	};
 	int	status;
=20
@@ -834,6 +849,7 @@ nfs3_proc_pathconf(struct nfs_server *server, struct nf=
s_fh *fhandle,
 		.rpc_proc	=3D &nfs3_procedures[NFS3PROC_PATHCONF],
 		.rpc_argp	=3D fhandle,
 		.rpc_resp	=3D info,
+		.rpc_xprt_hint	=3D nfs_fh_hash(fhandle),
 	};
 	int	status;
=20
diff --git a/fs/nfs/pagelist.c b/fs/nfs/pagelist.c
index 78c9c4bde..60578e9fd 100644
--- a/fs/nfs/pagelist.c
+++ b/fs/nfs/pagelist.c
@@ -762,6 +762,7 @@ int nfs_initiate_pgio(struct rpc_clnt *clnt, struct nfs=
_pgio_header *hdr,
 		.rpc_argp =3D &hdr->args,
 		.rpc_resp =3D &hdr->res,
 		.rpc_cred =3D cred,
+		.rpc_xprt_hint =3D nfs_fh_hash(hdr->args.fh),
 	};
 	struct rpc_task_setup task_setup_data =3D {
 		.rpc_client =3D clnt,
diff --git a/fs/nfs/unlink.c b/fs/nfs/unlink.c
index b27ebdcce..b1713d6fb 100644
--- a/fs/nfs/unlink.c
+++ b/fs/nfs/unlink.c
@@ -355,6 +355,12 @@ nfs_async_rename(struct inode *old_dir, struct inode *=
new_dir,
 	msg.rpc_resp =3D &data->res;
 	msg.rpc_cred =3D data->cred;
=20
+	if (data->args.new_dir)
+		msg.rpc_xprt_hint =3D nfs_fh_hash(data->args.new_dir);
+	else
+		msg.rpc_xprt_hint =3D nfs_fh_hash(data->args.old_dir);
+
+
 	/* set up nfs_renamedata */
 	data->old_dir =3D old_dir;
 	ihold(old_dir);
diff --git a/fs/nfs/write.c b/fs/nfs/write.c
index 639c34fec..284b364a6 100644
--- a/fs/nfs/write.c
+++ b/fs/nfs/write.c
@@ -1687,6 +1687,7 @@ int nfs_initiate_commit(struct rpc_clnt *clnt, struct=
 nfs_commit_data *data,
 		.rpc_argp =3D &data->args,
 		.rpc_resp =3D &data->res,
 		.rpc_cred =3D data->cred,
+		.rpc_xprt_hint =3D nfs_fh_hash(data->args.fh),
 	};
 	struct rpc_task_setup task_setup_data =3D {
 		.task =3D &data->task,
diff --git a/include/linux/nfs.h b/include/linux/nfs.h
index 0dc7ad38a..5d5b2d20b 100644
--- a/include/linux/nfs.h
+++ b/include/linux/nfs.h
@@ -10,6 +10,7 @@
=20
 #include <linux/sunrpc/msg_prot.h>
 #include <linux/string.h>
+#include <linux/jhash.h>
 #include <uapi/linux/nfs.h>
=20
 /*
@@ -36,6 +37,10 @@ static inline void nfs_copy_fh(struct nfs_fh *target, co=
nst struct nfs_fh *sourc
 	memcpy(target->data, source->data, source->size);
 }
=20
+static inline u32 nfs_fh_hash(const struct nfs_fh *fh)
+{
+	return (fh ? jhash(fh->data, fh->size, 0) : 0);
+}
=20
 /*
  * This is really a general kernel constant, but since nothing like
diff --git a/include/linux/sunrpc/sched.h b/include/linux/sunrpc/sched.h
index df696efdd..8f365280c 100644
--- a/include/linux/sunrpc/sched.h
+++ b/include/linux/sunrpc/sched.h
@@ -27,6 +27,7 @@ struct rpc_message {
 	void *			rpc_argp;	/* Arguments */
 	void *			rpc_resp;	/* Result */
 	const struct cred *	rpc_cred;	/* Credentials */
+	u32			rpc_xprt_hint;
 };
=20
 struct rpc_call_ops;
diff --git a/net/sunrpc/clnt.c b/net/sunrpc/clnt.c
index 612f0a641..fcf8e7962 100644
--- a/net/sunrpc/clnt.c
+++ b/net/sunrpc/clnt.c
@@ -1058,6 +1058,53 @@ rpc_task_get_next_xprt(struct rpc_clnt *clnt)
 	return rpc_task_get_xprt(clnt, xprt_iter_get_next(&clnt->cl_xpi));
 }
=20
+static
+bool xprt_is_active(const struct rpc_xprt *xprt)
+{
+	return kref_read(&xprt->kref) !=3D 0;
+}
+
+static struct rpc_xprt *
+rpc_task_get_hashed_xprt(struct rpc_clnt *clnt, const struct rpc_task *tas=
k)
+{
+	const struct rpc_xprt_switch *xps =3D NULL;
+	struct rpc_xprt *xprt =3D NULL;
+	const struct rpc_message *rpc_message =3D &task->tk_msg;
+	const u32 hash =3D rpc_message->rpc_xprt_hint;
+
+	if (!hash)
+		return rpc_task_get_next_xprt(clnt);
+
+	rcu_read_lock();
+	xps =3D rcu_dereference(clnt->cl_xpi.xpi_xpswitch);
+
+	if (xps && hash) {
+		const struct list_head *head =3D &xps->xps_xprt_list;
+		struct rpc_xprt *pos;
+		const u32 nactive =3D READ_ONCE(xps->xps_nactive);
+		const u32 xprt_idx =3D (hash % nactive);
+		u32 idx =3D 0;
+
+		list_for_each_entry_rcu(pos, head, xprt_switch) {
+			if (xprt_idx > idx++)
+				continue;
+			if (xprt_is_active(pos)) {
+				xprt =3D xprt_get(pos);
+				break;
+			}
+		}
+	}
+
+	/*
+	 * Use first transport, if not found any.
+	 */
+	if (!xprt)
+		xprt =3D xprt_get(rcu_dereference(clnt->cl_xprt));
+	rcu_read_unlock();
+
+	return rpc_task_get_xprt(clnt, xprt);
+}
+
 static
 void rpc_task_set_transport(struct rpc_task *task, struct rpc_clnt *clnt)
 {
@@ -1066,7 +1113,7 @@ void rpc_task_set_transport(struct rpc_task *task, st=
ruct rpc_clnt *clnt)
 	if (task->tk_flags & RPC_TASK_NO_ROUND_ROBIN)
 		task->tk_xprt =3D rpc_task_get_first_xprt(clnt);
 	else
-		task->tk_xprt =3D rpc_task_get_next_xprt(clnt);
+		task->tk_xprt =3D rpc_task_get_hashed_xprt(clnt, task);
 }
=20
 static
@@ -1100,6 +1147,7 @@ rpc_task_set_rpc_message(struct rpc_task *task, const=
 struct rpc_message *msg)
 		task->tk_msg.rpc_argp =3D msg->rpc_argp;
 		task->tk_msg.rpc_resp =3D msg->rpc_resp;
 		task->tk_msg.rpc_cred =3D msg->rpc_cred;
+		task->tk_msg.rpc_xprt_hint =3D msg->rpc_xprt_hint;
 		if (!(task->tk_flags & RPC_TASK_CRED_NOREF))
 			get_cred(task->tk_msg.rpc_cred);
 	}
@@ -1130,8 +1178,8 @@ struct rpc_task *rpc_run_task(const struct rpc_task_s=
etup *task_setup_data)
 	if (!RPC_IS_ASYNC(task))
 		task->tk_flags |=3D RPC_TASK_CRED_NOREF;
=20
-	rpc_task_set_client(task, task_setup_data->rpc_client);
 	rpc_task_set_rpc_message(task, task_setup_data->rpc_message);
+	rpc_task_set_client(task, task_setup_data->rpc_client);
=20
 	if (task->tk_action =3D=3D NULL)
 		rpc_call_start(task);