sunrpc: introduce the concept of a minimum number of threads per pool

Add a new pool->sp_nrthrmin field to track the minimum number of threads
in a pool. Add min_threads parameters to both svc_set_num_threads() and
svc_set_pool_threads(). If min_threads is non-zero and less than the
max, svc_set_num_threads() will ensure that the number of running
threads is between the min and the max.

If the min is 0 or greater than the max, then it is ignored, and the
maximum number of threads will be started, and never spun down.

For now, the min_threads is always 0, but a later patch will pass the
proper value through from nfsd.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/lockd/svc.c

@@ -340,7 +340,7 @@ static int lockd_get(void)
 		return -ENOMEM;
 	}
 
-	error = svc_set_num_threads(serv, 1);
+	error = svc_set_num_threads(serv, 0, 1);
 	if (error < 0) {
 		svc_destroy(&serv);
 		return error;
@@ -368,7 +368,7 @@ static void lockd_put(void)
 	unregister_inet6addr_notifier(&lockd_inet6addr_notifier);
 #endif
 
-	svc_set_num_threads(nlmsvc_serv, 0);
+	svc_set_num_threads(nlmsvc_serv, 0, 0);
 	timer_delete_sync(&nlmsvc_retry);
 	svc_destroy(&nlmsvc_serv);
 	dprintk("lockd_down: service destroyed\n");

fs/nfs/callback.c

@@ -119,9 +119,9 @@ static int nfs_callback_start_svc(int minorversion, struct rpc_xprt *xprt,
 	if (serv->sv_nrthreads == nrservs)
 		return 0;
 
-	ret = svc_set_num_threads(serv, nrservs);
+	ret = svc_set_num_threads(serv, 0, nrservs);
 	if (ret) {
-		svc_set_num_threads(serv, 0);
+		svc_set_num_threads(serv, 0, 0);
 		return ret;
 	}
 	dprintk("nfs_callback_up: service started\n");
@@ -242,7 +242,7 @@ int nfs_callback_up(u32 minorversion, struct rpc_xprt *xprt)
 	cb_info->users++;
 err_net:
 	if (!cb_info->users) {
-		svc_set_num_threads(cb_info->serv, 0);
+		svc_set_num_threads(cb_info->serv, 0, 0);
 		svc_destroy(&cb_info->serv);
 	}
 err_create:
@@ -268,7 +268,7 @@ void nfs_callback_down(int minorversion, struct net *net, struct rpc_xprt *xprt)
 	nfs_callback_down_net(minorversion, serv, net);
 	cb_info->users--;
 	if (cb_info->users == 0) {
-		svc_set_num_threads(serv, 0);
+		svc_set_num_threads(serv, 0, 0);
 		dprintk("nfs_callback_down: service destroyed\n");
 		xprt_svc_destroy_nullify_bc(xprt, &cb_info->serv);
 	}

fs/nfsd/nfssvc.c

@@ -580,7 +580,7 @@ void nfsd_shutdown_threads(struct net *net)
 	}
 
 	/* Kill outstanding nfsd threads */
-	svc_set_num_threads(serv, 0);
+	svc_set_num_threads(serv, 0, 0);
 	nfsd_destroy_serv(net);
 	mutex_unlock(&nfsd_mutex);
 }
@@ -690,7 +690,7 @@ int nfsd_set_nrthreads(int n, int *nthreads, struct net *net)
 
 	/* Special case: When n == 1, distribute threads equally among pools. */
 	if (n == 1)
-		return svc_set_num_threads(nn->nfsd_serv, nthreads[0]);
+		return svc_set_num_threads(nn->nfsd_serv, 0, nthreads[0]);
 
 	if (n > nn->nfsd_serv->sv_nrpools)
 		n = nn->nfsd_serv->sv_nrpools;
@@ -718,7 +718,7 @@ int nfsd_set_nrthreads(int n, int *nthreads, struct net *net)
 	for (i = 0; i < n; i++) {
 		err = svc_set_pool_threads(nn->nfsd_serv,
 					   &nn->nfsd_serv->sv_pools[i],
-					   nthreads[i]);
+					   0, nthreads[i]);
 		if (err)
 			goto out;
 	}
@@ -727,7 +727,7 @@ int nfsd_set_nrthreads(int n, int *nthreads, struct net *net)
 	for (i = n; i < nn->nfsd_serv->sv_nrpools; ++i) {
 		err = svc_set_pool_threads(nn->nfsd_serv,
 					   &nn->nfsd_serv->sv_pools[i],
-					   0);
+					   0, 0);
 		if (err)
 			goto out;
 	}

include/linux/sunrpc/svc.h

@@ -36,6 +36,7 @@
 struct svc_pool {
 	unsigned int		sp_id;		/* pool id; also node id on NUMA */
 	unsigned int		sp_nrthreads;	/* # of threads currently running in pool */
+	unsigned int		sp_nrthrmin;	/* Min number of threads to run per pool */
 	unsigned int		sp_nrthrmax;	/* Max requested number of threads in pool */
 	struct lwq		sp_xprts;	/* pending transports */
 	struct list_head	sp_all_threads;	/* all server threads */
@@ -72,7 +73,7 @@ struct svc_serv {
 	struct svc_stat *	sv_stats;	/* RPC statistics */
 	spinlock_t		sv_lock;
 	unsigned int		sv_nprogs;	/* Number of sv_programs */
-	unsigned int		sv_nrthreads;	/* # of server threads */
+	unsigned int		sv_nrthreads;	/* # of running server threads */
 	unsigned int		sv_max_payload;	/* datagram payload size */
 	unsigned int		sv_max_mesg;	/* max_payload + 1 page for overheads */
 	unsigned int		sv_xdrsize;	/* XDR buffer size */
@@ -448,8 +449,9 @@ struct svc_serv *  svc_create_pooled(struct svc_program *prog,
 				     unsigned int bufsize,
 				     int (*threadfn)(void *data));
 int		   svc_set_pool_threads(struct svc_serv *serv, struct svc_pool *pool,
-					unsigned int nrservs);
-int		   svc_set_num_threads(struct svc_serv *serv, unsigned int nrservs);
+					unsigned int min_threads, unsigned int max_threads);
+int		   svc_set_num_threads(struct svc_serv *serv, unsigned int min_threads,
+				       unsigned int nrservs);
 int		   svc_pool_stats_open(struct svc_info *si, struct file *file);
 void		   svc_process(struct svc_rqst *rqstp);
 void		   svc_process_bc(struct rpc_rqst *req, struct svc_rqst *rqstp);

net/sunrpc/svc.c

@@ -820,9 +820,14 @@ svc_stop_kthreads(struct svc_serv *serv, struct svc_pool *pool, int nrservs)
  * svc_set_pool_threads - adjust number of threads per pool
  * @serv: RPC service to adjust
  * @pool: Specific pool from which to choose threads
- * @nrservs: New number of threads for @serv (0 means kill all threads)
+ * @min_threads: min number of threads to run in @pool
+ * @max_threads: max number of threads in @pool (0 means kill all threads)
  *
- * Create or destroy threads in @pool to bring it to @nrservs.
+ * Create or destroy threads in @pool to bring it into an acceptable range
+ * between @min_threads and @max_threads.
+ *
+ * If @min_threads is 0 or larger than @max_threads, then it is ignored and
+ * the pool will be set to run a static @max_threads number of threads.
  *
  * Caller must ensure mutual exclusion between this and server startup or
  * shutdown.
@@ -832,16 +837,36 @@ svc_stop_kthreads(struct svc_serv *serv, struct svc_pool *pool, int nrservs)
  */
 int
 svc_set_pool_threads(struct svc_serv *serv, struct svc_pool *pool,
-		     unsigned int nrservs)
+		     unsigned int min_threads, unsigned int max_threads)
 {
-	int delta = nrservs;
+	int delta;
 
 	if (!pool)
 		return -EINVAL;
 
-	pool->sp_nrthrmax = nrservs;
-	delta -= pool->sp_nrthreads;
+	/* clamp min threads to the max */
+	if (min_threads > max_threads)
+		min_threads = max_threads;
 
+	pool->sp_nrthrmin = min_threads;
+	pool->sp_nrthrmax = max_threads;
+
+	/*
+	 * When min_threads is set, then only change the number of
+	 * threads to bring it within an acceptable range.
+	 */
+	if (min_threads) {
+		if (pool->sp_nrthreads > max_threads)
+			delta = max_threads;
+		else if (pool->sp_nrthreads < min_threads)
+			delta = min_threads;
+		else
+			return 0;
+	} else {
+		delta = max_threads;
+	}
+
+	delta -= pool->sp_nrthreads;
 	if (delta > 0)
 		return svc_start_kthreads(serv, pool, delta);
 	if (delta < 0)
@@ -853,6 +878,7 @@ EXPORT_SYMBOL_GPL(svc_set_pool_threads);
 /**
  * svc_set_num_threads - adjust number of threads in serv
  * @serv: RPC service to adjust
+ * @min_threads: min number of threads to run per pool
  * @nrservs: New number of threads for @serv (0 means kill all threads)
  *
  * Create or destroy threads in @serv to bring it to @nrservs. If there
@@ -867,20 +893,23 @@ EXPORT_SYMBOL_GPL(svc_set_pool_threads);
  * adjusted; the caller is responsible for recovery.
  */
 int
-svc_set_num_threads(struct svc_serv *serv, unsigned int nrservs)
+svc_set_num_threads(struct svc_serv *serv, unsigned int min_threads,
+		    unsigned int nrservs)
 {
 	unsigned int base = nrservs / serv->sv_nrpools;
 	unsigned int remain = nrservs % serv->sv_nrpools;
 	int i, err = 0;
 
 	for (i = 0; i < serv->sv_nrpools; ++i) {
+		struct svc_pool *pool = &serv->sv_pools[i];
 		int threads = base;
 
 		if (remain) {
 			++threads;
 			--remain;
 		}
-		err = svc_set_pool_threads(serv, &serv->sv_pools[i], threads);
+
+		err = svc_set_pool_threads(serv, pool, min_threads, threads);
 		if (err)
 			break;
 	}