[PATCH v2] sunrpc: hardcode pool_mode to pernode, remove other modes

[PATCH v2] sunrpc: hardcode pool_mode to pernode, remove other modes

[Jeff Layton]

The SVC_POOL_AUTO/GLOBAL/PERCPU/PERNODE pool mode selection machinery
was added when NUMA was new and the right default was unclear.  Today,
pernode is the right choice everywhere:

- On multi-NUMA hosts, it gives one pool per node with proper thread
  affinity and NUMA-local memory allocation.
- On single-node hosts, pernode degenerates to exactly one pool,
  identical to the old "global" mode -- svc_pool_for_cpu() short-
  circuits when sv_nrpools <= 1, no CPU affinity is set, and memory
  is allocated from the single node.

The percpu mode (one pool per CPU) created excessive pools relative to
the number of threads most deployments run, and was only auto-selected
in a narrow case (single node, >2 CPUs).

Remove the SVC_POOL_* enum, mode selection heuristic,
svc_pool_map_init_percpu(), and all mode-based switch statements.
Simplify pool map functions to always use the pernode path.  If pool
map allocation fails, svc_pool_map_get() now returns 0 and service
creation fails, rather than silently falling back to a single global
pool.

The module parameter and netlink interfaces are preserved for backward
compatibility:
- Writing any previously-accepted value succeeds silently
- Reading always returns "pernode"
- Writing to the module parameter emits a deprecation notice

Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
This version is essentially the same as v1, but allows the kernel to
accept any previously-accepted setting for pool-mode, which should
alleviate concerns about breakage.
---
Changes in v2:
- Accept any previously-accepted setting for pool_mode
- Link to v1: https://lore.kernel.org/r/20260423-sunrpc-pool-mode-v1-1-b7f20e35749b@kernel.org
---
 net/sunrpc/svc.c | 238 +++++++++----------------------------------------------
 1 file changed, 37 insertions(+), 201 deletions(-)

diff --git a/net/sunrpc/svc.c b/net/sunrpc/svc.c
index dd80a2eaaa74..6e3d509bf95a 100644
--- a/net/sunrpc/svc.c
+++ b/net/sunrpc/svc.c
@@ -38,19 +38,6 @@
 
 static void svc_unregister(const struct svc_serv *serv, struct net *net);
 
-#define SVC_POOL_DEFAULT	SVC_POOL_GLOBAL
-
-/*
- * Mode for mapping cpus to pools.
- */
-enum {
-	SVC_POOL_AUTO = -1,	/* choose one of the others */
-	SVC_POOL_GLOBAL,	/* no mapping, just a single global pool
-				 * (legacy & UP mode) */
-	SVC_POOL_PERCPU,	/* one pool per cpu */
-	SVC_POOL_PERNODE	/* one pool per numa node */
-};
-
 /*
  * Structure for mapping cpus to pools and vice versa.
  * Setup once during sunrpc initialisation.
@@ -58,62 +45,23 @@ enum {
 
 struct svc_pool_map {
 	int count;			/* How many svc_servs use us */
-	int mode;			/* Note: int not enum to avoid
-					 * warnings about "enumeration value
-					 * not handled in switch" */
 	unsigned int npools;
-	unsigned int *pool_to;		/* maps pool id to cpu or node */
-	unsigned int *to_pool;		/* maps cpu or node to pool id */
+	unsigned int *pool_to;		/* maps pool id to node */
+	unsigned int *to_pool;		/* maps node to pool id */
 };
 
-static struct svc_pool_map svc_pool_map = {
-	.mode = SVC_POOL_DEFAULT
-};
+static struct svc_pool_map svc_pool_map;
 
 static DEFINE_MUTEX(svc_pool_map_mutex);/* protects svc_pool_map.count only */
 
-static int
-__param_set_pool_mode(const char *val, struct svc_pool_map *m)
-{
-	int err, mode;
-
-	mutex_lock(&svc_pool_map_mutex);
-
-	err = 0;
-	if (!strncmp(val, "auto", 4))
-		mode = SVC_POOL_AUTO;
-	else if (!strncmp(val, "global", 6))
-		mode = SVC_POOL_GLOBAL;
-	else if (!strncmp(val, "percpu", 6))
-		mode = SVC_POOL_PERCPU;
-	else if (!strncmp(val, "pernode", 7))
-		mode = SVC_POOL_PERNODE;
-	else
-		err = -EINVAL;
-
-	if (err)
-		goto out;
-
-	if (m->count == 0)
-		m->mode = mode;
-	else if (mode != m->mode)
-		err = -EBUSY;
-out:
-	mutex_unlock(&svc_pool_map_mutex);
-	return err;
-}
-
-static int
-param_set_pool_mode(const char *val, const struct kernel_param *kp)
-{
-	struct svc_pool_map *m = kp->arg;
-
-	return __param_set_pool_mode(val, m);
-}
-
 int sunrpc_set_pool_mode(const char *val)
 {
-	return __param_set_pool_mode(val, &svc_pool_map);
+	if (!strncmp(val, "auto", 4) ||
+	    !strncmp(val, "global", 6) ||
+	    !strncmp(val, "percpu", 6) ||
+	    !strncmp(val, "pernode", 7))
+		return 0;
+	return -EINVAL;
 }
 EXPORT_SYMBOL(sunrpc_set_pool_mode);
 
@@ -122,84 +70,32 @@ EXPORT_SYMBOL(sunrpc_set_pool_mode);
  * @buf: where to write the current pool_mode
  * @size: size of @buf
  *
- * Grab the current pool_mode from the svc_pool_map and write
- * the resulting string to @buf. Returns the number of characters
+ * Write the pool_mode string to @buf. Returns the number of characters
  * written to @buf (a'la snprintf()).
  */
 int
 sunrpc_get_pool_mode(char *buf, size_t size)
 {
-	struct svc_pool_map *m = &svc_pool_map;
-
-	switch (m->mode)
-	{
-	case SVC_POOL_AUTO:
-		return snprintf(buf, size, "auto");
-	case SVC_POOL_GLOBAL:
-		return snprintf(buf, size, "global");
-	case SVC_POOL_PERCPU:
-		return snprintf(buf, size, "percpu");
-	case SVC_POOL_PERNODE:
-		return snprintf(buf, size, "pernode");
-	default:
-		return snprintf(buf, size, "%d", m->mode);
-	}
+	return snprintf(buf, size, "pernode");
 }
 EXPORT_SYMBOL(sunrpc_get_pool_mode);
 
 static int
-param_get_pool_mode(char *buf, const struct kernel_param *kp)
+param_set_pool_mode(const char *val, const struct kernel_param *kp)
 {
-	char str[16];
-	int len;
-
-	len = sunrpc_get_pool_mode(str, ARRAY_SIZE(str));
-
-	/* Ensure we have room for newline and NUL */
-	len = min_t(int, len, ARRAY_SIZE(str) - 2);
-
-	/* tack on the newline */
-	str[len] = '\n';
-	str[len + 1] = '\0';
-
-	return sysfs_emit(buf, "%s", str);
+	pr_notice("sunrpc: the pool_mode parameter is deprecated and will be removed in a future release.\n");
+	return sunrpc_set_pool_mode(val);
 }
 
-module_param_call(pool_mode, param_set_pool_mode, param_get_pool_mode,
-		  &svc_pool_map, 0644);
-
-/*
- * Detect best pool mapping mode heuristically,
- * according to the machine's topology.
- */
 static int
-svc_pool_map_choose_mode(void)
+param_get_pool_mode(char *buf, const struct kernel_param *kp)
 {
-	unsigned int node;
-
-	if (nr_online_nodes > 1) {
-		/*
-		 * Actually have multiple NUMA nodes,
-		 * so split pools on NUMA node boundaries
-		 */
-		return SVC_POOL_PERNODE;
-	}
-
-	node = first_online_node;
-	if (nr_cpus_node(node) > 2) {
-		/*
-		 * Non-trivial SMP, or CONFIG_NUMA on
-		 * non-NUMA hardware, e.g. with a generic
-		 * x86_64 kernel on Xeons.  In this case we
-		 * want to divide the pools on cpu boundaries.
-		 */
-		return SVC_POOL_PERCPU;
-	}
-
-	/* default: one global pool */
-	return SVC_POOL_GLOBAL;
+	return sysfs_emit(buf, "pernode\n");
 }
 
+module_param_call(pool_mode, param_set_pool_mode, param_get_pool_mode,
+		  NULL, 0644);
+
 /*
  * Allocate the to_pool[] and pool_to[] arrays.
  * Returns 0 on success or an errno.
@@ -224,35 +120,7 @@ svc_pool_map_alloc_arrays(struct svc_pool_map *m, unsigned int maxpools)
 }
 
 /*
- * Initialise the pool map for SVC_POOL_PERCPU mode.
- * Returns number of pools or <0 on error.
- */
-static int
-svc_pool_map_init_percpu(struct svc_pool_map *m)
-{
-	unsigned int maxpools = nr_cpu_ids;
-	unsigned int pidx = 0;
-	unsigned int cpu;
-	int err;
-
-	err = svc_pool_map_alloc_arrays(m, maxpools);
-	if (err)
-		return err;
-
-	for_each_online_cpu(cpu) {
-		BUG_ON(pidx >= maxpools);
-		m->to_pool[cpu] = pidx;
-		m->pool_to[pidx] = cpu;
-		pidx++;
-	}
-	/* cpus brought online later all get mapped to pool0, sorry */
-
-	return pidx;
-};
-
-
-/*
- * Initialise the pool map for SVC_POOL_PERNODE mode.
+ * Initialise the pool map for one pool per NUMA node.
  * Returns number of pools or <0 on error.
  */
 static int
@@ -284,14 +152,13 @@ svc_pool_map_init_pernode(struct svc_pool_map *m)
  * Add a reference to the global map of cpus to pools (and
  * vice versa) if pools are in use.
  * Initialise the map if we're the first user.
- * Returns the number of pools. If this is '1', no reference
- * was taken.
+ * Returns the number of pools, or 0 on failure.
  */
 static unsigned int
 svc_pool_map_get(void)
 {
 	struct svc_pool_map *m = &svc_pool_map;
-	int npools = -1;
+	int npools;
 
 	mutex_lock(&svc_pool_map_mutex);
 	if (m->count++) {
@@ -299,22 +166,11 @@ svc_pool_map_get(void)
 		return m->npools;
 	}
 
-	if (m->mode == SVC_POOL_AUTO)
-		m->mode = svc_pool_map_choose_mode();
-
-	switch (m->mode) {
-	case SVC_POOL_PERCPU:
-		npools = svc_pool_map_init_percpu(m);
-		break;
-	case SVC_POOL_PERNODE:
-		npools = svc_pool_map_init_pernode(m);
-		break;
-	}
-
+	npools = svc_pool_map_init_pernode(m);
 	if (npools <= 0) {
-		/* default, or memory allocation failure */
-		npools = 1;
-		m->mode = SVC_POOL_GLOBAL;
+		m->count--;
+		mutex_unlock(&svc_pool_map_mutex);
+		return 0;
 	}
 	m->npools = npools;
 	mutex_unlock(&svc_pool_map_mutex);
@@ -346,14 +202,11 @@ static int svc_pool_map_get_node(unsigned int pidx)
 {
 	const struct svc_pool_map *m = &svc_pool_map;
 
-	if (m->count) {
-		if (m->mode == SVC_POOL_PERCPU)
-			return cpu_to_node(m->pool_to[pidx]);
-		if (m->mode == SVC_POOL_PERNODE)
-			return m->pool_to[pidx];
-	}
+	if (m->count)
+		return m->pool_to[pidx];
 	return numa_mem_id();
 }
+
 /*
  * Set the given thread's cpus_allowed mask so that it
  * will only run on cpus in the given pool.
@@ -372,27 +225,15 @@ svc_pool_map_set_cpumask(struct task_struct *task, unsigned int pidx)
 	if (m->count == 0)
 		return;
 
-	switch (m->mode) {
-	case SVC_POOL_PERCPU:
-	{
-		set_cpus_allowed_ptr(task, cpumask_of(node));
-		break;
-	}
-	case SVC_POOL_PERNODE:
-	{
-		set_cpus_allowed_ptr(task, cpumask_of_node(node));
-		break;
-	}
-	}
+	set_cpus_allowed_ptr(task, cpumask_of_node(node));
 }
 
 /**
  * svc_pool_for_cpu - Select pool to run a thread on this cpu
  * @serv: An RPC service
  *
- * Use the active CPU and the svc_pool_map's mode setting to
- * select the svc thread pool to use. Once initialized, the
- * svc_pool_map does not change.
+ * Use the active CPU and the svc_pool_map to select the svc thread
+ * pool to use. Once initialized, the svc_pool_map does not change.
  *
  * Return value:
  *   A pointer to an svc_pool
@@ -400,20 +241,12 @@ svc_pool_map_set_cpumask(struct task_struct *task, unsigned int pidx)
 struct svc_pool *svc_pool_for_cpu(struct svc_serv *serv)
 {
 	struct svc_pool_map *m = &svc_pool_map;
-	int cpu = raw_smp_processor_id();
-	unsigned int pidx = 0;
+	unsigned int pidx;
 
 	if (serv->sv_nrpools <= 1)
 		return serv->sv_pools;
 
-	switch (m->mode) {
-	case SVC_POOL_PERCPU:
-		pidx = m->to_pool[cpu];
-		break;
-	case SVC_POOL_PERNODE:
-		pidx = m->to_pool[cpu_to_node(cpu)];
-		break;
-	}
+	pidx = m->to_pool[cpu_to_node(raw_smp_processor_id())];
 
 	return &serv->sv_pools[pidx % serv->sv_nrpools];
 }
@@ -617,6 +450,9 @@ struct svc_serv *svc_create_pooled(struct svc_program *prog,
 	struct svc_serv *serv;
 	unsigned int npools = svc_pool_map_get();
 
+	if (!npools)
+		return NULL;
+
 	serv = __svc_create(prog, nprogs, stats, bufsize, npools, threadfn);
 	if (!serv)
 		goto out_err;

---
base-commit: 2bb83225da8ee0383d17783b5c903589696faf90
change-id: 20260423-sunrpc-pool-mode-3e6b56320dc4

Best regards,
-- 
Jeff Layton <jlayton@kernel.org>

Re: [PATCH v2] sunrpc: hardcode pool_mode to pernode, remove other modes

[Chuck Lever]

On Thu, Jun 25, 2026, at 11:59 AM, Jeff Layton wrote:
> The SVC_POOL_AUTO/GLOBAL/PERCPU/PERNODE pool mode selection machinery
> was added when NUMA was new and the right default was unclear.  Today,
> pernode is the right choice everywhere:

> diff --git a/net/sunrpc/svc.c b/net/sunrpc/svc.c
> index dd80a2eaaa74..6e3d509bf95a 100644
> --- a/net/sunrpc/svc.c
> +++ b/net/sunrpc/svc.c
> @@ -38,19 +38,6 @@
> 
>  static void svc_unregister(const struct svc_serv *serv, struct net *net);
> 
> -#define SVC_POOL_DEFAULT	SVC_POOL_GLOBAL
> -
> -/*
> - * Mode for mapping cpus to pools.
> - */
> -enum {
> -	SVC_POOL_AUTO = -1,	/* choose one of the others */
> -	SVC_POOL_GLOBAL,	/* no mapping, just a single global pool
> -				 * (legacy & UP mode) */
> -	SVC_POOL_PERCPU,	/* one pool per cpu */
> -	SVC_POOL_PERNODE	/* one pool per numa node */
> -};
> -
>  /*
>   * Structure for mapping cpus to pools and vice versa.
>   * Setup once during sunrpc initialisation.

The commit message should call out the change in the default setting.


> @@ -299,22 +166,11 @@ svc_pool_map_get(void)
>  		return m->npools;
>  	}
>
> -	if (m->mode == SVC_POOL_AUTO)
> -		m->mode = svc_pool_map_choose_mode();
> -
> -	switch (m->mode) {
> -	case SVC_POOL_PERCPU:
> -		npools = svc_pool_map_init_percpu(m);
> -		break;
> -	case SVC_POOL_PERNODE:
> -		npools = svc_pool_map_init_pernode(m);
> -		break;
> -	}
> -
> +	npools = svc_pool_map_init_pernode(m);

With one pool per node now mandatory, can a server end up with pools
that have no threads?  svc_set_num_threads() spreads nrservs evenly
across the pools:

net/sunrpc/svc.c:svc_set_num_threads() {
	unsigned int base = nrservs / serv->sv_nrpools;
	unsigned int remain = nrservs % serv->sv_nrpools;
	...
}

so when nrservs is smaller than the number of NUMA nodes with CPUs,
base is 0 and the trailing pools get zero threads.

Work is still routed to the per-node pool selected from the running
CPU:

net/sunrpc/svc_xprt.c:svc_xprt_enqueue() {
	pool = svc_pool_for_cpu(xprt->xpt_server);
	...
	lwq_enqueue(&xprt->xpt_ready, &pool->sp_xprts);
	svc_pool_wake_idle_thread(pool);
}

and svc_pool_wake_idle_thread() only consults that one pool's idle
list, with no fallback to a sibling pool.  If a connection lands on a
node whose pool has no threads, does its transport sit on sp_xprts and
never get serviced?

The old global default kept a single pool, so this configuration could
not arise without an explicit pernode/percpu selection.  With global
mode removed, is there a way left for an admin to avoid empty pools on
a server that runs fewer threads than it has NUMA nodes, or should the
thread distribution or the enqueue path guarantee work only reaches a
populated pool?

Separately, the pool_mode knob is preserved but every accepted value
now behaves as pernode.  Documentation/admin-guide/kernel-parameters.txt
still presents the four modes as distinct:

	auto	    the server chooses an appropriate mode
		    automatically using heuristics
	global	    a single global pool contains all CPUs
	percpu	    one pool for each CPU
	pernode	    one pool for each NUMA node (equivalent
		    to global on non-NUMA machines)

and states the option "will affect which CPUs will do NFS serving."
The patch should update this documentation.


-- 
Chuck Lever

Re: [PATCH v2] sunrpc: hardcode pool_mode to pernode, remove other modes

[NeilBrown]

On Fri, 26 Jun 2026, Jeff Layton wrote:
> The SVC_POOL_AUTO/GLOBAL/PERCPU/PERNODE pool mode selection machinery
> was added when NUMA was new and the right default was unclear.  Today,
> pernode is the right choice everywhere:
> 
> - On multi-NUMA hosts, it gives one pool per node with proper thread
>   affinity and NUMA-local memory allocation.
> - On single-node hosts, pernode degenerates to exactly one pool,
>   identical to the old "global" mode -- svc_pool_for_cpu() short-
>   circuits when sv_nrpools <= 1, no CPU affinity is set, and memory
>   is allocated from the single node.
> 
> The percpu mode (one pool per CPU) created excessive pools relative to
> the number of threads most deployments run, and was only auto-selected
> in a narrow case (single node, >2 CPUs).
> 
> Remove the SVC_POOL_* enum, mode selection heuristic,
> svc_pool_map_init_percpu(), and all mode-based switch statements.
> Simplify pool map functions to always use the pernode path.  If pool
> map allocation fails, svc_pool_map_get() now returns 0 and service
> creation fails, rather than silently falling back to a single global
> pool.
> 
> The module parameter and netlink interfaces are preserved for backward
> compatibility:
> - Writing any previously-accepted value succeeds silently
> - Reading always returns "pernode"
> - Writing to the module parameter emits a deprecation notice
> 
> Assisted-by: Claude:claude-opus-4-8
> Signed-off-by: Jeff Layton <jlayton@kernel.org>
> ---
> This version is essentially the same as v1, but allows the kernel to
> accept any previously-accepted setting for pool-mode, which should
> alleviate concerns about breakage.
> ---
> Changes in v2:
> - Accept any previously-accepted setting for pool_mode
> - Link to v1: https://lore.kernel.org/r/20260423-sunrpc-pool-mode-v1-1-b7f20e35749b@kernel.org
> ---
>  net/sunrpc/svc.c | 238 +++++++++----------------------------------------------
>  1 file changed, 37 insertions(+), 201 deletions(-)
> 
> diff --git a/net/sunrpc/svc.c b/net/sunrpc/svc.c
> index dd80a2eaaa74..6e3d509bf95a 100644
> --- a/net/sunrpc/svc.c
> +++ b/net/sunrpc/svc.c
> @@ -38,19 +38,6 @@
>  
>  static void svc_unregister(const struct svc_serv *serv, struct net *net);
>  
> -#define SVC_POOL_DEFAULT	SVC_POOL_GLOBAL
> -
> -/*
> - * Mode for mapping cpus to pools.
> - */
> -enum {
> -	SVC_POOL_AUTO = -1,	/* choose one of the others */
> -	SVC_POOL_GLOBAL,	/* no mapping, just a single global pool
> -				 * (legacy & UP mode) */
> -	SVC_POOL_PERCPU,	/* one pool per cpu */
> -	SVC_POOL_PERNODE	/* one pool per numa node */
> -};
> -
>  /*
>   * Structure for mapping cpus to pools and vice versa.
>   * Setup once during sunrpc initialisation.
> @@ -58,62 +45,23 @@ enum {
>  
>  struct svc_pool_map {
>  	int count;			/* How many svc_servs use us */
> -	int mode;			/* Note: int not enum to avoid
> -					 * warnings about "enumeration value
> -					 * not handled in switch" */
>  	unsigned int npools;
> -	unsigned int *pool_to;		/* maps pool id to cpu or node */
> -	unsigned int *to_pool;		/* maps cpu or node to pool id */
> +	unsigned int *pool_to;		/* maps pool id to node */
> +	unsigned int *to_pool;		/* maps node to pool id */
>  };
>  
> -static struct svc_pool_map svc_pool_map = {
> -	.mode = SVC_POOL_DEFAULT
> -};
> +static struct svc_pool_map svc_pool_map;
>  
>  static DEFINE_MUTEX(svc_pool_map_mutex);/* protects svc_pool_map.count only */
>  
> -static int
> -__param_set_pool_mode(const char *val, struct svc_pool_map *m)
> -{
> -	int err, mode;
> -
> -	mutex_lock(&svc_pool_map_mutex);
> -
> -	err = 0;
> -	if (!strncmp(val, "auto", 4))
> -		mode = SVC_POOL_AUTO;
> -	else if (!strncmp(val, "global", 6))
> -		mode = SVC_POOL_GLOBAL;
> -	else if (!strncmp(val, "percpu", 6))
> -		mode = SVC_POOL_PERCPU;
> -	else if (!strncmp(val, "pernode", 7))
> -		mode = SVC_POOL_PERNODE;
> -	else
> -		err = -EINVAL;
> -
> -	if (err)
> -		goto out;
> -
> -	if (m->count == 0)
> -		m->mode = mode;
> -	else if (mode != m->mode)
> -		err = -EBUSY;
> -out:
> -	mutex_unlock(&svc_pool_map_mutex);
> -	return err;
> -}
> -
> -static int
> -param_set_pool_mode(const char *val, const struct kernel_param *kp)
> -{
> -	struct svc_pool_map *m = kp->arg;
> -
> -	return __param_set_pool_mode(val, m);
> -}
> -
>  int sunrpc_set_pool_mode(const char *val)
>  {
> -	return __param_set_pool_mode(val, &svc_pool_map);
> +	if (!strncmp(val, "auto", 4) ||
> +	    !strncmp(val, "global", 6) ||
> +	    !strncmp(val, "percpu", 6) ||
> +	    !strncmp(val, "pernode", 7))
> +		return 0;

"!strncmp" is one most disliked constructs....
What would you think of using match_string() or even
sysfs_match_string() ??

> +	return -EINVAL;
>  }
>  EXPORT_SYMBOL(sunrpc_set_pool_mode);
>  
> @@ -122,84 +70,32 @@ EXPORT_SYMBOL(sunrpc_set_pool_mode);
>   * @buf: where to write the current pool_mode
>   * @size: size of @buf
>   *
> - * Grab the current pool_mode from the svc_pool_map and write
> - * the resulting string to @buf. Returns the number of characters
> + * Write the pool_mode string to @buf. Returns the number of characters
>   * written to @buf (a'la snprintf()).
>   */
>  int
>  sunrpc_get_pool_mode(char *buf, size_t size)
>  {
> -	struct svc_pool_map *m = &svc_pool_map;
> -
> -	switch (m->mode)
> -	{
> -	case SVC_POOL_AUTO:
> -		return snprintf(buf, size, "auto");
> -	case SVC_POOL_GLOBAL:
> -		return snprintf(buf, size, "global");
> -	case SVC_POOL_PERCPU:
> -		return snprintf(buf, size, "percpu");
> -	case SVC_POOL_PERNODE:
> -		return snprintf(buf, size, "pernode");
> -	default:
> -		return snprintf(buf, size, "%d", m->mode);
> -	}
> +	return snprintf(buf, size, "pernode");
>  }
>  EXPORT_SYMBOL(sunrpc_get_pool_mode);
>  
>  static int
> -param_get_pool_mode(char *buf, const struct kernel_param *kp)
> +param_set_pool_mode(const char *val, const struct kernel_param *kp)
>  {
> -	char str[16];
> -	int len;
> -
> -	len = sunrpc_get_pool_mode(str, ARRAY_SIZE(str));
> -
> -	/* Ensure we have room for newline and NUL */
> -	len = min_t(int, len, ARRAY_SIZE(str) - 2);
> -
> -	/* tack on the newline */
> -	str[len] = '\n';
> -	str[len + 1] = '\0';
> -
> -	return sysfs_emit(buf, "%s", str);
> +	pr_notice("sunrpc: the pool_mode parameter is deprecated and will be removed in a future release.\n");

Which future release?  How will we remember?

> +	return sunrpc_set_pool_mode(val);
>  }
>  
> -module_param_call(pool_mode, param_set_pool_mode, param_get_pool_mode,
> -		  &svc_pool_map, 0644);
> -
> -/*
> - * Detect best pool mapping mode heuristically,
> - * according to the machine's topology.
> - */
>  static int
> -svc_pool_map_choose_mode(void)
> +param_get_pool_mode(char *buf, const struct kernel_param *kp)
>  {
> -	unsigned int node;
> -
> -	if (nr_online_nodes > 1) {
> -		/*
> -		 * Actually have multiple NUMA nodes,
> -		 * so split pools on NUMA node boundaries
> -		 */
> -		return SVC_POOL_PERNODE;
> -	}
> -
> -	node = first_online_node;
> -	if (nr_cpus_node(node) > 2) {
> -		/*
> -		 * Non-trivial SMP, or CONFIG_NUMA on
> -		 * non-NUMA hardware, e.g. with a generic
> -		 * x86_64 kernel on Xeons.  In this case we
> -		 * want to divide the pools on cpu boundaries.
> -		 */
> -		return SVC_POOL_PERCPU;
> -	}
> -
> -	/* default: one global pool */
> -	return SVC_POOL_GLOBAL;
> +	return sysfs_emit(buf, "pernode\n");
>  }
>  
> +module_param_call(pool_mode, param_set_pool_mode, param_get_pool_mode,
> +		  NULL, 0644);
> +
>  /*
>   * Allocate the to_pool[] and pool_to[] arrays.
>   * Returns 0 on success or an errno.
> @@ -224,35 +120,7 @@ svc_pool_map_alloc_arrays(struct svc_pool_map *m, unsigned int maxpools)
>  }
>  
>  /*
> - * Initialise the pool map for SVC_POOL_PERCPU mode.
> - * Returns number of pools or <0 on error.
> - */
> -static int
> -svc_pool_map_init_percpu(struct svc_pool_map *m)
> -{
> -	unsigned int maxpools = nr_cpu_ids;
> -	unsigned int pidx = 0;
> -	unsigned int cpu;
> -	int err;
> -
> -	err = svc_pool_map_alloc_arrays(m, maxpools);
> -	if (err)
> -		return err;
> -
> -	for_each_online_cpu(cpu) {
> -		BUG_ON(pidx >= maxpools);
> -		m->to_pool[cpu] = pidx;
> -		m->pool_to[pidx] = cpu;
> -		pidx++;
> -	}
> -	/* cpus brought online later all get mapped to pool0, sorry */
> -
> -	return pidx;
> -};
> -
> -
> -/*
> - * Initialise the pool map for SVC_POOL_PERNODE mode.
> + * Initialise the pool map for one pool per NUMA node.
>   * Returns number of pools or <0 on error.
>   */
>  static int
> @@ -284,14 +152,13 @@ svc_pool_map_init_pernode(struct svc_pool_map *m)
>   * Add a reference to the global map of cpus to pools (and
>   * vice versa) if pools are in use.
>   * Initialise the map if we're the first user.
> - * Returns the number of pools. If this is '1', no reference
> - * was taken.
> + * Returns the number of pools, or 0 on failure.
>   */
>  static unsigned int
>  svc_pool_map_get(void)
>  {
>  	struct svc_pool_map *m = &svc_pool_map;
> -	int npools = -1;
> +	int npools;
>  
>  	mutex_lock(&svc_pool_map_mutex);
>  	if (m->count++) {
> @@ -299,22 +166,11 @@ svc_pool_map_get(void)
>  		return m->npools;
>  	}
>  
> -	if (m->mode == SVC_POOL_AUTO)
> -		m->mode = svc_pool_map_choose_mode();
> -
> -	switch (m->mode) {
> -	case SVC_POOL_PERCPU:
> -		npools = svc_pool_map_init_percpu(m);
> -		break;
> -	case SVC_POOL_PERNODE:
> -		npools = svc_pool_map_init_pernode(m);
> -		break;
> -	}
> -
> +	npools = svc_pool_map_init_pernode(m);
>  	if (npools <= 0) {
> -		/* default, or memory allocation failure */
> -		npools = 1;
> -		m->mode = SVC_POOL_GLOBAL;
> +		m->count--;

Could we make this
   m->count = 0

to make it crystal clear that there are now no references?
It would be nice to rename ->count to ->refcount or similar
so it is obvious what is being counted, but that doesn't need to be part
of this change.

> +		mutex_unlock(&svc_pool_map_mutex);
> +		return 0;
>  	}
>  	m->npools = npools;
>  	mutex_unlock(&svc_pool_map_mutex);
> @@ -346,14 +202,11 @@ static int svc_pool_map_get_node(unsigned int pidx)
>  {
>  	const struct svc_pool_map *m = &svc_pool_map;
>  
> -	if (m->count) {
> -		if (m->mode == SVC_POOL_PERCPU)
> -			return cpu_to_node(m->pool_to[pidx]);
> -		if (m->mode == SVC_POOL_PERNODE)
> -			return m->pool_to[pidx];
> -	}
> +	if (m->count)
> +		return m->pool_to[pidx];
>  	return numa_mem_id();

Can this code run if m->count is zero?

>  }
> +
>  /*
>   * Set the given thread's cpus_allowed mask so that it
>   * will only run on cpus in the given pool.
> @@ -372,27 +225,15 @@ svc_pool_map_set_cpumask(struct task_struct *task, unsigned int pidx)
>  	if (m->count == 0)
>  		return;
>  
> -	switch (m->mode) {
> -	case SVC_POOL_PERCPU:
> -	{
> -		set_cpus_allowed_ptr(task, cpumask_of(node));
> -		break;
> -	}
> -	case SVC_POOL_PERNODE:
> -	{
> -		set_cpus_allowed_ptr(task, cpumask_of_node(node));
> -		break;
> -	}
> -	}
> +	set_cpus_allowed_ptr(task, cpumask_of_node(node));
>  }
>  
>  /**
>   * svc_pool_for_cpu - Select pool to run a thread on this cpu
>   * @serv: An RPC service
>   *
> - * Use the active CPU and the svc_pool_map's mode setting to
> - * select the svc thread pool to use. Once initialized, the
> - * svc_pool_map does not change.
> + * Use the active CPU and the svc_pool_map to select the svc thread
> + * pool to use. Once initialized, the svc_pool_map does not change.
>   *
>   * Return value:
>   *   A pointer to an svc_pool
> @@ -400,20 +241,12 @@ svc_pool_map_set_cpumask(struct task_struct *task, unsigned int pidx)
>  struct svc_pool *svc_pool_for_cpu(struct svc_serv *serv)
>  {
>  	struct svc_pool_map *m = &svc_pool_map;
> -	int cpu = raw_smp_processor_id();
> -	unsigned int pidx = 0;
> +	unsigned int pidx;
>  
>  	if (serv->sv_nrpools <= 1)
>  		return serv->sv_pools;
>  
> -	switch (m->mode) {
> -	case SVC_POOL_PERCPU:
> -		pidx = m->to_pool[cpu];
> -		break;
> -	case SVC_POOL_PERNODE:
> -		pidx = m->to_pool[cpu_to_node(cpu)];
> -		break;
> -	}
> +	pidx = m->to_pool[cpu_to_node(raw_smp_processor_id())];
>  
>  	return &serv->sv_pools[pidx % serv->sv_nrpools];
>  }
> @@ -617,6 +450,9 @@ struct svc_serv *svc_create_pooled(struct svc_program *prog,
>  	struct svc_serv *serv;
>  	unsigned int npools = svc_pool_map_get();
>  
> +	if (!npools)
> +		return NULL;
> +
>  	serv = __svc_create(prog, nprogs, stats, bufsize, npools, threadfn);
>  	if (!serv)
>  		goto out_err;
> 

I think this is a good a worthwhile change.
Regarding Chucks comments, I think a minimum of one thread per node,
rather than a minimum of one thread total, is the sensible approach.
Of course if there exist memory-only nodes, they wouldn't need a thread.

Thanks,
NeilBrown

Re: [PATCH v2] sunrpc: hardcode pool_mode to pernode, remove other modes

[Chuck Lever]

On Thu, Jun 25, 2026, at 10:14 PM, NeilBrown wrote:
> Regarding Chucks comments, I think a minimum of one thread per node,
> rather than a minimum of one thread total, is the sensible approach.
> Of course if there exist memory-only nodes, they wouldn't need a thread.

What happens when transport work is scheduled on a memory-only
pool? Obviously there's no execution engine there, even if we
do provide one or more threads.


-- 
Chuck Lever

Re: [PATCH v2] sunrpc: hardcode pool_mode to pernode, remove other modes

[Jeff Layton]

On Fri, 2026-06-26 at 09:18 -0400, Chuck Lever wrote:
> On Thu, Jun 25, 2026, at 10:14 PM, NeilBrown wrote:
> > Regarding Chucks comments, I think a minimum of one thread per node,
> > rather than a minimum of one thread total, is the sensible approach.
> > Of course if there exist memory-only nodes, they wouldn't need a thread.
> 
> What happens when transport work is scheduled on a memory-only
> pool? Obviously there's no execution engine there, even if we
> do provide one or more threads.
> 

The hitch here is that we have traditionally allowed userland to
dictate how many threads each pool gets. You're allowed to send down an
array like this with (4 pools):

     0 128 0 0

That would give you exactly that layout (128 threads in pool 2 and none
elsewhere). I think we can continue to honor that by saying that if you
set a thread count of 0 in a pool then we should not spawn any threads
there.

Because of this, I favor a belt and bracers approach:

1/ when min-threads is set, assign at least one thread to each node
that has a cpu as a minimum, but only when we are auto-distributing
threads between nodes (the typical case). This allows us to preserve
the wishes of userland when it sends down a more complex pool array. We
should not override the user's wishes in that case.

2/ add in a fallback in the queueing path so that when there are no
threads in a node, we queue it to a populated one. We could even add a
pr_notice_once() that fires in that case to warn that the configuration
is sub-optimal. The cost is an extra check for sp_nrthreads == 0 when
enqueueing, but it should almost always come back false when things are
properly configured.

-- 
Jeff Layton <jlayton@kernel.org>

Re: [PATCH v2] sunrpc: hardcode pool_mode to pernode, remove other modes

[Jeff Layton]

On Fri, 2026-06-26 at 12:14 +1000, NeilBrown wrote:
> On Fri, 26 Jun 2026, Jeff Layton wrote:
> > The SVC_POOL_AUTO/GLOBAL/PERCPU/PERNODE pool mode selection machinery
> > was added when NUMA was new and the right default was unclear.  Today,
> > pernode is the right choice everywhere:
> > 
> > - On multi-NUMA hosts, it gives one pool per node with proper thread
> >   affinity and NUMA-local memory allocation.
> > - On single-node hosts, pernode degenerates to exactly one pool,
> >   identical to the old "global" mode -- svc_pool_for_cpu() short-
> >   circuits when sv_nrpools <= 1, no CPU affinity is set, and memory
> >   is allocated from the single node.
> > 
> > The percpu mode (one pool per CPU) created excessive pools relative to
> > the number of threads most deployments run, and was only auto-selected
> > in a narrow case (single node, >2 CPUs).
> > 
> > Remove the SVC_POOL_* enum, mode selection heuristic,
> > svc_pool_map_init_percpu(), and all mode-based switch statements.
> > Simplify pool map functions to always use the pernode path.  If pool
> > map allocation fails, svc_pool_map_get() now returns 0 and service
> > creation fails, rather than silently falling back to a single global
> > pool.
> > 
> > The module parameter and netlink interfaces are preserved for backward
> > compatibility:
> > - Writing any previously-accepted value succeeds silently
> > - Reading always returns "pernode"
> > - Writing to the module parameter emits a deprecation notice
> > 
> > Assisted-by: Claude:claude-opus-4-8
> > Signed-off-by: Jeff Layton <jlayton@kernel.org>
> > ---
> > This version is essentially the same as v1, but allows the kernel to
> > accept any previously-accepted setting for pool-mode, which should
> > alleviate concerns about breakage.
> > ---
> > Changes in v2:
> > - Accept any previously-accepted setting for pool_mode
> > - Link to v1: https://lore.kernel.org/r/20260423-sunrpc-pool-mode-v1-1-b7f20e35749b@kernel.org
> > ---
> >  net/sunrpc/svc.c | 238 +++++++++----------------------------------------------
> >  1 file changed, 37 insertions(+), 201 deletions(-)
> > 
> > diff --git a/net/sunrpc/svc.c b/net/sunrpc/svc.c
> > index dd80a2eaaa74..6e3d509bf95a 100644
> > --- a/net/sunrpc/svc.c
> > +++ b/net/sunrpc/svc.c
> > @@ -38,19 +38,6 @@
> >  
> >  static void svc_unregister(const struct svc_serv *serv, struct net *net);
> >  
> > -#define SVC_POOL_DEFAULT	SVC_POOL_GLOBAL
> > -
> > -/*
> > - * Mode for mapping cpus to pools.
> > - */
> > -enum {
> > -	SVC_POOL_AUTO = -1,	/* choose one of the others */
> > -	SVC_POOL_GLOBAL,	/* no mapping, just a single global pool
> > -				 * (legacy & UP mode) */
> > -	SVC_POOL_PERCPU,	/* one pool per cpu */
> > -	SVC_POOL_PERNODE	/* one pool per numa node */
> > -};
> > -
> >  /*
> >   * Structure for mapping cpus to pools and vice versa.
> >   * Setup once during sunrpc initialisation.
> > @@ -58,62 +45,23 @@ enum {
> >  
> >  struct svc_pool_map {
> >  	int count;			/* How many svc_servs use us */
> > -	int mode;			/* Note: int not enum to avoid
> > -					 * warnings about "enumeration value
> > -					 * not handled in switch" */
> >  	unsigned int npools;
> > -	unsigned int *pool_to;		/* maps pool id to cpu or node */
> > -	unsigned int *to_pool;		/* maps cpu or node to pool id */
> > +	unsigned int *pool_to;		/* maps pool id to node */
> > +	unsigned int *to_pool;		/* maps node to pool id */
> >  };
> >  
> > -static struct svc_pool_map svc_pool_map = {
> > -	.mode = SVC_POOL_DEFAULT
> > -};
> > +static struct svc_pool_map svc_pool_map;
> >  
> >  static DEFINE_MUTEX(svc_pool_map_mutex);/* protects svc_pool_map.count only */
> >  
> > -static int
> > -__param_set_pool_mode(const char *val, struct svc_pool_map *m)
> > -{
> > -	int err, mode;
> > -
> > -	mutex_lock(&svc_pool_map_mutex);
> > -
> > -	err = 0;
> > -	if (!strncmp(val, "auto", 4))
> > -		mode = SVC_POOL_AUTO;
> > -	else if (!strncmp(val, "global", 6))
> > -		mode = SVC_POOL_GLOBAL;
> > -	else if (!strncmp(val, "percpu", 6))
> > -		mode = SVC_POOL_PERCPU;
> > -	else if (!strncmp(val, "pernode", 7))
> > -		mode = SVC_POOL_PERNODE;
> > -	else
> > -		err = -EINVAL;
> > -
> > -	if (err)
> > -		goto out;
> > -
> > -	if (m->count == 0)
> > -		m->mode = mode;
> > -	else if (mode != m->mode)
> > -		err = -EBUSY;
> > -out:
> > -	mutex_unlock(&svc_pool_map_mutex);
> > -	return err;
> > -}
> > -
> > -static int
> > -param_set_pool_mode(const char *val, const struct kernel_param *kp)
> > -{
> > -	struct svc_pool_map *m = kp->arg;
> > -
> > -	return __param_set_pool_mode(val, m);
> > -}
> > -
> >  int sunrpc_set_pool_mode(const char *val)
> >  {
> > -	return __param_set_pool_mode(val, &svc_pool_map);
> > +	if (!strncmp(val, "auto", 4) ||
> > +	    !strncmp(val, "global", 6) ||
> > +	    !strncmp(val, "percpu", 6) ||
> > +	    !strncmp(val, "pernode", 7))
> > +		return 0;
> 
> "!strncmp" is one most disliked constructs....
> What would you think of using match_string() or even
> sysfs_match_string() ??
> 

Thanks. Switched to sysfs_match_string().

> > +	return -EINVAL;
> >  }
> >  EXPORT_SYMBOL(sunrpc_set_pool_mode);
> >  
> > @@ -122,84 +70,32 @@ EXPORT_SYMBOL(sunrpc_set_pool_mode);
> >   * @buf: where to write the current pool_mode
> >   * @size: size of @buf
> >   *
> > - * Grab the current pool_mode from the svc_pool_map and write
> > - * the resulting string to @buf. Returns the number of characters
> > + * Write the pool_mode string to @buf. Returns the number of characters
> >   * written to @buf (a'la snprintf()).
> >   */
> >  int
> >  sunrpc_get_pool_mode(char *buf, size_t size)
> >  {
> > -	struct svc_pool_map *m = &svc_pool_map;
> > -
> > -	switch (m->mode)
> > -	{
> > -	case SVC_POOL_AUTO:
> > -		return snprintf(buf, size, "auto");
> > -	case SVC_POOL_GLOBAL:
> > -		return snprintf(buf, size, "global");
> > -	case SVC_POOL_PERCPU:
> > -		return snprintf(buf, size, "percpu");
> > -	case SVC_POOL_PERNODE:
> > -		return snprintf(buf, size, "pernode");
> > -	default:
> > -		return snprintf(buf, size, "%d", m->mode);
> > -	}
> > +	return snprintf(buf, size, "pernode");
> >  }
> >  EXPORT_SYMBOL(sunrpc_get_pool_mode);
> >  
> >  static int
> > -param_get_pool_mode(char *buf, const struct kernel_param *kp)
> > +param_set_pool_mode(const char *val, const struct kernel_param *kp)
> >  {
> > -	char str[16];
> > -	int len;
> > -
> > -	len = sunrpc_get_pool_mode(str, ARRAY_SIZE(str));
> > -
> > -	/* Ensure we have room for newline and NUL */
> > -	len = min_t(int, len, ARRAY_SIZE(str) - 2);
> > -
> > -	/* tack on the newline */
> > -	str[len] = '\n';
> > -	str[len + 1] = '\0';
> > -
> > -	return sysfs_emit(buf, "%s", str);
> > +	pr_notice("sunrpc: the pool_mode parameter is deprecated and will be removed in a future release.\n");
> 
> Which future release?  How will we remember?
> 

I'll make this more vague. I'm not sure what the guidance is on removal
of knobs like this. A few years, I guess?

> > +	return sunrpc_set_pool_mode(val);
> >  }
> >  
> > -module_param_call(pool_mode, param_set_pool_mode, param_get_pool_mode,
> > -		  &svc_pool_map, 0644);
> > -
> > -/*
> > - * Detect best pool mapping mode heuristically,
> > - * according to the machine's topology.
> > - */
> >  static int
> > -svc_pool_map_choose_mode(void)
> > +param_get_pool_mode(char *buf, const struct kernel_param *kp)
> >  {
> > -	unsigned int node;
> > -
> > -	if (nr_online_nodes > 1) {
> > -		/*
> > -		 * Actually have multiple NUMA nodes,
> > -		 * so split pools on NUMA node boundaries
> > -		 */
> > -		return SVC_POOL_PERNODE;
> > -	}
> > -
> > -	node = first_online_node;
> > -	if (nr_cpus_node(node) > 2) {
> > -		/*
> > -		 * Non-trivial SMP, or CONFIG_NUMA on
> > -		 * non-NUMA hardware, e.g. with a generic
> > -		 * x86_64 kernel on Xeons.  In this case we
> > -		 * want to divide the pools on cpu boundaries.
> > -		 */
> > -		return SVC_POOL_PERCPU;
> > -	}
> > -
> > -	/* default: one global pool */
> > -	return SVC_POOL_GLOBAL;
> > +	return sysfs_emit(buf, "pernode\n");
> >  }
> >  
> > +module_param_call(pool_mode, param_set_pool_mode, param_get_pool_mode,
> > +		  NULL, 0644);
> > +
> >  /*
> >   * Allocate the to_pool[] and pool_to[] arrays.
> >   * Returns 0 on success or an errno.
> > @@ -224,35 +120,7 @@ svc_pool_map_alloc_arrays(struct svc_pool_map *m, unsigned int maxpools)
> >  }
> >  
> >  /*
> > - * Initialise the pool map for SVC_POOL_PERCPU mode.
> > - * Returns number of pools or <0 on error.
> > - */
> > -static int
> > -svc_pool_map_init_percpu(struct svc_pool_map *m)
> > -{
> > -	unsigned int maxpools = nr_cpu_ids;
> > -	unsigned int pidx = 0;
> > -	unsigned int cpu;
> > -	int err;
> > -
> > -	err = svc_pool_map_alloc_arrays(m, maxpools);
> > -	if (err)
> > -		return err;
> > -
> > -	for_each_online_cpu(cpu) {
> > -		BUG_ON(pidx >= maxpools);
> > -		m->to_pool[cpu] = pidx;
> > -		m->pool_to[pidx] = cpu;
> > -		pidx++;
> > -	}
> > -	/* cpus brought online later all get mapped to pool0, sorry */
> > -
> > -	return pidx;
> > -};
> > -
> > -
> > -/*
> > - * Initialise the pool map for SVC_POOL_PERNODE mode.
> > + * Initialise the pool map for one pool per NUMA node.
> >   * Returns number of pools or <0 on error.
> >   */
> >  static int
> > @@ -284,14 +152,13 @@ svc_pool_map_init_pernode(struct svc_pool_map *m)
> >   * Add a reference to the global map of cpus to pools (and
> >   * vice versa) if pools are in use.
> >   * Initialise the map if we're the first user.
> > - * Returns the number of pools. If this is '1', no reference
> > - * was taken.
> > + * Returns the number of pools, or 0 on failure.
> >   */
> >  static unsigned int
> >  svc_pool_map_get(void)
> >  {
> >  	struct svc_pool_map *m = &svc_pool_map;
> > -	int npools = -1;
> > +	int npools;
> >  
> >  	mutex_lock(&svc_pool_map_mutex);
> >  	if (m->count++) {
> > @@ -299,22 +166,11 @@ svc_pool_map_get(void)
> >  		return m->npools;
> >  	}
> >  
> > -	if (m->mode == SVC_POOL_AUTO)
> > -		m->mode = svc_pool_map_choose_mode();
> > -
> > -	switch (m->mode) {
> > -	case SVC_POOL_PERCPU:
> > -		npools = svc_pool_map_init_percpu(m);
> > -		break;
> > -	case SVC_POOL_PERNODE:
> > -		npools = svc_pool_map_init_pernode(m);
> > -		break;
> > -	}
> > -
> > +	npools = svc_pool_map_init_pernode(m);
> >  	if (npools <= 0) {
> > -		/* default, or memory allocation failure */
> > -		npools = 1;
> > -		m->mode = SVC_POOL_GLOBAL;
> > +		m->count--;
> 
> Could we make this
>    m->count = 0
> 
> to make it crystal clear that there are now no references?
> It would be nice to rename ->count to ->refcount or similar
> so it is obvious what is being counted, but that doesn't need to be part
> of this change.
> 

Yep, fixed. I didn't rename the counter though. That can be future
cleanup.

> > +		mutex_unlock(&svc_pool_map_mutex);
> > +		return 0;
> >  	}
> >  	m->npools = npools;
> >  	mutex_unlock(&svc_pool_map_mutex);
> > @@ -346,14 +202,11 @@ static int svc_pool_map_get_node(unsigned int pidx)
> >  {
> >  	const struct svc_pool_map *m = &svc_pool_map;
> >  
> > -	if (m->count) {
> > -		if (m->mode == SVC_POOL_PERCPU)
> > -			return cpu_to_node(m->pool_to[pidx]);
> > -		if (m->mode == SVC_POOL_PERNODE)
> > -			return m->pool_to[pidx];
> > -	}
> > +	if (m->count)
> > +		return m->pool_to[pidx];
> >  	return numa_mem_id();
> 
> Can this code run if m->count is zero?
> 

Yes, it can. It's called unconditionally from svc_new_thread(), which
runs for non-pooled services created via svc_create() (e.g. lockd) that
never call svc_pool_map_get(). In that case m->count == 0 and
m->pool_to == NULL.

> >  }
> > +
> >  /*
> >   * Set the given thread's cpus_allowed mask so that it
> >   * will only run on cpus in the given pool.
> > @@ -372,27 +225,15 @@ svc_pool_map_set_cpumask(struct task_struct *task, unsigned int pidx)
> >  	if (m->count == 0)
> >  		return;
> >  
> > -	switch (m->mode) {
> > -	case SVC_POOL_PERCPU:
> > -	{
> > -		set_cpus_allowed_ptr(task, cpumask_of(node));
> > -		break;
> > -	}
> > -	case SVC_POOL_PERNODE:
> > -	{
> > -		set_cpus_allowed_ptr(task, cpumask_of_node(node));
> > -		break;
> > -	}
> > -	}
> > +	set_cpus_allowed_ptr(task, cpumask_of_node(node));
> >  }
> >  
> >  /**
> >   * svc_pool_for_cpu - Select pool to run a thread on this cpu
> >   * @serv: An RPC service
> >   *
> > - * Use the active CPU and the svc_pool_map's mode setting to
> > - * select the svc thread pool to use. Once initialized, the
> > - * svc_pool_map does not change.
> > + * Use the active CPU and the svc_pool_map to select the svc thread
> > + * pool to use. Once initialized, the svc_pool_map does not change.
> >   *
> >   * Return value:
> >   *   A pointer to an svc_pool
> > @@ -400,20 +241,12 @@ svc_pool_map_set_cpumask(struct task_struct *task, unsigned int pidx)
> >  struct svc_pool *svc_pool_for_cpu(struct svc_serv *serv)
> >  {
> >  	struct svc_pool_map *m = &svc_pool_map;
> > -	int cpu = raw_smp_processor_id();
> > -	unsigned int pidx = 0;
> > +	unsigned int pidx;
> >  
> >  	if (serv->sv_nrpools <= 1)
> >  		return serv->sv_pools;
> >  
> > -	switch (m->mode) {
> > -	case SVC_POOL_PERCPU:
> > -		pidx = m->to_pool[cpu];
> > -		break;
> > -	case SVC_POOL_PERNODE:
> > -		pidx = m->to_pool[cpu_to_node(cpu)];
> > -		break;
> > -	}
> > +	pidx = m->to_pool[cpu_to_node(raw_smp_processor_id())];
> >  
> >  	return &serv->sv_pools[pidx % serv->sv_nrpools];
> >  }
> > @@ -617,6 +450,9 @@ struct svc_serv *svc_create_pooled(struct svc_program *prog,
> >  	struct svc_serv *serv;
> >  	unsigned int npools = svc_pool_map_get();
> >  
> > +	if (!npools)
> > +		return NULL;
> > +
> >  	serv = __svc_create(prog, nprogs, stats, bufsize, npools, threadfn);
> >  	if (!serv)
> >  		goto out_err;
> > 
> 
> I think this is a good a worthwhile change.
> Regarding Chucks comments, I think a minimum of one thread per node,
> rather than a minimum of one thread total, is the sensible approach.
> Of course if there exist memory-only nodes, they wouldn't need a thread.
> 

I responded to Chuck's reply on this point. I think that's part of a
solution, but we probably also need to avoid queueing to pools (nodes)
that have no threads in them, to help catch misconfigurations.

Thanks for the review!
-- 
Jeff Layton <jlayton@kernel.org>