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

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 <[email protected]>
> > ---
> > 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-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.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 <[email protected]>