[PATCH v4 3/4] sunrpc: guarantee a thread per CPU-bearing node when auto-distributing

[Jeff Layton <jlayton@kernel.org>]

svc_set_num_threads() spreads the requested thread count evenly across
the service's pools. In pernode mode each pool maps to a NUMA node, and
svc_pool_for_cpu() steers an incoming transport to the pool for the node
it arrived on. When fewer threads than pools are requested, even
distribution leaves some nodes' pools empty, and a transport steered to
an empty pool has no thread to service it.

Floor each CPU-bearing node's pool at one thread when auto-distributing a
non-zero count, so no such pool is left empty. The resulting total may
exceed the requested count. This only affects the auto-distribute path
(a single-value array, i.e. svc_set_num_threads()); callers that set
per-pool counts explicitly via svc_set_pool_threads() are unchanged and
may still set a pool to zero. Nodes without CPUs (e.g. memory-only nodes)
get no thread, as nothing is steered to them.

Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 net/sunrpc/svc.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/net/sunrpc/svc.c b/net/sunrpc/svc.c
index c9fba7edaace..ae93a6f51087 100644
--- a/net/sunrpc/svc.c
+++ b/net/sunrpc/svc.c
@@ -837,6 +837,12 @@ EXPORT_SYMBOL_GPL(svc_set_pool_threads);
  * are multiple pools then the new threads or victims will be distributed
  * evenly among them.
  *
+ * When @nrservs is non-zero but smaller than the number of pools, even
+ * distribution would leave some pools empty. Since each pool maps to a
+ * NUMA node and only services transports steered to that node, every
+ * pool whose node has CPUs is instead guaranteed at least one thread.
+ * The resulting total may therefore exceed @nrservs.
+ *
  * Caller must ensure mutual exclusion between this and server startup or
  * shutdown.
  *
@@ -861,6 +867,15 @@ svc_set_num_threads(struct svc_serv *serv, unsigned int min_threads,
 			--remain;
 		}
 
+		/*
+		 * Don't let a node's pool sit empty while threads are
+		 * being auto-distributed: a transport steered there would
+		 * have nothing to service it.
+		 */
+		if (threads == 0 && nrservs &&
+		    nr_cpus_node(svc_pool_map_get_node(pool->sp_id)))
+			threads = 1;
+
 		err = svc_set_pool_threads(serv, pool, min_threads, threads);
 		if (err)
 			break;

-- 
2.54.0