diff --git a/lib/libthr/thread/thr_attr.c b/lib/libthr/thread/thr_attr.c
--- a/lib/libthr/thread/thr_attr.c
+++ b/lib/libthr/thread/thr_attr.c
@@ -599,9 +599,11 @@
 		size_t len;
 
 		len = sizeof(kern_cpuset_size);
-		if (sysctlbyname("kern.sched.cpusetsize", &kern_cpuset_size,
-		    &len, NULL, 0))
-			PANIC("failed to get sysctl kern.sched.cpusetsize");
+		if (sysctlbyname("kern.sched.cpusetsizemin", &kern_cpuset_size,
+		    &len, NULL, 0) != 0)
+			if (sysctlbyname("kern.sched.cpusetsize",
+			    &kern_cpuset_size, &len, NULL, 0) != 0)
+				PANIC("failed to get sysctl kern.sched.cpusetsize");
 	}
 
 	return (kern_cpuset_size);
diff --git a/sys/kern/kern_cpuset.c b/sys/kern/kern_cpuset.c
--- a/sys/kern/kern_cpuset.c
+++ b/sys/kern/kern_cpuset.c
@@ -136,16 +136,31 @@
 static struct unrhdr *cpuset_unr;
 static struct cpuset *cpuset_zero, *cpuset_default, *cpuset_kernel;
 static struct domainset *domainset0, *domainset2;
+u_int cpusetsizemin;
 
 /* Return the size of cpuset_t at the kernel level */
 SYSCTL_INT(_kern_sched, OID_AUTO, cpusetsize, CTLFLAG_RD | CTLFLAG_CAPRD,
     SYSCTL_NULL_INT_PTR, sizeof(cpuset_t), "sizeof(cpuset_t)");
 
+/* Return the minimum size of cpuset_t allowed by the kernel */
+SYSCTL_UINT(_kern_sched, OID_AUTO, cpusetsizemin,
+    CTLFLAG_RD | CTLFLAG_CAPRD, &cpusetsizemin, 0,
+    "The minimum size of cpuset_t allowed by the kernel");
+
 cpuset_t *cpuset_root;
 cpuset_t cpuset_domain[MAXMEMDOM];
 
 static int domainset_valid(const struct domainset *, const struct domainset *);
 
+static void
+cpu_calc_cpusetsize(void *dummy)
+{
+
+	cpusetsizemin = howmany(mp_maxid + 1, NBBY);
+}
+SYSINIT(cpu_calc_cpusetsize, SI_SUB_TUNABLES, SI_ORDER_ANY,
+    cpu_calc_cpusetsize, NULL);
+
 /*
  * Find the first non-anonymous set starting from 'set'.
  */
diff --git a/sys/sys/smp.h b/sys/sys/smp.h
--- a/sys/sys/smp.h
+++ b/sys/sys/smp.h
@@ -177,6 +177,7 @@
 
 extern cpuset_t all_cpus;
 extern cpuset_t cpuset_domain[MAXMEMDOM]; 	/* CPUs in each NUMA domain. */
+extern u_int cpusetsizemin;
 
 /*
  * Macro allowing us to determine whether a CPU is absent at any given