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
@@ -599,8 +599,10 @@
 		size_t len;
 
 		len = sizeof(kern_cpuset_size);
-		if (sysctlbyname("kern.sched.cpusetsize", &kern_cpuset_size,
-		    &len, NULL, 0))
+		if (sysctlbyname("kern.sched.cpusetsizemin", &kern_cpuset_size,
+		    &len, NULL, 0) != 0 &&
+		    sysctlbyname("kern.sched.cpusetsize", &kern_cpuset_size,
+		    &len, NULL, 0) != 0)
 			PANIC("failed to get sysctl kern.sched.cpusetsize");
 	}
 
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
@@ -136,11 +136,17 @@
 static struct unrhdr *cpuset_unr;
 static struct cpuset *cpuset_zero, *cpuset_default, *cpuset_kernel;
 static struct domainset *domainset0, *domainset2;
+u_int cpusetsizemin = 1;
 
 /* 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];
 
diff --git a/sys/kern/subr_smp.c b/sys/kern/subr_smp.c
--- a/sys/kern/subr_smp.c
+++ b/sys/kern/subr_smp.c
@@ -148,6 +148,8 @@
 	KASSERT(mp_maxid >= mp_ncpus - 1,
 	    ("%s: counters out of sync: max %d, count %d", __func__,
 		mp_maxid, mp_ncpus));
+
+	cpusetsizemin = howmany(mp_maxid + 1, NBBY);
 }
 SYSINIT(cpu_mp_setmaxid, SI_SUB_TUNABLES, SI_ORDER_FIRST, mp_setmaxid, NULL);
 
diff --git a/sys/sys/cpuset.h b/sys/sys/cpuset.h
--- a/sys/sys/cpuset.h
+++ b/sys/sys/cpuset.h
@@ -120,6 +120,7 @@
 #include <sys/queue.h>
 
 LIST_HEAD(setlist, cpuset);
+extern u_int cpusetsizemin;
 
 /*
  * cpusets encapsulate cpu binding information for one or more threads.