runtime: allow more CPUs on FreeBSD

Currently the FreeBSD CPU affinity code assumes that the maximum
GOMAXPROCS is 256, but we just removed that limit.

This commit rewrites the FreeBSD CPU affinity code to raise the CPU
count limit to 65,536, like the Linux CPU affinity code, and to
degrade more gracefully if we do somehow go over that.

Change-Id: Ic4ca7f88bd8b9448aae4dbd43ef21a6c1b8fea63
Reviewed-on: https://go-review.googlesource.com/66291
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

Author: aclements

src/runtime/os_freebsd.go

@@ -69,15 +69,19 @@ func sysctlnametomib(name []byte, mib *[_CTL_MAXNAME]uint32) uint32 {
 }
 
 const (
-	_CPU_SETSIZE_MAX = 32 // Limited by _MaxGomaxprocs(256) in runtime2.go.
 	_CPU_CURRENT_PID = -1 // Current process ID.
 )
 
 //go:noescape
 func cpuset_getaffinity(level int, which int, id int64, size int, mask *byte) int32
 
+//go:systemstack
 func getncpu() int32 {
-	var mask [_CPU_SETSIZE_MAX]byte
+	// Use a large buffer for the CPU mask. We're on the system
+	// stack, so this is fine, and we can't allocate memory for a
+	// dynamically-sized buffer at this point.
+	const maxCPUs = 64 * 1024
+	var mask [maxCPUs / 8]byte
 	var mib [_CTL_MAXNAME]uint32
 
 	// According to FreeBSD's /usr/src/sys/kern/kern_cpuset.c,
@@ -99,21 +103,20 @@ func getncpu() int32 {
 		return 1
 	}
 
-	size := maxcpus / _NBBY
-	ptrsize := uint32(unsafe.Sizeof(uintptr(0)))
-	if size < ptrsize {
-		size = ptrsize
+	maskSize := int(maxcpus+7) / 8
+	if maskSize < sys.PtrSize {
+		maskSize = sys.PtrSize
 	}
-	if size > _CPU_SETSIZE_MAX {
-		return 1
+	if maskSize > len(mask) {
+		maskSize = len(mask)
 	}
 
 	if cpuset_getaffinity(_CPU_LEVEL_WHICH, _CPU_WHICH_PID, _CPU_CURRENT_PID,
-		int(size), (*byte)(unsafe.Pointer(&mask[0]))) != 0 {
+		maskSize, (*byte)(unsafe.Pointer(&mask[0]))) != 0 {
 		return 1
 	}
 	n := int32(0)
-	for _, v := range mask[:size] {
+	for _, v := range mask[:maskSize] {
 		for v != 0 {
 			n += int32(v & 1)
 			v >>= 1