runtime: handle linux CPU masks up to 64k CPUs

Fixes #11823.

Change-Id: Ic949ccb9657478f8ca34fdf1a6fe88f57db69f24
Reviewed-on: https://go-review.googlesource.com/12535
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/os1_linux.go

@@ -75,11 +75,19 @@ func futexwakeup(addr *uint32, cnt uint32) {
 }
 
 func getproccount() int32 {
-	var buf [16]uintptr
+	// This buffer is huge (8 kB) but we are on the system stack
+	// and there should be plenty of space (64 kB).
+	// Also this is a leaf, so we're not holding up the memory for long.
+	// See golang.org/issue/11823.
+	// The suggested behavior here is to keep trying with ever-larger
+	// buffers, but we don't have a dynamic memory allocator at the
+	// moment, so that's a bit tricky and seems like overkill.
+	const maxCPUs = 64 * 1024
+	var buf [maxCPUs / (ptrSize * 8)]uintptr
 	r := sched_getaffinity(0, unsafe.Sizeof(buf), &buf[0])
 	n := int32(0)
 	for _, v := range buf[:r/ptrSize] {
-		for i := 0; i < 64; i++ {
+		for v != 0 {
 			n += int32(v & 1)
 			v >>= 1
 		}