Please answer these questions before submitting your issue. Thanks!

1. What version of Go are you using (go version)?
   go version

   go version devel +9d4efdf Sat Apr 9 07:15:22 2016 +0000 freebsd/amd64
   

2. What operating system and processor architecture are you using (go env)?
   go env

   GOARCH="amd64"
   GOBIN="/home/rhinofly/home/sandspace/go/bin"
   GOEXE=""
   GOHOSTARCH="amd64"
   GOHOSTOS="freebsd"
   GOOS="freebsd"
   GOPATH="/home/rhinofly/home/sandspace/external/:/home/rhinofly/home/sandspace/golibs/"
   GORACE=""
   GOROOT="/home/rhinofly/home/sandspace/go"
   GOTOOLDIR="/home/rhinofly/home/sandspace/go/pkg/tool/freebsd_amd64"
   CC="clang"
   GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -gno-record-gcc-switches"
   CXX="clang++"
   CGO_ENABLED="1"
   

   uname -a

   FreeBSD freebsd-i5-box.localdomain 10.2-STABLE FreeBSD 10.2-STABLE #0 r292855: Tue Dec 29 06:17:50 UTC 2015     root@releng1.nyi.freebsd.org:/usr/obj/usr/src/sys/GENERIC  amd64
   

3. What did you do?
   If possible, provide a recipe for reproducing the error.
   A complete runnable program is good.
   A link on play.golang.org is best.

cpuset is a util to configure/limit processes available for a processor, just like taskset in Linux.

DESCRIPTION
     The cpuset command can be used to assign processor sets to processes, run
     commands constrained to a given set or list of processors, and query
     information about processor binding, sets, and available processors in
     the system.

save follow code to freebsdncpu.go

go build -o freebsdncpu freebsdncpu.go and run as:

/usr/bin/cpuset -l 0 ./freebsdncpu

//
package main

import (
    "fmt"
    "os"
    "os/exec"
    "runtime"
    "strconv"
    "sync/atomic"
)

func main() {
    fmt.Printf("runtime.GOMAXPROCS(-1) return %d CPUs\n", runtime.GOMAXPROCS(-1))
    fmt.Printf("runtime.NumCPU() return %d CPUs\n", runtime.NumCPU())
    cpuset := exec.Command("cpuset", "-g", "-p", strconv.Itoa(os.Getpid()))
    output, err := cpuset.CombinedOutput()
    if err != nil {
        fmt.Printf("exec cpuset failed: %s\n", err.Error())
        os.Exit(1)
    }
    fmt.Printf("cpuset return: %s\n", output)
    var cnt int64
    for i := 0; i < runtime.NumCPU(); i++ {
        go func() {
            for {
                atomic.AddInt64(&cnt, 1)
            }
        }()
    }
    fmt.Printf("%d CPU burnner running ...\n", runtime.NumCPU())
    fmt.Printf("Press +C to exit ...\n")
    select {}
}

1. What did you expect to see?

runtime.GOMAXPROCS(-1) return 1 CPUs
runtime.NumCPU() return 1 CPUs
cpuset return: pid 40291 mask: 0

1 CPU burnner running ...
Press +C to exit ...

from top -IaP we see Go processor using only CPU0

1. What did you see instead?

runtime.GOMAXPROCS(-1) return 2 CPUs
runtime.NumCPU() return 2 CPUs
cpuset return: pid 40291 mask: 0

2 CPU burnner running ...
Press +C to exit ...

from top -IaP we see Go processor using only CPU0

1. more information:

as https://github.com/golang/go/blob/master/src/runtime/os1_freebsd.go#L20 showed, NumCPU()/ncpu is direct probe from sysctl item hw.ncpu, but in
https://www.freebsd.org/cgi/man.cgi?sysctl(3) and /usr/include/sys/sysctl.h, it's say "hw.ncpu" is "The number of cpus", is not the "usable by the current process" in https://github.com/golang/go/blob/master/src/runtime/debug.go#L32

And please note, cpuset already limit the CPU list "befor" Go processes start, not the case in #11609 (change usable cpu list on the fly).