x/sys: SchedGetaffinity fails on Linux machines with enough (potential) CPUs #23639

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

go version devel +926f278 Tue Jan 30 15:22:28 2018 +0000 linux/amd64

And also the current version of golang.org/x/sys from git.

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

GOARCH="amd64"
GOOS="linux"

What did you do?

Here is a trivial program to call unix.SchedGetaffinity() to count the number of currently usable CPUs in the system:

package main

import (
        "fmt"
        "os"

        "golang.org/x/sys/unix"
)

func main() {
        var cpuset unix.CPUSet
        err := unix.SchedGetaffinity(0, &cpuset)
        if err != nil {
                fmt.Printf("numcpus: cannot get affinity: %s\n", err)
                os.Exit(1)
        }
        fmt.Printf("%d\n", cpuset.Count())
}

On many of our amd64 machines this will work fine. However, on some of them (with large CPU counts) it will fail with:

$ numcpus
numcpus: cannot get affinity: invalid argument

Using strace says that sched_getaffinity() is being called as follows:

sched_getaffinity(0, 8, 0xc420075ed8)   = -1 EINVAL (Invalid argument)

The second argument here is the number of bytes in the cpuset. Eight bytes is not very many and looking at how unix.CPUSet is defined in the source code suggests that there is supposed to be room for 1024 CPUs. However, inspection of the actual code making the system call in affinity_linux.go suggests that it is making a size of pointer versus size of object pointed to confusion. The set argument is a pointer to a CPUSet not a CPUSet, so unsafe.Sizeof(set) is the size of the pointer (8 bytes on a 64-bit machine) instead of the much larger size of the CPUSet.