$ strace -E GLIBC_TUNABLES=glibc.pthread.rseq=1 ./tst
execve("./tst", ["./tst"], 0xaaab0a13a860 /* 48 vars */) = 0
brk(NULL)                               = 0x3c75a000
brk(0x3c75a024)                         = 0x3c75a024
brk(NULL)                               = 0x3c75a024
faccessat(AT_FDCWD, "/etc/ld.so.preload", R_OK) = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=28123, ...}) = 0
mmap(NULL, 28123, PROT_READ, MAP_PRIVATE, 3, 0) = 0xffffbbd20000
close(3)                                = 0
openat(AT_FDCWD, "/lib64/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\3\0\0\0\0\0\0\0\0\3\0\267\0\1\0\0\0\360t\2\0\0\0\0\0"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0755, st_size=2300336, ...}) = 0
mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xffffbbd1e000
mmap(NULL, 1826768, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0xffffbbb2e000
mprotect(0xffffbbcbc000, 122880, PROT_NONE) = 0
mmap(0xffffbbcda000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x19c000) = 0xffffbbcda000
mmap(0xffffbbce0000, 49104, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0xffffbbce0000
close(3)                                = 0
set_tid_address(0xffffbbd1ed90)         = 62758
set_robust_list(0xffffbbd1eda0, 24)     = 0
rseq(0xffffbbd1f460, 0x20, 0, 0xd428bc00) = 0
mprotect(0xffffbbcda000, 16384, PROT_READ) = 0
mprotect(0x41f000, 4096, PROT_READ)     = 0
mprotect(0xffffbbd2a000, 8192, PROT_READ) = 0
prlimit64(0, RLIMIT_STACK, NULL, {rlim_cur=8192*1024, rlim_max=RLIM64_INFINITY}) = 0
munmap(0xffffbbd20000, 28123)           = 0
getcpu([2], NULL, NULL)                 = 0
fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(0x88, 0), ...}) = 0
getrandom("\x79\x55\x48\xc0\x0f\x33\x6b\xab", 8, GRND_NONBLOCK) = 8
brk(NULL)                               = 0x3c75a024
brk(0x3c77b024)                         = 0x3c77b024
brk(0x3c77c000)                         = 0x3c77c000
write(1, "2\n", 22
)                      = 2
exit_group(0)                           = ?
+++ exited with 0 +++

#define _GNU_SOURCE
#include <stdio.h>
#include <sched.h>

int
main (void)
{
  printf ("%d\n", sched_getcpu ());
}