for i in $LIST_OF_LINUX_MACHINES
do
   ssh $LINUX_MACHINE /var/tmp/restart.sh
done

numprocs=25
timeout=5
xargs -P "$numprocs" -J '{}' -n 1 --
  perl -e 'alarm shift; exec @ARGV' -- "$timeout"
    ssh -nxaq -o ConnectTimeout=5 -o StrictHostKeyChecking=no '{}' /tmp/reboot.sh
  <hostnames # if a file; use < <(awk ...) if a script providing per-line info