Ubuntu更改CPU频率

在使用afl对Python程序进行fuzz时，遇到需要更改CPU频率的问题，error如下：

    Whoops, your system uses on-demand CPU frequency scaling, adjusted
    between 1171 and 3027 MHz. Unfortunately, the scaling algorithm in the
    kernel is imperfect and can miss the short-lived processes spawned by
    afl-fuzz. To keep things moving, run these commands as root:

    cd /sys/devices/system/cpu
    echo performance | tee cpu*/cpufreq/scaling_governor

执行完操作之后，报错：

    cd /sys/devices/system/cpu
    echo performance | tee cpu*/cpufreq/scaling_governor

tee: cpu0/cpufreq/scaling_governor: Permission denied
tee: cpu10/cpufreq/scaling_governor: Permission denied
tee: cpu11/cpufreq/scaling_governor: Permission denied
tee: cpu12/cpufreq/scaling_governor: Permission denied
tee: cpu13/cpufreq/scaling_governor: Permission denied
tee: cpu14/cpufreq/scaling_governor: Permission denied
tee: cpu15/cpufreq/scaling_governor: Permission denied
tee: cpu16/cpufreq/scaling_governor: Permission denied
tee: cpu17/cpufreq/scaling_governor: Permission denied
tee: cpu18/cpufreq/scaling_governor: Permission denied
tee: cpu19/cpufreq/scaling_governor: Permission denied
tee: cpu1/cpufreq/scaling_governor: Permission denied
tee: cpu2/cpufreq/scaling_governor: Permission denied
tee: cpu3/cpufreq/scaling_governor: Permission denied
tee: cpu4/cpufreq/scaling_governor: Permission denied
tee: cpu5/cpufreq/scaling_governor: Permission denied
tee: cpu6/cpufreq/scaling_governor: Permission denied
tee: cpu7/cpufreq/scaling_governor: Permission denied
tee: cpu8/cpufreq/scaling_governor: Permission denied
tee: cpu9/cpufreq/scaling_governor: Permission denied
performance

是权限问题，切换至root账户，提示如下error：

Password: 
su: Authentication failure

是因为提示输入密码时，输入的是当前账号的密码而非root账户，之前没有登陆过root账户，不知道密码，因此解决如下：

$ sudo paawd root
Enter new UNIX password: 
Retype new UNIX password: 
passwd: password updated successfully

接着切换至root用户即可成功执行上述命令。
使用exit即可退出root用户。