:CPU frequency discrepancy in /proc/cpuinfo

Problem description

When an instance is idle, the /proc/cpuinfo file might report a CPU frequency of 800 MHz for some CPUs. This value differs from the frequency listed in the instance specification, as shown below.

cpu MHz		: 800.000
cpu MHz		: 800.000
cpu MHz		: 800.000
cpu MHz		: 2996.976
cpu MHz		: 800.000
cpu MHz		: 800.000
cpu MHz		: 800.000
cpu MHz		: 800.000
cpu MHz		: 800.000
cpu MHz		: 800.000
cpu MHz		: 800.000
cpu MHz		: 800.000
cpu MHz		: 3192.046
cpu MHz		: 800.000
cpu MHz		: 3200.000
cpu MHz		: 800.000
cpu MHz		: 2975.717
cpu MHz		: 800.000
cpu MHz		: 3005.653
cpu MHz		: 800.000
cpu MHz		: 800.000
cpu MHz		: 800.000
cpu MHz		: 800.000
cpu MHz		: 800.000
cpu MHz		: 800.000
cpu MHz		: 800.000
cpu MHz		: 800.000

This solution also applies to the following kernels and operating systems:

Cause

Starting with Linux kernel 5.11, a patch (commit ID: 3fcd6a230fa7d03bffcb831a81b40435c146c12b) was added to improve system performance and CPU response.

 x86/cpu: Avoid cpuinfo-induced IPIing of idle CPUs
    Currently, accessing /proc/cpuinfo sends IPIs to idle CPUs in order to
    learn their clock frequency.  Which is a bit strange, given that waking
    them from idle likely significantly changes their clock frequency.
    This commit therefore avoids sending /proc/cpuinfo-induced IPIs to
    idle CPUs.

When /proc/cpuinfo is accessed, the rcu_is_idle_cpu function checks whether the CPU is idle. If so, it returns min_freq, which defaults to 800 MHz. Some kernel versions report this min_freq value as the CPU base frequency.

Solution

For accurate frequency readings, use tools such as cpupower or turbostat.

For example, run cpupower to check the frequency:

sudo cpupower frequency-info

The output shows that the current CPU frequency is 3.20 GHz.

[ecs-user@iZ2zxxx187kyZ ~]$ sudo cpupower frequency-info
analyzing CPU 0:
  driver: intel_pstate
  CPUs which run at the same hardware frequency: 0
  CPUs which need to have their frequency coordinated by software: 0
  maximum transition latency:  Cannot determine or is not supported.
  hardware limits: 800 MHz - 3.80 GHz
  available cpufreq governors: performance powersave
  current policy: frequency should be within 800 MHz and 3.80 GHz.
                  The governor "performance" may decide which speed to use
                  within this range.
  current CPU frequency: Unable to call hardware
  current CPU frequency: 3.20 GHz (asserted by call to kernel)
  boost state support:
    Supported: yes
    Active: yes