Unstable CPU frequency delivered by intel_pstate on isolated CPUs
Issue
When long-running CPU-bound task that performs almost zero number of syscalls is placed on isolated CPU core, then frequency of that CPU doesn't rise to higher level and stuck at lowest level or fluctuates around lowest level and never rises to highest allowed performance level however it's visible in OS that load is 100% on that isolated CPU.
Environment
- Red Hat Enterprise Linux 7
- intel_pstate cpufreq driver with "powersave" governor is used
- some CPUs are isolated and their ticks are stopped using nohz_full=... option
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