cpufreq: cpufreq_driver_lock is quite hot on 512p systems

[Nathan Zimmer]

I am noticing the cpufreq_driver_lock is quite hot.
Currently on an idle 512 system perf shows me most of the system time is 
spent on this lock.

-  84.18%  [kernel]                 [k] _raw_spin_lock_irqsave
   - _raw_spin_lock_irqsave
      - 99.97% __cpufreq_cpu_get
           cpufreq_cpu_get
           __cpufreq_driver_getavg
           dbs_check_cpu
           od_dbs_timer
           process_one_work
           worker_thread
           kthread
           ret_from_fork
 
The data this lock protects is used in a read mostly situation.
While exprimenting I found turning the lock to a rwlock fixed the problem.
It seems like this might be amenable to using the RCU.

Any other ideas on how to properly resolve the issue?

Thanks,
Nate

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