Dynamic configure max_cstate

["Zhang, Yanmin" <yanmin_zhang@linux.intel.com>]

When running a fio workload, I found sometimes cpu C state has
big impact on the result. Mostly, fio is a disk I/O workload
which doesn't spend much time with cpu, so cpu switch to C2/C3
freqently and the latency is big.

If I start kernel with idle=poll or processor.max_cstate=1,
the result is quite good. Consider a scenario that machine is
busy at daytime and free at night. Could we add a dynamic
configuration interface for processor.max_cstate or something
similiar with sysfs? So user applications could change the
max_cstate dynamically? For example, we could add a new
parameter to function cpuidle_governor->select to mark the
highest c state.

Any idea?

Yanmin