Since commit 7d5905dc ("x86 / CPU: Always show current CPU frequency
in /proc/cpuinfo") open and read of /proc/cpuinfo sends IPI to all CPUs.
Many applications read /proc/cpuinfo at the start for trivial reasons like
counting cores or detecting cpu features. While sensitive workloads like
DPDK network polling don't like any interrupts.

Integrates this feature with cpu isolation and do not send IPIs to CPUs
without housekeeping flag HK_FLAG_MISC (set by nohz_full).

Code that requests cpu frequency like show_cpuinfo() falls back to the last
frequency set by the cpufreq driver if this method returns 0.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Len Brown <len.brown@intel.com>
Cc: Frederic Weisbecker <frederic@kernel.org>
Cc: "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Link: https://lkml.kernel.org/r/155790354043.1104.15333317408370209.stgit@buzz