While looking through the (ab)use of the clockevents_notify()
function I stumbled over the following gem in the acpi_pad code:

  if (lapic_detected_unstable && !lapic_marked_unstable) {
     /* LAPIC could halt in idle, so notify users */
     for_each_online_cpu(i)
       clockevents_notify(CLOCK_EVT_NOTIFY_BROADCAST_ON, &i);
     lapic_marked_unstable = 1;
  }

This code calls on the cpu which detects the lapic unstable
condition first clockevents_notify() to tell the core code that
the broadcast should be enabled on all online cpus. Brilliant
stuff that as it notifies the core code a num_online_cpus()
times that the broadcast should be enabled on the current cpu.

This probably has never been noticed because that code got never
tested with NOHZ=n and HIGHRES_TIMER=n or it just worked by
chance because one of the other mechanisms told the core in the
right way that the local apic timer is wreckaged.

Sigh, this is:

 - The 4th incarnation of idle drivers which has their own mechanism
   to detect and deal with X86_FEATURE_ARAT.

 - The 2nd incarnation of fake idle mechanisms with a different set of
   brainmelting bugs.

 - Has been merged against an explicit NAK of the scheduler
   maintainer with the promise to improve it over time.

 - Another example of featuritis driven trainwreck engineering.

 - Another pointless waste of my time.

Fix this nonsense by removing that lapic detection and
notification logic and simply call into the clockevents code
unconditonally. The ARAT feature is marked in the lapic
clockevent already so the core code will just ignore the
requests and return.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1887788.RObRuI4tSv@vostro.rjw.lanSigned-off-by: Ingo Molnar <mingo@kernel.org>