x86/sched: Make mwait_usable() heed to "idle=" kernel parameters properly

The checks that exist in mwait_usable() for "idle=" kernel
parameters are insufficient. As a result, mwait_usable() can
return 1 even if "idle=nomwait" or "idle=poll" or "idle=halt"
parameters are passed.

Of these cases, incorrect handling of idle=nomwait is a
universal problem since mwait can get used for usual CPU idling.
However the rest of the cases are problematic only during CPU
Hotplug (offline) because, in the CPU offline path, the function
mwait_play_dead() is called, which might result in mwait being
used in the offline CPUs, if mwait_usable() happens to return 1.

Fix these issues by checking for the boot time "idle=" kernel
parameter properly in mwait_usable().

The first issue (usual cpu idling) is demonstrated below:

Before applying the patch (dmesg snippet):

 [    0.000000] Command line: [...] idle=nomwait
 [    0.000000] Kernel command line: [...] idle=nomwait
 [    0.000000]  RCU dyntick-idle grace-period acceleration is enabled.
 [    0.140606] using mwait in idle threads.  <======= mwait being used
 [    4.303986] cpuidle: using governor ladder
 [    4.308232] cpuidle: using governor menu

After applying the patch:

 [    0.000000] Command line: [...] idle=nomwait
 [    0.000000] Kernel command line: [...] idle=nomwait
 [    0.000000]  RCU dyntick-idle grace-period acceleration is enabled.
 [    4.264100] cpuidle: using governor ladder
 [    4.268342] cpuidle: using governor menu
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Acked-by: Deepthi Dharwar <deepthi@linux.vnet.ibm.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Cc: venki@google.com
Cc: suresh.b.siddha@intel.com
Cc: Borislav Petkov <bp@amd64.org>
Cc: lenb@kernel.org
Cc: Rafael J. Wysocki <rjw@sisk.pl>
Link: http://lkml.kernel.org/r/4F9E37B8.30001@linux.vnet.ibm.comSigned-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/process.c

@@ -594,9 +594,17 @@ int mwait_usable(const struct cpuinfo_x86 *c)
 {
 	u32 eax, ebx, ecx, edx;
 
+	/* Use mwait if idle=mwait boot option is given */
 	if (boot_option_idle_override == IDLE_FORCE_MWAIT)
 		return 1;
 
+	/*
+	 * Any idle= boot option other than idle=mwait means that we must not
+	 * use mwait. Eg: idle=halt or idle=poll or idle=nomwait
+	 */
+	if (boot_option_idle_override != IDLE_NO_OVERRIDE)
+		return 0;
+
 	if (c->cpuid_level < MWAIT_INFO)
 		return 0;