[PATCH] i386 Time: Avoid PIT SMP lockups

Avoid possible PIT livelock issues seen on SMP systems (and reported by
Andi), by not allowing it as a clocksource on SMP boxes.

However, since the PIT may no longer be present, we have to properly handle
the cases where SMP systems have TSC skew and fall back from the TSC.
Since the PIT isn't there, it would "fall back" to the TSC again.  So this
changes the jiffies rating to 1, and the TSC-bad rating value to 0.

Thus you will get the following behavior priority on i386 systems:

tsc		[if present & stable]
hpet		[if present]
cyclone		[if present]
acpi_pm		[if present]
pit		[if UP]
jiffies

Rather then the current more complicated:
tsc		[if present & stable]
hpet		[if present]
cyclone		[if present]
acpi_pm		[if present]
pit		[if cpus < 4]
tsc		[if present & unstable]
jiffies

Signed-off-by: John Stultz <johnstul@us.ibm.com>
Cc: Andi Kleen <ak@suse.de>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

kernel/time/jiffies.c

@@ -57,7 +57,7 @@ static cycle_t jiffies_read(void)
 
 struct clocksource clocksource_jiffies = {
 	.name		= "jiffies",
-	.rating		= 0, /* lowest rating*/
+	.rating		= 1, /* lowest valid rating*/
 	.read		= jiffies_read,
 	.mask		= 0xffffffff, /*32bits*/
 	.mult		= NSEC_PER_JIFFY << JIFFIES_SHIFT, /* details above */