[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>

arch/i386/kernel/i8253.c

@@ -109,7 +109,7 @@ static struct clocksource clocksource_pit = {
 
 static int __init init_pit_clocksource(void)
 {
-	if (num_possible_cpus() > 4) /* PIT does not scale! */
+	if (num_possible_cpus() > 1) /* PIT does not scale! */
 		return 0;
 
 	clocksource_pit.mult = clocksource_hz2mult(CLOCK_TICK_RATE, 20);

arch/i386/kernel/tsc.c

@@ -349,8 +349,8 @@ static int tsc_update_callback(void)
 	int change = 0;
 
 	/* check to see if we should switch to the safe clocksource: */
-	if (clocksource_tsc.rating != 50 && check_tsc_unstable()) {
-		clocksource_tsc.rating = 50;
+	if (clocksource_tsc.rating != 0 && check_tsc_unstable()) {
+		clocksource_tsc.rating = 0;
 		clocksource_reselect();
 		change = 1;
 	}
@@ -461,7 +461,7 @@ static int __init init_tsc_clocksource(void)
 							clocksource_tsc.shift);
 		/* lower the rating if we already know its unstable: */
 		if (check_tsc_unstable())
-			clocksource_tsc.rating = 50;
+			clocksource_tsc.rating = 0;
 
 		init_timer(&verify_tsc_freq_timer);
 		verify_tsc_freq_timer.function = verify_tsc_freq;

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 */