[PATCH] fix lost_tick detector for speedstep

From: john stultz <johnstul@us.ibm.com>

The patch tries to resolve issues caused by running the TSC based lost
tick compensation code on CPUs that change frequency (speedstep, etc).

Should the CPU be in slow mode when calibrate_tsc() executes, the kernel
will assume we have so many cycles per tick.  Later when the cpu speeds up,
the kernel will start noting that too many cycles have past since the last
interrupt.  Since this can occasionally happen, the lost tick compensation
code then tries to fix this by incrementing jiffies.  Thus every tick we
end up incrementing jiffies many times, causing timers to expire too
quickly and time to rush ahead.

This patch detects when there has been 100 consecutive interrupts where we
had to compensate for lost ticks.  If this occurs, we spit out a warning
and fall back to using the PIT as a time source.

I've tested this on my speedstep enabled laptop with success, and others
laptop users seeing this problem have reported it works for them.  Also to
ensure we don't fall back to the slower PIT too quickly, I tested the code
on a system I have that looses ~30 ticks about every second and it can
still manage to use the TSC as a good time source.

This solves most of the "time doubling" problems seen on laptops.
Additionally this revision has been modified to use the cleanups made in
rename-timer_A1.

arch/i386/kernel/timers/timer.c

@@ -23,6 +23,15 @@ static int __init clock_setup(char* str)
 }
 __setup("clock=", clock_setup);
 
+
+/* The chosen timesource has been found to be bad.
+ * Fall back to a known good timesource (the PIT)
+ */
+void clock_fallback(void)
+{
+	cur_timer = &timer_pit;
+}
+
 /* iterates through the list of timers, returning the first 
  * one that initializes successfully.
  */

arch/i386/kernel/timers/timer_tsc.c

@@ -124,6 +124,7 @@ static void mark_offset_tsc(void)
 	int countmp;
 	static int count1 = 0;
 	unsigned long long this_offset, last_offset;
+	static int lost_count = 0;
 	
 	write_lock(&monotonic_lock);
 	last_offset = ((unsigned long long)last_tsc_high<<32)|last_tsc_low;
@@ -178,9 +179,19 @@ static void mark_offset_tsc(void)
 	delta += delay_at_last_interrupt;
 	lost = delta/(1000000/HZ);
 	delay = delta%(1000000/HZ);
-	if (lost >= 2)
+	if (lost >= 2) {
 		jiffies += lost-1;
 
+		/* sanity check to ensure we're not always loosing ticks */
+		if (lost_count++ > 100) {
+			printk(KERN_WARNING "Loosing too many ticks!\n");
+			printk(KERN_WARNING "TSC cannot be used as a timesource."
+					" (Are you running with SpeedStep?)\n");
+			printk(KERN_WARNING "Falling back to a sane timesource.\n");
+			clock_fallback();
+		}
+	} else
+		lost_count = 0;
 	/* update the monotonic base value */
 	this_offset = ((unsigned long long)last_tsc_high<<32)|last_tsc_low;
 	monotonic_base += cycles_2_ns(this_offset - last_offset);

include/asm-i386/timer.h

@@ -21,6 +21,7 @@ struct timer_opts{
 #define TICK_SIZE (tick_nsec / 1000)
 
 extern struct timer_opts* select_timer(void);
+extern void clock_fallback(void);
 
 /* Modifiers for buggy PIT handling */