system timer: fix crash in <100Hz system timer

The kernel has a divide by zero crash when trying to run the system timer
less than 100Hz.  The problem is x/(HZ/USER_HZ) and related.  Now
x*(USER_HZ/HZ) will be used if HZ<USER_HZ.

I'm running the Linux kernel under qemu and went to run a slower system
timer to take less CPU (and battery) on the host.  I found that the kernel
paniced under emulation because of a divide by zero in three places.  Here
is the patch.  The base git was updated today 01-05-2008.  I went for a
20Hz system time by adding config HZ_20 etc to kernel/Kconfig.hz.  With
this patch I verified the system timer by looking at /proc/interrupts.

[akpm@linux-foundation.org: partially clean up the macro maze]
Signed-off-by: David Fries <david@fries.net>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/acct.h

@@ -173,7 +173,11 @@ typedef struct acct acct_t;
 static inline u32 jiffies_to_AHZ(unsigned long x)
 {
 #if (TICK_NSEC % (NSEC_PER_SEC / AHZ)) == 0
+# if HZ < AHZ
+	return x * (AHZ / HZ);
+# else
 	return x / (HZ / AHZ);
+# endif
 #else
         u64 tmp = (u64)x * TICK_NSEC;
         do_div(tmp, (NSEC_PER_SEC / AHZ));

kernel/time.c

@@ -566,7 +566,11 @@ EXPORT_SYMBOL(jiffies_to_timeval);
 clock_t jiffies_to_clock_t(long x)
 {
 #if (TICK_NSEC % (NSEC_PER_SEC / USER_HZ)) == 0
+# if HZ < USER_HZ
+	return x * (USER_HZ / HZ);
+# else
 	return x / (HZ / USER_HZ);
+# endif
 #else
 	u64 tmp = (u64)x * TICK_NSEC;
 	do_div(tmp, (NSEC_PER_SEC / USER_HZ));
@@ -599,7 +603,12 @@ EXPORT_SYMBOL(clock_t_to_jiffies);
 u64 jiffies_64_to_clock_t(u64 x)
 {
 #if (TICK_NSEC % (NSEC_PER_SEC / USER_HZ)) == 0
+# if HZ < USER_HZ
+	x *= USER_HZ;
+	do_div(x, HZ);
+# else
 	do_div(x, HZ / USER_HZ);
+# endif
 #else
 	/*
 	 * There are better ways that don't overflow early,
@@ -611,7 +620,6 @@ u64 jiffies_64_to_clock_t(u64 x)
 #endif
 	return x;
 }
-
 EXPORT_SYMBOL(jiffies_64_to_clock_t);
 
 u64 nsec_to_clock_t(u64 x)
@@ -646,7 +654,6 @@ u64 get_jiffies_64(void)
 	} while (read_seqretry(&xtime_lock, seq));
 	return ret;
 }
-
 EXPORT_SYMBOL(get_jiffies_64);
 #endif