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. Signed-off-by: David Fries <[EMAIL PROTECTED]> -- David Fries <[EMAIL PROTECTED]> http://fries.net/~david/ (PGP encryption key available) diff --git a/include/linux/acct.h b/include/linux/acct.h index 302eb72..86b848d 100644 --- a/include/linux/acct.h +++ b/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 - return x / (HZ / AHZ); + #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)); diff --git a/kernel/time.c b/kernel/time.c index 09d3c45..23af26f 100644 --- a/kernel/time.c +++ b/kernel/time.c @@ -565,7 +565,11 @@ 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)); @@ -598,7 +602,12 @@ EXPORT_SYMBOL(clock_t_to_jiffies); u64 jiffies_64_to_clock_t(u64 x) { #if (TICK_NSEC % (NSEC_PER_SEC / USER_HZ)) == 0 - do_div(x, HZ / USER_HZ); + #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,
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