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Subject[RFC PATCH] x86:Use cpu_khz for loops_per_jiffy calculation
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On X86 platform we can use the value of cpu_khz computed during tsc calibration
to calculate the loops_per_jiffy value. Its very important to keep the error in
lpj values to minimum as any error in that may result in kernel panic in
check_timer.
In virtualization environment on a highly overloaded host, the guest delay
calibration may sometimes result in errors beyond the ~50% that timer_irq_works
can handle, resulting in the guest panicking.

This change could also help large MP systems in reducing their booting time.

Patch also does some formating changes to lpj_setup code to now have a single
printk to print the calculated bogomips value.

On top of current git.

Signed-off-by: Alok N Kataria <akataria@vmware.com>

Index: linux-2.6/arch/x86/kernel/time_64.c
===================================================================
--- linux-2.6.orig/arch/x86/kernel/time_64.c 2008-05-23 16:56:24.000000000 -0700
+++ linux-2.6/arch/x86/kernel/time_64.c 2008-06-03 17:28:46.000000000 -0700
@@ -123,6 +123,8 @@
(boot_cpu_data.x86_vendor == X86_VENDOR_AMD))
cpu_khz = calculate_cpu_khz();

+ lpj_tsc = ((unsigned long)cpu_khz * 1000)/HZ;
+
if (unsynchronized_tsc())
mark_tsc_unstable("TSCs unsynchronized");

Index: linux-2.6/include/linux/delay.h
===================================================================
--- linux-2.6.orig/include/linux/delay.h 2008-05-23 16:56:24.000000000 -0700
+++ linux-2.6/include/linux/delay.h 2008-06-03 17:28:46.000000000 -0700
@@ -41,6 +41,7 @@
#define ndelay(x) ndelay(x)
#endif

+extern unsigned long lpj_tsc;
void calibrate_delay(void);
void msleep(unsigned int msecs);
unsigned long msleep_interruptible(unsigned int msecs);
Index: linux-2.6/init/calibrate.c
===================================================================
--- linux-2.6.orig/init/calibrate.c 2008-05-23 16:56:24.000000000 -0700
+++ linux-2.6/init/calibrate.c 2008-06-03 17:28:46.000000000 -0700
@@ -9,6 +9,7 @@
#include <linux/init.h>
#include <linux/timex.h>

+unsigned long lpj_tsc;
unsigned long preset_lpj;
static int __init lpj_setup(char *str)
{
@@ -118,16 +119,14 @@

if (preset_lpj) {
loops_per_jiffy = preset_lpj;
- printk("Calibrating delay loop (skipped)... "
- "%lu.%02lu BogoMIPS preset\n",
- loops_per_jiffy/(500000/HZ),
- (loops_per_jiffy/(5000/HZ)) % 100);
+ printk("Calibrating delay loop skipped, "
+ "using preset value.. ");
+ } else if (lpj_tsc) {
+ loops_per_jiffy = lpj_tsc;
+ printk("Calibrating delay loop skipped, "
+ "using value calculated from tsc.. ");
} else if ((loops_per_jiffy = calibrate_delay_direct()) != 0) {
printk("Calibrating delay using timer specific routine.. ");
- printk("%lu.%02lu BogoMIPS (lpj=%lu)\n",
- loops_per_jiffy/(500000/HZ),
- (loops_per_jiffy/(5000/HZ)) % 100,
- loops_per_jiffy);
} else {
loops_per_jiffy = (1<<12);

@@ -161,12 +160,7 @@
if (jiffies != ticks) /* longer than 1 tick */
loops_per_jiffy &= ~loopbit;
}
-
- /* Round the value and print it */
- printk("%lu.%02lu BogoMIPS (lpj=%lu)\n",
- loops_per_jiffy/(500000/HZ),
- (loops_per_jiffy/(5000/HZ)) % 100,
- loops_per_jiffy);
}
-
+ printk("%lu.%02lu BogoMIPS (lpj=%lu)\n", loops_per_jiffy/(500000/HZ),
+ (loops_per_jiffy/(5000/HZ)) % 100, loops_per_jiffy);
}
Index: linux-2.6/arch/x86/kernel/tsc_32.c
===================================================================
--- linux-2.6.orig/arch/x86/kernel/tsc_32.c 2008-05-27 12:28:16.000000000 -0700
+++ linux-2.6/arch/x86/kernel/tsc_32.c 2008-06-03 17:28:46.000000000 -0700
@@ -401,6 +401,7 @@
void __init tsc_init(void)
{
int cpu;
+ u64 lpj;

if (!cpu_has_tsc || tsc_disabled) {
/* Disable the TSC in case of !cpu_has_tsc */
@@ -421,6 +422,10 @@
return;
}

+ lpj = ((u64)cpu_khz * 1000);
+ do_div(lpj, HZ);
+ lpj_tsc = lpj;
+
printk("Detected %lu.%03lu MHz processor.\n",
(unsigned long)cpu_khz / 1000,
(unsigned long)cpu_khz % 1000);



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