Re: [PATCH] i386: Selectable Frequency of the Timer Interrupt

[Jesper Juhl <jesper.juhl@gmail.com>]

[-- Attachment #1: Type: text/plain, Size: 6110 bytes --]

Linus Torvalds wrote:
> 
> On Thu, 14 Jul 2005, Lee Revell wrote:
> 
>>I don't think this will fly because we take a big performance hit by
>>calculating HZ at runtime.
> 
> 
> I think it might be an acceptable solution for a distribution that really 
> needed it, since it should be fairly simple. However, it's definitely not 
> the right solution.
> 
> HOWEVER. I bet that somebody who really really cares (hint hint) could
> easily make HZ be 1000, and then dynamically tweak the divisor at bootup
> to be either 1000, 250, or 100, and then increment "jiffies" by 1, 4 or
> 10.
> 
> My wild guess is that this is 20 lines of code, plus another 20 for 
> "setup", so that you can choose between 100/250/1000 Hz with a kernel 
> command line.
> 
> It wouldn't be "dynamic" at first - you'd just set it up at bootup, and 
> set a "jiffies_increment" variable, and change the
> 
> 	jiffies_64++;
> 
> into
> 
> 	jiffies_64 += jiffies_increment;
> 
> and you'd be done. 
> 
> Really. I dare you guys. First one to send me a tested patch gets a gold 
> star.
> 

I don't know if this will earn me that gold star or not, but it's what I 
have at this point.
It's buggy, that I know. setting kernel_hz (the new boot parameter) to 
250 causes my system clock to run at something like 4-5 times normal 
speed, and key repeat is all funny (super fast) and various other nasty 
things - however, it does build and it does boot (i386 only as that's 
all I have) and apart from the kernels notion of time being way off it 
does seem to be a step in the right direction.
Me never having looked at the kernels timer code before is most likely 
the explanation for the bugs - that and the fact that I didn't try to 
tackle the bogomips calculation at all...  Ohh, and it'll probably bee 
even more strange if you build the kernel with CONFIG_HZ set to anything 
other than 1000 - if this is what we want to do, then I guess CONFIG_HZ 
needs to go away completely and just always be 1000 and then people 
should use the boot option to modify if needed/wanted.

Anyway, is this somewhere along the lines of what you were thinking?

The patch is below, but I've also attached it since I suspect 
thunderbird will mangle it.


diff -upr linux-2.6.13-rc3-orig/arch/i386/kernel/time.c 
linux-2.6.13-rc3/arch/i386/kernel/time.c
--- linux-2.6.13-rc3-orig/arch/i386/kernel/time.c	2005-07-14 
20:33:35.000000000 +0200
+++ linux-2.6.13-rc3/arch/i386/kernel/time.c	2005-07-15 
04:02:50.000000000 +0200
@@ -75,6 +75,7 @@ int pit_latch_buggy;              /* ext
  #include "do_timer.h"

  u64 jiffies_64 = INITIAL_JIFFIES;
+u16 jiffies_increment = 1;

  EXPORT_SYMBOL(jiffies_64);

@@ -481,3 +482,27 @@ void __init time_init(void)

  	time_init_hook();
  }
+
+static int __init jiffies_increment_setup(char *str)
+{
+	printk(KERN_NOTICE "setting up jiffies_increment : ");
+	if (str) {
+		printk("kernel_hz = %s, ", str);
+	} else {
+		printk("kernel_hz is unset, ");
+	}
+	if (!strncmp("100", str, 3)) {
+		jiffies_increment = 10;
+		printk("jiffies_increment set to 10, effective HZ will be 100\n");
+	} else if (!strncmp("250", str, 3)) {
+		jiffies_increment = 4;
+		printk("jiffies_increment set to 4, effective HZ will be 250\n");
+	} else {
+		jiffies_increment = 1;
+		printk("jiffies_increment set to 1, effective HZ will be 1000\n");
+	}
+
+	return 1;
+}
+
+__setup("kernel_hz=", jiffies_increment_setup);
diff -upr linux-2.6.13-rc3-orig/arch/i386/kernel/timers/timer_pm.c 
linux-2.6.13-rc3/arch/i386/kernel/timers/timer_pm.c
--- linux-2.6.13-rc3-orig/arch/i386/kernel/timers/timer_pm.c	2005-07-14 
20:33:35.000000000 +0200
+++ linux-2.6.13-rc3/arch/i386/kernel/timers/timer_pm.c	2005-07-15 
02:59:39.000000000 +0200
@@ -176,7 +176,7 @@ static void mark_offset_pmtmr(void)

  	/* compensate for lost ticks */
  	if (lost >= 2)
-		jiffies_64 += lost - 1;
+		jiffies_64 += (lost * jiffies_increment) - 1;

  	/* don't calculate delay for first run,
  	   or if we've got less then a tick */
diff -upr linux-2.6.13-rc3-orig/arch/i386/kernel/timers/timer_tsc.c 
linux-2.6.13-rc3/arch/i386/kernel/timers/timer_tsc.c
--- linux-2.6.13-rc3-orig/arch/i386/kernel/timers/timer_tsc.c	2005-07-14 
20:33:35.000000000 +0200
+++ linux-2.6.13-rc3/arch/i386/kernel/timers/timer_tsc.c	2005-07-15 
02:59:13.000000000 +0200
@@ -193,7 +193,7 @@ static void mark_offset_tsc_hpet(void)
  	offset = hpet_readl(HPET_T0_CMP) - hpet_tick;
  	if (unlikely(((offset - hpet_last) > hpet_tick) && (hpet_last != 0))) {
  		int lost_ticks = (offset - hpet_last) / hpet_tick;
-		jiffies_64 += lost_ticks;
+		jiffies_64 += lost_ticks * jiffies_increment;
  	}
  	hpet_last = hpet_current;

@@ -415,7 +415,7 @@ static void mark_offset_tsc(void)
  	lost = delta/(1000000/HZ);
  	delay = delta%(1000000/HZ);
  	if (lost >= 2) {
-		jiffies_64 += lost-1;
+		jiffies_64 += (lost * jiffies_increment) - 1;

  		/* sanity check to ensure we're not always losing ticks */
  		if (lost_count++ > 100) {
@@ -448,7 +448,7 @@ static void mark_offset_tsc(void)
  	 * usec delta is > 90% # of usecs/tick)
  	 */
  	if (lost && abs(delay - delay_at_last_interrupt) > (900000/HZ))
-		jiffies_64++;
+		jiffies_64 += jiffies_increment;
  }

  static int __init init_tsc(char* override)
diff -upr linux-2.6.13-rc3-orig/include/linux/jiffies.h 
linux-2.6.13-rc3/include/linux/jiffies.h
--- linux-2.6.13-rc3-orig/include/linux/jiffies.h	2005-06-17 
21:48:29.000000000 +0200
+++ linux-2.6.13-rc3/include/linux/jiffies.h	2005-07-15 
03:42:57.000000000 +0200
@@ -83,6 +83,7 @@
   */
  extern u64 __jiffy_data jiffies_64;
  extern unsigned long volatile __jiffy_data jiffies;
+extern u16 jiffies_increment;

  #if (BITS_PER_LONG < 64)
  u64 get_jiffies_64(void);
diff -upr linux-2.6.13-rc3-orig/kernel/timer.c 
linux-2.6.13-rc3/kernel/timer.c
--- linux-2.6.13-rc3-orig/kernel/timer.c	2005-07-14 20:34:24.000000000 +0200
+++ linux-2.6.13-rc3/kernel/timer.c	2005-07-15 02:55:45.000000000 +0200
@@ -948,7 +948,7 @@ static inline void update_times(void)

  void do_timer(struct pt_regs *regs)
  {
-	jiffies_64++;
+	jiffies_64 += jiffies_increment;
  	update_times();
  }



-- 
Jesper Juhl


[-- Attachment #2: jiffies_interval.patch --]
[-- Type: text/x-patch, Size: 3847 bytes --]

diff -upr linux-2.6.13-rc3-orig/arch/i386/kernel/time.c linux-2.6.13-rc3/arch/i386/kernel/time.c
--- linux-2.6.13-rc3-orig/arch/i386/kernel/time.c	2005-07-14 20:33:35.000000000 +0200
+++ linux-2.6.13-rc3/arch/i386/kernel/time.c	2005-07-15 04:02:50.000000000 +0200
@@ -75,6 +75,7 @@ int pit_latch_buggy;              /* ext
 #include "do_timer.h"
 
 u64 jiffies_64 = INITIAL_JIFFIES;
+u16 jiffies_increment = 1;
 
 EXPORT_SYMBOL(jiffies_64);
 
@@ -481,3 +482,27 @@ void __init time_init(void)
 
 	time_init_hook();
 }
+
+static int __init jiffies_increment_setup(char *str)
+{
+	printk(KERN_NOTICE "setting up jiffies_increment : ");
+	if (str) {
+		printk("kernel_hz = %s, ", str);
+	} else {
+		printk("kernel_hz is unset, ");
+	}
+	if (!strncmp("100", str, 3)) {
+		jiffies_increment = 10;
+		printk("jiffies_increment set to 10, effective HZ will be 100\n");
+	} else if (!strncmp("250", str, 3)) {
+		jiffies_increment = 4;
+		printk("jiffies_increment set to 4, effective HZ will be 250\n");
+	} else {
+		jiffies_increment = 1;
+		printk("jiffies_increment set to 1, effective HZ will be 1000\n");
+	}
+
+	return 1;
+}
+
+__setup("kernel_hz=", jiffies_increment_setup);
diff -upr linux-2.6.13-rc3-orig/arch/i386/kernel/timers/timer_pm.c linux-2.6.13-rc3/arch/i386/kernel/timers/timer_pm.c
--- linux-2.6.13-rc3-orig/arch/i386/kernel/timers/timer_pm.c	2005-07-14 20:33:35.000000000 +0200
+++ linux-2.6.13-rc3/arch/i386/kernel/timers/timer_pm.c	2005-07-15 02:59:39.000000000 +0200
@@ -176,7 +176,7 @@ static void mark_offset_pmtmr(void)
 
 	/* compensate for lost ticks */
 	if (lost >= 2)
-		jiffies_64 += lost - 1;
+		jiffies_64 += (lost * jiffies_increment) - 1;
 
 	/* don't calculate delay for first run,
 	   or if we've got less then a tick */
diff -upr linux-2.6.13-rc3-orig/arch/i386/kernel/timers/timer_tsc.c linux-2.6.13-rc3/arch/i386/kernel/timers/timer_tsc.c
--- linux-2.6.13-rc3-orig/arch/i386/kernel/timers/timer_tsc.c	2005-07-14 20:33:35.000000000 +0200
+++ linux-2.6.13-rc3/arch/i386/kernel/timers/timer_tsc.c	2005-07-15 02:59:13.000000000 +0200
@@ -193,7 +193,7 @@ static void mark_offset_tsc_hpet(void)
 	offset = hpet_readl(HPET_T0_CMP) - hpet_tick;
 	if (unlikely(((offset - hpet_last) > hpet_tick) && (hpet_last != 0))) {
 		int lost_ticks = (offset - hpet_last) / hpet_tick;
-		jiffies_64 += lost_ticks;
+		jiffies_64 += lost_ticks * jiffies_increment;
 	}
 	hpet_last = hpet_current;
 
@@ -415,7 +415,7 @@ static void mark_offset_tsc(void)
 	lost = delta/(1000000/HZ);
 	delay = delta%(1000000/HZ);
 	if (lost >= 2) {
-		jiffies_64 += lost-1;
+		jiffies_64 += (lost * jiffies_increment) - 1;
 
 		/* sanity check to ensure we're not always losing ticks */
 		if (lost_count++ > 100) {
@@ -448,7 +448,7 @@ static void mark_offset_tsc(void)
 	 * usec delta is > 90% # of usecs/tick)
 	 */
 	if (lost && abs(delay - delay_at_last_interrupt) > (900000/HZ))
-		jiffies_64++;
+		jiffies_64 += jiffies_increment;
 }
 
 static int __init init_tsc(char* override)
diff -upr linux-2.6.13-rc3-orig/include/linux/jiffies.h linux-2.6.13-rc3/include/linux/jiffies.h
--- linux-2.6.13-rc3-orig/include/linux/jiffies.h	2005-06-17 21:48:29.000000000 +0200
+++ linux-2.6.13-rc3/include/linux/jiffies.h	2005-07-15 03:42:57.000000000 +0200
@@ -83,6 +83,7 @@
  */
 extern u64 __jiffy_data jiffies_64;
 extern unsigned long volatile __jiffy_data jiffies;
+extern u16 jiffies_increment;
 
 #if (BITS_PER_LONG < 64)
 u64 get_jiffies_64(void);
diff -upr linux-2.6.13-rc3-orig/kernel/timer.c linux-2.6.13-rc3/kernel/timer.c
--- linux-2.6.13-rc3-orig/kernel/timer.c	2005-07-14 20:34:24.000000000 +0200
+++ linux-2.6.13-rc3/kernel/timer.c	2005-07-15 02:55:45.000000000 +0200
@@ -948,7 +948,7 @@ static inline void update_times(void)
 
 void do_timer(struct pt_regs *regs)
 {
-	jiffies_64++;
+	jiffies_64 += jiffies_increment;
 	update_times();
 }