Re: boot time, process start time, and NOW time

[OGAWA Hirofumi]

Albert Cahalan <albert@xxxxxxxxxxxx> writes:

> Even with the 2.6.7 kernel, I'm still getting reports of process
> start times wandering. Here is an example:
> 
>    "About 12 hours since reboot to 2.6.7 there was already a
>    difference of about 7 seconds between the real start time
>    and the start time reported by ps. Now, 24 hours since reboot
>    the difference is 10 seconds."
> 
> The calculation used is:
> 
>    now - uptime + time_from_boot_to_process_start

Start-time and uptime is using different source. Looks like the
jiffies was added bogus lost counts.

quick hack. Does this change the behavior?
-- 
OGAWA Hirofumi <hirofumi@xxxxxxxxxxxxxxxxxx>

---

 arch/i386/kernel/smpboot.c          |   16 +++++++++-------
 arch/i386/kernel/timers/timer_tsc.c |    8 ++++++--
 include/linux/time.h                |    2 +-
 3 files changed, 16 insertions(+), 10 deletions(-)

diff -puN arch/i386/kernel/timers/timer_tsc.c~uptime-fix arch/i386/kernel/timers/timer_tsc.c
--- linux-2.6.7/arch/i386/kernel/timers/timer_tsc.c~uptime-fix	2004-06-29 01:21:26.000000000 +0900
+++ linux-2.6.7-hirofumi/arch/i386/kernel/timers/timer_tsc.c	2004-06-29 01:21:26.000000000 +0900
@@ -467,8 +467,6 @@ static int __init init_tsc(char* overrid
  	 *	moaned if you have the only one in the world - you fix it!
  	 */
 
-	count2 = LATCH; /* initialize counter for mark_offset_tsc() */
-
 	if (cpu_has_tsc) {
 		unsigned long tsc_quotient;
 #ifdef CONFIG_HPET_TIMER
@@ -512,6 +510,12 @@ static int __init init_tsc(char* overrid
 				printk("Detected %lu.%03lu MHz processor.\n", cpu_khz / 1000, cpu_khz % 1000);
 			}
 			set_cyc2ns_scale(cpu_khz/1000);
+
+			/* initialize for mark_offset_tsc() */
+			count2 = LATCH;
+			rdtsc(last_tsc_low, last_tsc_high);
+			printk("initial tsc: %lu.%lu\n",
+				last_tsc_high, last_tsc_low);
 			return 0;
 		}
 	}
diff -puN include/linux/time.h~uptime-fix include/linux/time.h
--- linux-2.6.7/include/linux/time.h~uptime-fix	2004-06-29 01:21:26.000000000 +0900
+++ linux-2.6.7-hirofumi/include/linux/time.h	2004-06-29 01:21:26.000000000 +0900
@@ -41,7 +41,7 @@ struct timezone {
  * Have the 32 bit jiffies value wrap 5 minutes after boot
  * so jiffies wrap bugs show up earlier.
  */
-#define INITIAL_JIFFIES ((unsigned long)(unsigned int) (-300*HZ))
+#define INITIAL_JIFFIES		(-300L*HZ)
 
 /*
  * Change timeval to jiffies, trying to avoid the
diff -puN arch/i386/kernel/smpboot.c~uptime-fix arch/i386/kernel/smpboot.c
--- linux-2.6.7/arch/i386/kernel/smpboot.c~uptime-fix	2004-06-29 01:25:59.000000000 +0900
+++ linux-2.6.7-hirofumi/arch/i386/kernel/smpboot.c	2004-06-29 01:34:55.000000000 +0900
@@ -210,6 +210,8 @@ static unsigned long long __init div64 (
 	return res;
 }
 
+static unsigned long __initdata sync_tsc_high, sync_tsc_low;
+
 static void __init synchronize_tsc_bp (void)
 {
 	int i;
@@ -251,11 +253,6 @@ static void __init synchronize_tsc_bp (v
 		atomic_inc(&tsc_count_start);
 
 		rdtscll(tsc_values[smp_processor_id()]);
-		/*
-		 * We clear the TSC in the last loop:
-		 */
-		if (i == NR_LOOPS-1)
-			write_tsc(0, 0);
 
 		/*
 		 * Wait for all APs to leave the synchronization point:
@@ -264,8 +261,14 @@ static void __init synchronize_tsc_bp (v
 			mb();
 		atomic_set(&tsc_count_start, 0);
 		wmb();
+
+		/* We save the TSC in the last loop: */
+		if (i == NR_LOOPS-1)
+			rdtsc(sync_tsc_low, sync_tsc_high);
+
 		atomic_inc(&tsc_count_stop);
 	}
+	write_tsc(sync_tsc_low, sync_tsc_high);
 
 	sum = 0;
 	for (i = 0; i < NR_CPUS; i++) {
@@ -323,12 +326,11 @@ static void __init synchronize_tsc_ap (v
 			mb();
 
 		rdtscll(tsc_values[smp_processor_id()]);
-		if (i == NR_LOOPS-1)
-			write_tsc(0, 0);
 
 		atomic_inc(&tsc_count_stop);
 		while (atomic_read(&tsc_count_stop) != num_booting_cpus()) mb();
 	}
+	write_tsc(sync_tsc_low, sync_tsc_high);
 }
 #undef NR_LOOPS
 

_
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