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
The code shown below works great on a 2.4.xx or earlier kernel.
It generally relys on USER_HZ, which is supposedly in our ABI.
I have a feeling we'll forever be chasing bugs related to not
using a PLL to drive the clock tick at exactly HZ ticks per second.
Perhaps the DragonflyBSD code could be stolen. Anyway, the code:
///////////////////////////////////////////////////////////////////////////
unsigned long seconds_since_1970 = time(NULL);
unsigned long seconds_since_boot = uptime(0,0);
unsigned long time_of_boot = seconds_since_1970 - seconds_since_boot;
int pr_stime(char *restrict const outbuf, const proc_t *restrict const pp){
struct tm *proc_time;
struct tm *our_time;
time_t t;
const char *fmt;
int tm_year;
int tm_yday;
our_time = localtime(&seconds_since_1970); /* not reentrant */
tm_year = our_time->tm_year;
tm_yday = our_time->tm_yday;
t = time_of_boot + pp->start_time / Hertz;
proc_time = localtime(&t); /* not reentrant, this corrupts our_time */
fmt = "%H:%M"; /* 03:02 23:59 */
if(tm_yday != proc_time->tm_yday) fmt = "%b%d"; /* Jun06 Aug27 */
if(tm_year != proc_time->tm_year) fmt = "%Y"; /* 1991 2001 */
return strftime(outbuf, 42, fmt, proc_time);
}
///////////////////////////////////////////////////////////////////////////
Albert Cahalan <[email protected]> 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 <[email protected]>
---
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
_