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

[Albert Cahalan <albert@users.sf.net>]

On Mon, 2004-08-16 at 20:31, George Anzinger wrote:

> Hm...  That patch was for a reason...  It seems to me that doing anything short 
> of putting "xtime" (or better, clock_gettime() :)) in at fork time is not going 
> to fix anything.   As written the start_time in the task_struct is fixed.  If 
> "now - uptime + time_from_boot_to_process_start" it is wandering, it must be the 
> fault of "now - uptime".  Since this seems to be wandering, and we corrected 
> uptime in the referenced patch, is it safe to assume that "now" is actually 
> being computed from "jiffies" rather than a gettimeofday()?
> 
> Seems like that is where we should be changing things.

That's userspace, which works fine on a 2.4.xx kernel.
If userspace were to change, it wouldn't work OK for
a 2.4.xx kernel anymore. So consider that cast in stone.

"now" is the time() function. Using gettimeofday()
would only make sense if I decided to pay the cost
of asking for the time every time I look at a task.

Here is the "now - uptime + time_from_boot_to_process_start"
calculation, unsimplified, ripped from the procps code:

////////////////////////////////////////////////////////////////
unsigned long   seconds_since_boot = -1;
static unsigned long seconds_since_1970;
static unsigned long time_of_boot;

some_init_function(){
  seconds_since_boot = uptime(0,0);
  seconds_since_1970 = time(NULL);
  time_of_boot = seconds_since_1970 - seconds_since_boot;
}

static 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);
}
////////////////////////////////////////////////////////////////