[RFC][PATCH] linux-2.5.70_btime-fix_A0

[RFC][PATCH] linux-2.5.70_btime-fix_A0

[john stultz]

All,

	Since jiffies didn't necessarily start incrementing at a second
boundary, jiffies/HZ doesn't increment at the same moment as
xtime.tv_sec. This causes one second wobbles in the calculation of btime
(xtime.tv_sec - jiffies/HZ).  

This fix increases the precision of the calculation so the usec
component of xtime is used as well. Additionally it fixes some of the
non-atomic reading of time values. 


This is a fix for bugme bug #764.
http://bugme.osdl.org/show_bug.cgi?id=764


Let me know if you have any comments

thanks
-john

--- 1.77/fs/proc/proc_misc.c	Sun May 25 14:08:09 2003
+++ edited/fs/proc/proc_misc.c	Tue Jun  3 15:52:41 2003
@@ -378,8 +378,22 @@
 {
 	int i, len;
 	extern unsigned long total_forks;
-	u64 jif = get_jiffies_64() - INITIAL_JIFFIES;
+	u64 jif;
 	unsigned int sum = 0, user = 0, nice = 0, system = 0, idle = 0, iowait = 0;
+	struct timeval now; 
+	unsigned long seq;
+
+	/* Atomically read jiffies and time of day */ 
+	do {
+		seq = read_seqbegin(&xtime_lock);
+
+		jif = get_jiffies_64() - INITIAL_JIFFIES;
+		do_gettimeofday(&now);
+	} while (read_seqretry(&xtime_lock, seq));
+
+	/* calc # of seconds since boot time */
+	jif = ((u64)now.tv_sec * HZ) + (now.tv_usec/(1000000/HZ)) - jif;
+	do_div(jif, HZ);
 
 	for (i = 0 ; i < NR_CPUS; i++) {
 		int j;
@@ -419,7 +433,6 @@
 		len += sprintf(page + len, " %u", kstat_irqs(i));
 #endif
 
-	do_div(jif, HZ);
 	len += sprintf(page + len,
 		"\nctxt %lu\n"
 		"btime %lu\n"
@@ -427,7 +440,7 @@
 		"procs_running %lu\n"
 		"procs_blocked %lu\n",
 		nr_context_switches(),
-		xtime.tv_sec - (unsigned long) jif,
+		(unsigned long)jif,
 		total_forks,
 		nr_running(),
 		nr_iowait());

Re: [RFC][PATCH] linux-2.5.70_btime-fix_A0

[jw schultz]

On Tue, Jun 03, 2003 at 04:00:59PM -0700, john stultz wrote:
> All,
> 
> 	Since jiffies didn't necessarily start incrementing at a second
> boundary, jiffies/HZ doesn't increment at the same moment as
> xtime.tv_sec. This causes one second wobbles in the calculation of btime
> (xtime.tv_sec - jiffies/HZ).  
> 
> This fix increases the precision of the calculation so the usec
> component of xtime is used as well. Additionally it fixes some of the
> non-atomic reading of time values. 
> 
> 
> This is a fix for bugme bug #764.
> http://bugme.osdl.org/show_bug.cgi?id=764
> 
> 
> Let me know if you have any comments

Might it not be cheaper to start jiffies at the 1 second
boundary or with a value that simulates that?

-- 
________________________________________________________________
	J.W. Schultz            Pegasystems Technologies
	email address:		jw@pegasys.ws

		Remember Cernan and Schmitt

Re: [RFC][PATCH] linux-2.5.70_btime-fix_A0

[john stultz]

jw schultz wrote:
> On Tue, Jun 03, 2003 at 04:00:59PM -0700, john stultz wrote:
>> Since jiffies didn't necessarily start incrementing at a second
>> boundary, jiffies/HZ doesn't increment at the same moment as
>> xtime.tv_sec. This causes one second wobbles in the calculation of btime
>> (xtime.tv_sec - jiffies/HZ).
>
> Might it not be cheaper to start jiffies at the 1 second
> boundary or with a value that simulates that?

Ehhh... Mucking with the meaning of jiffies (# of timer ticks) just to fix a
math error in /proc/stat seems like a bit much. 

But I'll look to see if I can improve the math some. 

thanks
-john