counting clock ticks linux 2.4.14

Question

hello
I am trying to count the total number of clock ticks for each process (only when its actually running).
I inserted the following code in schedule() (sched.h):

...  
switch_tasks:  
 prefetch(next);  
 clear_tsk_need_resched(prev);  
 if (likely(prev != next)) {  
        rq->nr_switches++;  
        rq->curr = next;  
            /*my code start here*/  
        if(next->start_count==1)  next->start_run=jiffies;  
        if(prev->start_count==1)  
        {  
            prev->total_running += (jiffies-prev->start_run);  
                    printk("total running = %lu, jif-start = %lu\n", \  
                    prev->total_running, jiffies-prev->start_run);  
        }  
...  

I added the printk because I got weird results. here is some of the output:
total running = 1522, jif-start = 1
total running = 1522, jif-start = 0
total running = 1523, jif-start = 1
total running = 1, jif-start = 1
total running = 0, jif-start = 0
total running = 0, jif-start = 0
total running = 0, jif-start = 0
total running = 0, jif-start = 0

It does not make sense to me....
is something wrong with my code?

Answer 1

The trace output is showing per-task state, so it might make more sense if you also printed prev->pid to identify which task you're talking about.

EDIT: OK, if you're concerned about getting "jif-start = 0" : note that jiffies only increments on every timer interrupt, which I think is almost certainly every 10ms for a 2.4.14 kernel. It's quite possible (or even likely) that your I/O-bound processes could wake up, and then block on I/O very quickly, causing another reschedule before the jiffy counter increases.

It's possible that you may be able to get higher-resolution time intervals from do_gettimeofday() (declared in<linux/time.h>) but the actual resolution you'll get out of that depends on the platform.

Answer 2

output with pid:
proc no. 3238, total running = 82, jif-start = 1
proc no. 3246, total running = 0, jif-start = 0
proc no. 3242, total running = 0, jif-start = 0
proc no. 3245, total running = 0, jif-start = 0
proc no. 3244, total running = 0, jif-start = 0
proc no. 3243, total running = 0, jif-start = 0
proc no. 3238, total running = 83, jif-start = 1
proc no. 3238, total running = 84, jif-start = 1
proc no. 3246, total running = 0, jif-start = 0

proc. 3242-3246 - I/O bound processes
proc. 3238 - CPU bound process