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SubjectDrift when measuring time using add_timer
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Hello All,

I am testing the timers on a OMAP4 based board. I am seeing a drift or
the time difference between the timer value and when the function callback
happens. This drift is around 1000ms or 1s for a expiry value of >=5mins.
Is the time drift expected? Is this way of measuring right or is there
any other way?

OMAP4 as you may know is a multi-core processor and CONFIG_SMP is enabled
along with the following:
CONFIG_NO_HZ, CONFIG_HIGH_RES_TIMERS

My kernel configurations:
kernel: 2.6.37-rc7 (commit id: ffc96d)
config: omap2plus_defconfig

My driver does the following:

struct timeval tv;
test_timer_proc_write()
{
del_timer_sync(&my_timer);
period = 300000; /* 5min */
my_timer.expires = jiffies + msecs_to_jiffies(period);
my_timer.function = func_timeout; /* Call back function*/
/* Save the current time in global variable */
do_gettimeofday(&timeval);
/* add timer with an expiry value of 5mins*/
add_timer(&my_timer);
}
/* Get the current time and print the difference between the two values */
func_timeout()
{
struct timeval tv1;
do_gettimeofday(&tv);
/* take the difference between current time and saved time */
ms = (tv.tv_sec - tv1.tv_sec) * 1000 + \
(tv.tv_usec - tv1.tv_usec) / 1000;
printk("Interval = %d\n", ms);
}
Thanks for your time.

Regards,
Mohan


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