linux-kernel - Drift when measuring time using add

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Message-ID: <AANLkTim4zbBvTyc4hNurCmPN7GisXe1mmzR6P4aj3xzA@mail.gmail.com>
Date:	Tue, 28 Dec 2010 17:35:22 +0530
From:	Mohan V <mohanvforum@...il.com>
To:	linux-kernel@...r.kernel.org
Subject: Drift when measuring time using add_timer

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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