On Thu, 2013-02-21 at 00:39 +0800, buyitian wrote:

i am confused about my test. in one device driver, 
i put below code:

    printk("start to test test jiffies\n");

    local_irq_save(flags);

    jf1 = jiffies; // read jiffies first time

    // hold cpu for about 2 seconds(do some calculation)

    jf2 = jiffies; // read jiffies after 2 seconds

    local_irq_restore(flags);

    printk("jf1:%lu, jf2:%lu\n", jf1, jf2);

and the output is as below:

    <4>[  108.551124]start to test test jiffies
    <4>[  110.367604]jf1:4294948151, jf2:4294948151

the jf1 and jf2 are the same value, although they are
read between 2 seconds interval, i think this is because
i disabled local interrupt.
but the printk timestamp is from 108.551124 to 110.367604,
which is about 2 seconds. and on my platform, printk timestamp
is got from the function read_sched_clock:
   static u32 __read_mostly (*read_sched_clock)(void) = jiffy_sched_clock_read;

and function jiffy_sched_clock_read() is to read from jiffies.

it seems that the jiffies is frozen when local irq is disabled,
but after local_irq_restore(), the jiffies not only start
to run, but also recover the lost 2 seconds.

is the jiffies updated from another cpu when irq is disabled on
local cpu? 

is there some internel processor interrupt between cpu1 and cpu0
after local irq is re-enabled so that jiffies recover the lost 2 seconds? 		 	   		  

I think it is because of the fact that some RTC registers keep the
elapsed time and when the irq is re-enabled the kernel reads these
registers to be in sync with time.
However I am not that sure which registers as I think that depends on
the RTC chip and chip changes based on different boards.
I hope some expert on these matters can pitch in here.

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