test jiffies on ARM SMP board

[anish kumar]

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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