test jiffies on ARM SMP board

[Russell King - ARM Linux]

On Wed, Feb 20, 2013 at 10:54:41PM +0530, anish kumar wrote:
> 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

The RTC has nothing to do with this.

As soon as the IRQs are allowed again (immediately after the
local_irq_restore()) the pending interrupt - including the timer
interrupt will be processed.

At this point, because we read the clocksource, we can see that two
seconds have passed, and so we advance jiffies by the elapsed time.

This means printk() sees that the two seconds have passed.  But because
you're reading from jiffies within the interrupt disabled region, that
code can't see the missed ticks.