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. _______________________________________________ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies