On 2013年02月21日 01:30, Russell King - ARM Linux wrote:
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.
80 /*
81 * Event handler for periodic ticks
82 */
83 void tick_handle_periodic(struct clock_event_device *dev)
84 {
85 int cpu = smp_processor_id();
86 ktime_t next;
87
88 tick_periodic(cpu);
89
90 if (dev->mode != CLOCK_EVT_MODE_ONESHOT)
91 return;
92 /*
93 * Setup the next period for devices, which do not have
94 * periodic mode:
95 */
96 next = ktime_add(dev->next_event, tick_period);
97 for (;;) {
98 if (!clockevents_program_event(dev, next, ktime_get())) <--- once
irq enabled, here we got -ETIME, then
99 return;
100 /*
101 * Have to be careful here. If we're in oneshot mode,
102 * before we call tick_periodic() in a loop, we need
103 * to be sure we're using a real hardware clocksource.
104 * Otherwise we could get trapped in an infinite
105 * loop, as the tick_periodic() increments jiffies,
106 * when then will increment time, posibly causing
107 * the loop to trigger again and again.
108 */
109 if (timekeeping_valid_for_hres())
110 tick_periodic(cpu); <----
here, we add missing jiffies
111 next = ktime_add(next, tick_period);
112 }
113 }
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.
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