Re: too many timer retries happen when do local timer swtich withbroadcast timer

[Jason Liu]

2013/2/21 Thomas Gleixner <tglx@xxxxxxxxxxxxx>:
> On Thu, 21 Feb 2013, Jason Liu wrote:
>> 2013/2/20 Thomas Gleixner <tglx@xxxxxxxxxxxxx>:
>> > On Wed, 20 Feb 2013, Jason Liu wrote:
>> >> void arch_idle(void)
>> >> {
>> >> ....
>> >> clockevents_notify(CLOCK_EVT_NOTIFY_BROADCAST_ENTER, &cpu);
>> >>
>> >> enter_the_wait_mode();
>> >>
>> >> clockevents_notify(CLOCK_EVT_NOTIFY_BROADCAST_EXIT, &cpu);
>> >> }
>> >>
>> >> when the broadcast timer interrupt arrives(this interrupt just wakeup
>> >> the ARM, and ARM has no chance
>> >> to handle it since local irq is disabled. In fact it's disabled in
>> >> cpu_idle() of arch/arm/kernel/process.c)
>> >>
>> >> the broadcast timer interrupt will wake up the CPU and run:
>> >>
>> >> clockevents_notify(CLOCK_EVT_NOTIFY_BROADCAST_EXIT, &cpu);    ->
>> >> tick_broadcast_oneshot_control(...);
>> >> ->
>> >> tick_program_event(dev->next_event, 1);
>> >> ->
>> >> tick_dev_program_event(dev, expires, force);
>> >> ->
>> >> for (i = 0;;) {
>> >>                 int ret = clockevents_program_event(dev, expires, now);
>> >>                 if (!ret || !force)
>> >>                         return ret;
>> >>
>> >>                 dev->retries++;
>> >>                 ....
>> >>                 now = ktime_get();
>> >>                 expires = ktime_add_ns(now, dev->min_delta_ns);
>> >> }
>> >> clockevents_program_event(dev, expires, now);
>> >>
>> >>         delta = ktime_to_ns(ktime_sub(expires, now));
>> >>
>> >>         if (delta <= 0)
>> >>                 return -ETIME;
>> >>
>> >> when the bc timer interrupt arrives,  which means the last local timer
>> >> expires too. so,
>> >> clockevents_program_event will return -ETIME, which will cause the
>> >> dev->retries++
>> >> when retry to program the expired timer.
>> >>
>> >> Even under the worst case, after the re-program the expired timer,
>> >> then CPU enter idle
>> >> quickly before the re-progam timer expired, it will make system
>> >> ping-pang forever,
>> >
>> > That's nonsense.
>>
>> I don't think so.
>>
>> >
>> > The timer IPI brings the core out of the deep idle state.
>> >
>> > So after returning from enter_wait_mode() and after calling
>> > clockevents_notify() it returns from arch_idle() to cpu_idle().
>> >
>> > In cpu_idle() interrupts are reenabled, so the timer IPI handler is
>> > invoked. That calls the event_handler of the per cpu local clockevent
>> > device (the one which stops in C3). That ends up in the generic timer
>> > code which expires timers and reprograms the local clock event device
>> > with the next pending timer.
>> >
>> > So you cannot go idle again, before the expired timers of this event
>> > are handled and their callbacks invoked.
>>
>> That's true for the CPUs which not response to the global timer interrupt.
>> Take our platform as example: we have 4CPUs(CPU0, CPU1,CPU2,CPU3)
>> The global timer device will keep running even in the deep idle mode, so, it
>> can be used as the broadcast timer device, and the interrupt of this device
>> just raised to CPU0 when the timer expired, then, CPU0 will broadcast the
>> IPI timer to other CPUs which is in deep idle mode.
>>
>> So for CPU1, CPU2, CPU3, you are right, the IPI timer will bring it out of idle
>> state, after running clockevents_notify() it returns from arch_idle()
>> to cpu_idle(),
>> then local_irq_enable(), the IPI handler will be invoked and handle
>> the expires times
>> and re-program the next pending timer.
>>
>> But, that's not true for the CPU0. The flow for CPU0 is:
>> the global timer interrupt wakes up CPU0 and then call:
>> clockevents_notify(CLOCK_EVT_NOTIFY_BROADCAST_EXIT, &cpu);
>>
>> which will cpumask_clear_cpu(cpu, tick_get_broadcast_oneshot_mask());
>> in the function tick_broadcast_oneshot_control(),
>
> Now your explanation makes sense.
>
> I have no fast solution for this, but I think that I have an idea how
> to fix it. Stay tuned.

Thanks Thomas, wait for your fix. :)

>
> Thanks,
>
>         tglx
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