Hi Jason, On Thu, Feb 21, 2013 at 06:16:51AM +0000, Jason Liu wrote: > 2013/2/20 Thomas Gleixner <t...@linutronix.de>: > > 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(),
For my own understanding: at this point in time CPU0 local timer is also reprogrammed, with min_delta (ie 1us) if I got your description right. > > After return from clockevents_notify(), it will return to cpu_idle > from arch_idle, > then local_irq_enable(), the CPU0 will response to the global timer > interrupt, and > call the interrupt handler: tick_handle_oneshot_broadcast() > > static void tick_handle_oneshot_broadcast(struct clock_event_device *dev) > { > struct tick_device *td; > ktime_t now, next_event; > int cpu; > > raw_spin_lock(&tick_broadcast_lock); > again: > dev->next_event.tv64 = KTIME_MAX; > next_event.tv64 = KTIME_MAX; > cpumask_clear(to_cpumask(tmpmask)); > now = ktime_get(); > /* Find all expired events */ > for_each_cpu(cpu, tick_get_broadcast_oneshot_mask()) { > td = &per_cpu(tick_cpu_device, cpu); > if (td->evtdev->next_event.tv64 <= now.tv64) > cpumask_set_cpu(cpu, to_cpumask(tmpmask)); > else if (td->evtdev->next_event.tv64 < next_event.tv64) > next_event.tv64 = td->evtdev->next_event.tv64; > } > > /* > * Wakeup the cpus which have an expired event. > */ > tick_do_broadcast(to_cpumask(tmpmask)); > ... > } > > since cpu0 has been removed from the tick_get_broadcast_oneshot_mask(), and if > all the other cpu1/2/3 state in idle, and no expired timers, then the > tmpmask will be 0, > when call tick_do_broadcast(). > > static void tick_do_broadcast(struct cpumask *mask) > { > int cpu = smp_processor_id(); > struct tick_device *td; > > /* > * Check, if the current cpu is in the mask > */ > if (cpumask_test_cpu(cpu, mask)) { > cpumask_clear_cpu(cpu, mask); > td = &per_cpu(tick_cpu_device, cpu); > td->evtdev->event_handler(td->evtdev); > } > > if (!cpumask_empty(mask)) { > /* > * It might be necessary to actually check whether the devices > * have different broadcast functions. For now, just use the > * one of the first device. This works as long as we have this > * misfeature only on x86 (lapic) > */ > td = &per_cpu(tick_cpu_device, cpumask_first(mask)); > td->evtdev->broadcast(mask); > } > } > > If the mask is empty, then tick_do_broadcast will do nothing and return, which > will make cpu0 enter idle quickly, and then system will ping-pang there. This means that the local timer reprogrammed above (to actually emulate the expired local timer on CPU0, likely to be set to min_delta == 1us) does not have time to expire before the idle thread disables IRQs and goes idle again. Is this a correct description of what's happening ? Thanks a lot, Lorenzo -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/