On Wed, Oct 02, 2013 at 06:22:29PM +0200, Daniel Lezcano wrote: > On 10/02/2013 05:57 PM, Frederic Weisbecker wrote: > >2013/10/2 Daniel Lezcano <daniel.lezc...@linaro.org>: > >>The sleep_length is computed in the tick_nohz_stop_sched_tick function but > >>it > >>is used later in the code with in between the local irq enabled. > >> > >>cpu_idle_loop > >> tick_nohz_idle_enter [ exits with local irq enabled ] > >> __tick_nohz_idle_enter > >> tick_nohz_stop_sched_tick > >> ... > >> > >> arch_cpu_idle > >> menu_select [ uses here 'sleep_length' ] > >> ... > >> > >>Between the computation of the sleep length and its usage, some interrupts > >>may occur, making the sleep length shorter than actually it is because of > >>the > >>interrupt processing > > > >So, do you mean that the ts->sleep_length would return a value that is too > >long > >given that the CPU already spent some time to service the irqs since we > >computed > >the sleep length in tick_nohz_idle_enter()? > > > >But then tick_nohz_irq_exit() should take care of that as it calls > >again tick_nohz_stop_sched_tick(). > >So I'm a bit confused. > > > >>or different if the timer itself expired. > > > >Same here, if the timer expired, it triggers an interrupt which can do > >two things: > > > >1) reprogram a new timer and this recompute sleep_length > >2) set_need_resched() and then exit the idle loop, so arch_cpu_idle() won't > >even > >be called. Or the timer interrupts hlt, but then menu_select() was > >called before. > > > >So I probably missed something here. > > No you did not :) > > Indeed... At the first glance, this issue sounded so obvious I > suspected there must be a trick somewhere but I did not think to > look at the irq_exit, the code is very complex. Thanks for > clarifying this. > > For my personal information, is there any particular reason to set > an intermediate 'sleep_length' in tick_nohz_stop_sched_tick instead > of doing what does this patch ?
May be we could do it that way yeah. Is menu_select() called only there? I don't know how much difference that would make. > > Thanks > -- Daniel > > -- > <http://www.linaro.org/> Linaro.org │ Open source software for ARM SoCs > > Follow Linaro: <http://www.facebook.com/pages/Linaro> Facebook | > <http://twitter.com/#!/linaroorg> Twitter | > <http://www.linaro.org/linaro-blog/> Blog > -- 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/
