Messages in this thread | | | Date | Wed, 02 Oct 2013 20:03:39 +0200 | From | Daniel Lezcano <> | Subject | Re: [PATCH V2] tick: Make sleep length calculation more accurate |
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On 10/02/2013 06:42 PM, Frederic Weisbecker wrote: > 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.lezcano@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.
Yes, it is called just one time in all the code. The benefit would be just to cleanup a field in the struct tick_sched.
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