Re: [PATCH V2] tick: Make sleep length calculation more accurate

[Frederic Weisbecker]

On Wed, Oct 02, 2013 at 08:03:39PM +0200, Daniel Lezcano wrote:
> 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@xxxxxxxxxx>:
> >>>>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.

Yeah, why not.

Thanks.

> 
> 
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