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

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Date:	Wed, 2 Oct 2013 18:42:23 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Stephen Boyd <sboyd@...eaurora.org>,
	LKML <linux-kernel@...r.kernel.org>, patches@...aro.org,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>
Subject: Re: [PATCH V2] tick: Make sleep length calculation more accurate

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@...aro.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
> 
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