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

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Message-ID: <524C4845.5060800@linaro.org>
Date:	Wed, 02 Oct 2013 18:22:29 +0200
From:	Daniel Lezcano <daniel.lezcano@...aro.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
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 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 ?

Thanks
   -- Daniel

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