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

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Date:	Wed, 02 Oct 2013 20:03:39 +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 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@...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.

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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