Date:	Wed, 12 Feb 2014 13:45:01 +0100
From:	Daniel Lezcano <daniel.lezcano@...aro.org>
To:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
CC:	mingo@...nel.org, peterz@...radead.org, tglx@...utronix.de,
	rjw@...ysocki.net, nicolas.pitre@...aro.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/5] idle: Reorganize the idle loop

On 02/12/2014 12:00 PM, Preeti U Murthy wrote:
> Hi Daniel,
>
> Find below a couple of comments.
>
> On 02/11/2014 08:41 PM, Daniel Lezcano wrote:
>> Now that we have the main cpuidle function in idle.c, move some code from
>> the idle mainloop to this function for the sake of clarity.
>>
>> That removes if then else indentation difficult to follow when looking at the
>> code. This patch does not the change the current behavior.
>>
>> Signed-off-by: Daniel Lezcano <daniel.lezcano@...aro.org>
>> ---
>>   include/linux/cpuidle.h |    2 ++
>>   kernel/sched/idle.c     |   39 ++++++++++++++++++++-------------------
>>   2 files changed, 22 insertions(+), 19 deletions(-)
>>
>> diff --git a/include/linux/cpuidle.h b/include/linux/cpuidle.h
>> index 0befaff..a8d5bd3 100644
>> --- a/include/linux/cpuidle.h
>> +++ b/include/linux/cpuidle.h
>> @@ -175,6 +175,8 @@ static inline int cpuidle_enable_device(struct cpuidle_device *dev)
>>   {return -ENODEV; }
>>   static inline void cpuidle_disable_device(struct cpuidle_device *dev) { }
>>   static inline int cpuidle_play_dead(void) {return -ENODEV; }
>> +static inline struct cpuidle_driver *cpuidle_get_cpu_driver(
>> +	struct cpuidle_device *dev) {return NULL; }
>>   #endif
>>
>>   #ifdef CONFIG_ARCH_NEEDS_CPU_IDLE_COUPLED
>> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
>> index 6963822..8b10891 100644
>> --- a/kernel/sched/idle.c
>> +++ b/kernel/sched/idle.c
>> @@ -63,7 +63,6 @@ void __weak arch_cpu_idle(void)
>>   	local_irq_enable();
>>   }
>>
>> -#ifdef CONFIG_CPU_IDLE
>>   /**
>>    * cpuidle_idle_call - the main idle function
>>    *
>> @@ -76,17 +75,26 @@ static int cpuidle_idle_call(void)
>>   	struct cpuidle_driver *drv = cpuidle_get_cpu_driver(dev);
>>   	int next_state, entered_state;
>>
>> -	/* ask the governor for the next state */
>> +	stop_critical_timings();
>> +	rcu_idle_enter();
>> +
>> +	/* Ask the governor for the next state, this call can fail for
>> +	 * different reasons: cpuidle is not enabled or an idle state
>> +	 * fulfilling the constraints was not found. In this case, we fall
>> +	 * back to the default idle function
>> +	 */
>>   	next_state = cpuidle_select(drv, dev);
>> -	if (next_state < 0)
>> -		return next_state;
>> +	if (next_state < 0) {
>> +		arch_cpu_idle();
>> +		goto out;
>> +	}
>>
>>   	if (need_resched()) {
>>   		dev->last_residency = 0;
>>   		/* give the governor an opportunity to reflect on the outcome */
>>   		cpuidle_reflect(dev, next_state);
>>   		local_irq_enable();
>> -		return 0;
>> +		goto out;
>>   	}
>>
>>   	trace_cpu_idle_rcuidle(next_state, dev->cpu);
>> @@ -97,15 +105,15 @@ static int cpuidle_idle_call(void)
>>
>>   	/* give the governor an opportunity to reflect on the outcome */
>>   	cpuidle_reflect(dev, entered_state);
>> +out:
>> +	if (WARN_ON_ONCE(irqs_disabled()))
>> +		local_irq_enable();
>> +
>> +	rcu_idle_exit();
>> +	start_critical_timings();
>>
>>   	return 0;
>>   }
>> -#else
>> -static inline int cpuidle_idle_call(void)
>> -{
>> -	return -ENODEV;
>> -}
>> -#endif
>>
>>   /*
>>    * Generic idle loop implementation
>> @@ -138,14 +146,7 @@ static void cpu_idle_loop(void)
>>   				cpu_idle_poll();
>>   			} else {
>>   				if (!current_clr_polling_and_test()) {
>> -					stop_critical_timings();
>> -					rcu_idle_enter();
>> -					if (cpuidle_idle_call())
>> -						arch_cpu_idle();
>> -					if (WARN_ON_ONCE(irqs_disabled()))
>> -						local_irq_enable();
>> -					rcu_idle_exit();
>> -					start_critical_timings();
>> +					cpuidle_idle_call();
>>   				} else {
>>   					local_irq_enable();
>>   				}
>
> Is this really necessary? It seems better to let the cpuidle_idle_loop()
> to handle the cpuidle specific tasks and the generic idle loop to handle
> the peripheral functions like stop_critical_timings(), rcu_idle_enter()
> and their counterparts. I am unable to see what we are gaining by doing
> this.
>
> Besides, cpuidle_idle_call() is expected to just call into the cpuidle
> governor and driver. What I would have expected this patchset to do is
> simply move this call under kernel/sched like you have done in your
> first two patches so as to gain the benefit of using the parameters that
> the scheduler keeps track of to make better cpuidle state entry decisions.
>
> But going one step too far by moving the other idle handling functions
> into it would result in some confusion and add more code than it is
> meant to handle. This will avoid having to add  comments in the
> cpuidle_idle_call() function as currently being done in Patch[5/5], to
> clarify what each function is meant to do.
>
> So IMO, Patches[1/5] and [2/5] by themselves are sufficient to increase
> the proximity between scheduler and cpuidle.

May be patches[1/5] and [2/5] are enough to increase the proximity but 
the next 3 patches do clearly simplify the code readability.

 From my POV, the conditions in the idle loop are much more confusing 
than making a linear code in cpuidle_idle_call.

What I agree on is the confusion around the cpuidle_idle_call function 
name which does not reflect what it does after these patches and maybe 
we should rename 'cpuidle_idle_call' to 'idle()' (or whatever).


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