linux-kernel - Re: [RFC PATCH 2/2] sched: idle: IRQ based next prediction for idle period

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Date:	Tue, 12 Jan 2016 16:12:12 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Daniel Lezcano <daniel.lezcano@...aro.org>
cc:	peterz@...radead.org, rafael@...nel.org, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org, nicolas.pitre@...aro.org,
	vincent.guittot@...aro.org
Subject: Re: [RFC PATCH 2/2] sched: idle: IRQ based next prediction for idle
 period

On Tue, 12 Jan 2016, Daniel Lezcano wrote:
> On 01/12/2016 03:26 PM, Thomas Gleixner wrote:
> > You better implement the switching part in the cpuidle core first, i.e.
> > proper
> > callbacks when a governor is switched in/out. Then make use of this
> > switcheroo
> > right away. Doing it the other way round is just wrong.
> 
> The problem is this code is not another governor but a 'predictor' where the
> scheduler will use the information to ask the cpuidle to go to a specific idle
> state without going through the governor code, so into the governor's
> callbacks. It is on top of cpuidle. The scheduler will become the governor.
> 
> The current straightforward code, does the switch in the cpu_idle_loop
> idle_task's function:
> 
> [ ... ]
> 
> if (cpu_idle_force_poll || tick_check_broadcast_expired())
> 	cpu_idle_poll();
> else {
> 	if (sched_idle_enabled()) {
> 		int latency = pm_qos_request(PM_QOS_CPU_DMA_LATENCY);
> 		s64 duration = sched_idle_next_wakeup();
> 		sched_idle(duration, latency);
> 	} else {
> 		cpuidle_idle_call();
> 	}
> }
> 
> Due to the complexity of the code, this first step introduce a mechanism to
> predict the next event and re-use it trivially in the idle task.

This looks really wrong. Why on earth don't you implement a proper governor
and just get rid of this extra hackery?

Thanks,

	tglx