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Date:	Fri, 19 Sep 2014 14:30:55 -0400 (EDT)
From:	Nicolas Pitre <nicolas.pitre@...aro.org>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...hat.com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org, linaro-kernel@...ts.linaro.org
Subject: Re: [PATCH v2 1/2] sched: let the scheduler see CPU idle states

On Fri, 19 Sep 2014, Peter Zijlstra wrote:

> On Fri, Sep 19, 2014 at 01:17:15AM +0200, Peter Zijlstra wrote:
> > On Thu, Sep 18, 2014 at 02:32:25PM -0400, Nicolas Pitre wrote:
> > > On Thu, 18 Sep 2014, Paul E. McKenney wrote:
> > 
> > > > So what is it that you really need to do here?
> > > 
> > > In short, we don't want the cpufreq data to go away (see the 2 scenarios 
> > > above) while the scheduler is looking at it.  The scheduler uses the 
> > > provided accessors (see patch 2/2) so we can put any protection 
> > > mechanism we want in them.  A simple spinlock could do just as well 
> > > which should be good enough.
> > 
> > rq->lock disables interrupts so on that something like
> > kick_all_cpus_sync() will guarantee what you need --
> > wake_up_all_idle_cpus() will not.
> 
> Something like so then?

I'll trust you for anything that relates to RCU as its subtleties are 
still escaping my mind.

Still, the commit log refers to idle_put_state() which is no more, and 
that should be adjusted.

> 
> ---
> Subject: sched: let the scheduler see CPU idle states
> From: Daniel Lezcano <daniel.lezcano@...aro.org>
> Date: Thu, 04 Sep 2014 11:32:09 -0400
> 
> When the cpu enters idle, it stores the cpuidle state pointer in its
> struct rq instance which in turn could be used to make a better decision
> when balancing tasks.
> 
> As soon as the cpu exits its idle state, the struct rq reference is
> cleared.
> 
> There are a couple of situations where the idle state pointer could be changed
> while it is being consulted:
> 
> 1. For x86/acpi with dynamic c-states, when a laptop switches from battery
>    to AC that could result on removing the deeper idle state. The acpi driver
>    triggers:
> 	'acpi_processor_cst_has_changed'
> 		'cpuidle_pause_and_lock'
> 			'cpuidle_uninstall_idle_handler'
> 				'kick_all_cpus_sync'.
> 
> All cpus will exit their idle state and the pointed object will be set to
> NULL.
> 
> 2. The cpuidle driver is unloaded. Logically that could happen but not
> in practice because the drivers are always compiled in and 95% of them are
> not coded to unregister themselves.  In any case, the unloading code must
> call 'cpuidle_unregister_device', that calls 'cpuidle_pause_and_lock'
> leading to 'kick_all_cpus_sync' as mentioned above.
> 
> A race can happen if we use the pointer and then one of these two scenarios
> occurs at the same moment.
> 
> In order to be safe, the idle state pointer stored in the rq must be
> used inside a rcu_read_lock section where we are protected with the
> 'rcu_barrier' in the 'cpuidle_uninstall_idle_handler' function. The
> idle_get_state() and idle_put_state() accessors should be used to that
> effect.
> 
> Cc: "Rafael J. Wysocki" <rjw@...ysocki.net>
> Cc: Ingo Molnar <mingo@...hat.com>
> Signed-off-by: Daniel Lezcano <daniel.lezcano@...aro.org>
> Signed-off-by: Nicolas Pitre <nico@...aro.org>
> ---
>  drivers/cpuidle/cpuidle.c |    6 ++++++
>  kernel/sched/idle.c       |    6 ++++++
>  kernel/sched/sched.h      |   29 +++++++++++++++++++++++++++++
>  3 files changed, 41 insertions(+)
> 
> --- a/drivers/cpuidle/cpuidle.c
> +++ b/drivers/cpuidle/cpuidle.c
> @@ -225,6 +225,12 @@ void cpuidle_uninstall_idle_handler(void
>  		initialized = 0;
>  		wake_up_all_idle_cpus();
>  	}
> +
> +	/*
> +	 * Make sure external observers (such as the scheduler)
> +	 * are done looking at pointed idle states.
> +	 */
> +	kick_all_cpus_sync();
>  }
>  
>  /**
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -147,6 +147,9 @@ static void cpuidle_idle_call(void)
>  	    clockevents_notify(CLOCK_EVT_NOTIFY_BROADCAST_ENTER, &dev->cpu))
>  		goto use_default;
>  
> +	/* Take note of the planned idle state. */
> +	idle_set_state(this_rq(), &drv->states[next_state]);
> +
>  	/*
>  	 * Enter the idle state previously returned by the governor decision.
>  	 * This function will block until an interrupt occurs and will take
> @@ -154,6 +157,9 @@ static void cpuidle_idle_call(void)
>  	 */
>  	entered_state = cpuidle_enter(drv, dev, next_state);
>  
> +	/* The cpu is no longer idle or about to enter idle. */
> +	idle_set_state(this_rq(), NULL);
> +
>  	if (broadcast)
>  		clockevents_notify(CLOCK_EVT_NOTIFY_BROADCAST_EXIT, &dev->cpu);
>  
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -14,6 +14,7 @@
>  #include "cpuacct.h"
>  
>  struct rq;
> +struct cpuidle_state;
>  
>  /* task_struct::on_rq states: */
>  #define TASK_ON_RQ_QUEUED	1
> @@ -640,6 +641,11 @@ struct rq {
>  #ifdef CONFIG_SMP
>  	struct llist_head wake_list;
>  #endif
> +
> +#ifdef CONFIG_CPU_IDLE
> +	/* Must be inspected within a rcu lock section */
> +	struct cpuidle_state *idle_state;
> +#endif
>  };
>  
>  static inline int cpu_of(struct rq *rq)
> @@ -1193,6 +1199,29 @@ static inline void idle_exit_fair(struct
>  
>  #endif
>  
> +#ifdef CONFIG_CPU_IDLE
> +static inline void idle_set_state(struct rq *rq,
> +				  struct cpuidle_state *idle_state)
> +{
> +	rq->idle_state = idle_state;
> +}
> +
> +static inline struct cpuidle_state *idle_get_state(struct rq *rq)
> +{
> +	return rq->idle_state;
> +}
> +#else
> +static inline void idle_set_state(struct rq *rq,
> +				  struct cpuidle_state *idle_state)
> +{
> +}
> +
> +static inline struct cpuidle_state *idle_get_state(struct rq *rq)
> +{
> +	return NULL;
> +}
> +#endif
> +
>  extern void sysrq_sched_debug_show(void);
>  extern void sched_init_granularity(void);
>  extern void update_max_interval(void);
> 
> 
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