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Date:	Thu, 18 Sep 2014 10:37:33 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Nicolas Pitre <nicolas.pitre@...aro.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	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 Thu, Sep 04, 2014 at 11:32:09AM -0400, Nicolas Pitre wrote:
> From: Daniel Lezcano <daniel.lezcano@...aro.org>
> 
> 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.
> 
> 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      | 39 +++++++++++++++++++++++++++++++++++++++
>  3 files changed, 51 insertions(+)
> 
> diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c
> index ee9df5e3f5..530e3055a2 100644
> --- a/drivers/cpuidle/cpuidle.c
> +++ b/drivers/cpuidle/cpuidle.c
> @@ -225,6 +225,12 @@ void cpuidle_uninstall_idle_handler(void)
>  		initialized = 0;
>  		kick_all_cpus_sync();
>  	}
> +
> +	/*
> +	 * Make sure external observers (such as the scheduler)
> +	 * are done looking at pointed idle states.
> +	 */
> +	rcu_barrier();

Actually, all rcu_barrier() does is to make sure that all previously
queued RCU callbacks have been invoked.  And given the current
implementation, if there are no callbacks queued anywhere in the system,
rcu_barrier() is an extended no-op.  "Has CPU 0 any callbacks?" "Nope!"
"Has CPU 1 any callbacks?"  "Nope!" ... "Has CPU nr_cpu_ids-1 any
callbacks?"  "Nope!"  "OK, done!"

This is all done with the current task looking at per-CPU data structures,
with no interaction with the scheduler and with no need to actually make
those other CPUs do anything.

So what is it that you really need to do here?

A synchronize_sched() will wait for all non-idle online CPUs to pass
through the scheduler, where "idle" includes usermode execution in
CONFIG_NO_HZ_FULL=y kernels.  But it won't wait for CPUs executing
in the idle loop.

A synchronize_rcu_tasks() will wait for all non-idle tasks that are
currently on a runqueue to do a voluntary context switch.  There has
been some discussion about extending this to idle tasks, but the current
prospective users can live without this.  But if you need it, I can push
on getting it set up.  (Current plans are that synchronize_rcu_tasks()
goes into the v3.18 merge window.)  And one caveat: There is long
latency associated with synchronize_rcu_tasks() by design.  Grace
periods are measured in seconds.

A stop_cpus() will force a context switch on all CPUs, though it is
a rather big hammer.

So again, what do you really need?

							Thanx, Paul

>  }
> 
>  /**
> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> index 11e7bc434f..c47fce75e6 100644
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -147,6 +147,9 @@ use_default:
>  	    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 @@ use_default:
>  	 */
>  	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);
> 
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 579712f4e9..aea8baa7a5 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -14,6 +14,7 @@
>  #include "cpuacct.h"
> 
>  struct rq;
> +struct cpuidle_state;
> 
>  extern __read_mostly int scheduler_running;
> 
> @@ -636,6 +637,11 @@ 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)
> @@ -1180,6 +1186,39 @@ static inline void idle_exit_fair(struct rq *rq) { }
> 
>  #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)
> +{
> +	rcu_read_lock();
> +	return rq->idle_state;
> +}
> +
> +static inline void cpuidle_put_state(struct rq *rq)
> +{
> +	rcu_read_unlock();
> +}
> +#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;
> +}
> +
> +static inline void cpuidle_put_state(struct rq *rq)
> +{
> +}
> +#endif
> +
>  extern void sysrq_sched_debug_show(void);
>  extern void sched_init_granularity(void);
>  extern void update_max_interval(void);
> -- 
> 1.8.4.108.g55ea5f6
> 
> --
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