linux-kernel - Re: [PATCHv4 4/4] cpuidle: coupled: add parallel barrier function

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Message-Id: <201205092331.24487.rjw@sisk.pl>
Date:	Wed, 9 May 2012 23:31:24 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Colin Cross <ccross@...roid.com>
Cc:	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-pm@...ts.linux-foundation.org, Kevin Hilman <khilman@...com>,
	Len Brown <len.brown@...el.com>,
	Trinabh Gupta <g.trinabh@...il.com>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Deepthi Dharwar <deepthi@...ux.vnet.ibm.com>,
	"Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
	Kay Sievers <kay.sievers@...y.org>,
	Santosh Shilimkar <santosh.shilimkar@...com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Amit Kucheria <amit.kucheria@...aro.org>,
	Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
	Arnd Bergmann <arnd.bergmann@...aro.org>,
	Russell King <linux@....linux.org.uk>
Subject: Re: [PATCHv4 4/4] cpuidle: coupled: add parallel barrier function

On Tuesday, May 08, 2012, Colin Cross wrote:
> Adds cpuidle_coupled_parallel_barrier, which can be used by coupled
> cpuidle state enter functions to handle resynchronization after
> determining if any cpu needs to abort.  The normal use case will
> be:
> 
> static bool abort_flag;
> static atomic_t abort_barrier;
> 
> int arch_cpuidle_enter(struct cpuidle_device *dev, ...)
> {
> 	if (arch_turn_off_irq_controller()) {
> 	   	/* returns an error if an irq is pending and would be lost
> 		   if idle continued and turned off power */
> 		abort_flag = true;
> 	}
> 
> 	cpuidle_coupled_parallel_barrier(dev, &abort_barrier);
>
> 	if (abort_flag) {
> 	   	/* One of the cpus didn't turn off it's irq controller */
> 	   	arch_turn_on_irq_controller();
> 		return -EINTR;
> 	}
> 
> 	/* continue with idle */
> 	...
> }
> 
> This will cause all cpus to abort idle together if one of them needs
> to abort.
> 
> Reviewed-by: Santosh Shilimkar <santosh.shilimkar@...com>
> Tested-by: Santosh Shilimkar <santosh.shilimkar@...com>
> Reviewed-by: Kevin Hilman <khilman@...com>
> Tested-by: Kevin Hilman <khilman@...com>
> Signed-off-by: Colin Cross <ccross@...roid.com>
> ---
>  drivers/cpuidle/coupled.c |   37 +++++++++++++++++++++++++++++++++++++
>  include/linux/cpuidle.h   |    4 ++++
>  2 files changed, 41 insertions(+), 0 deletions(-)
> 
> diff --git a/drivers/cpuidle/coupled.c b/drivers/cpuidle/coupled.c
> index 93101fb..3e65de1 100644
> --- a/drivers/cpuidle/coupled.c
> +++ b/drivers/cpuidle/coupled.c
> @@ -130,6 +130,43 @@ struct cpuidle_coupled {
>  static cpumask_t cpuidle_coupled_poked_mask;
>  
>  /**
> + * cpuidle_coupled_parallel_barrier - synchronize all online coupled cpus
> + * @dev: cpuidle_device of the calling cpu
> + * @a:   atomic variable to hold the barrier
> + *
> + * No caller to this function will return from this function until all online
> + * cpus in the same coupled group have called this function.  Once any caller
> + * has returned from this function, the barrier is immediately available for
> + * reuse.
> + *
> + * The atomic variable a must be initialized to 0 before any cpu calls
> + * this function, will be reset to 0 before any cpu returns from this function.
> + *
> + * Must only be called from within a coupled idle state handler
> + * (state.enter when state.flags has CPUIDLE_FLAG_COUPLED set).
> + *
> + * Provides full smp barrier semantics before and after calling.
> + */
> +void cpuidle_coupled_parallel_barrier(struct cpuidle_device *dev, atomic_t *a)
> +{
> +	int n = dev->coupled->online_count;
> +
> +	smp_mb__before_atomic_inc();
> +	atomic_inc(a);
> +
> +	while (atomic_read(a) < n)
> +		cpu_relax();
> +
> +	if (atomic_inc_return(a) == n * 2) {
> +		atomic_set(a, 0);
> +		return;
> +	}
> +
> +	while (atomic_read(a) > n)
> +		cpu_relax();
> +}

Well, this looks like "wait until all CPUs execute this code".  Don't we have
anything like this already somewhere?

> +
> +/**
>   * cpuidle_state_is_coupled - check if a state is part of a coupled set
>   * @dev: struct cpuidle_device for the current cpu
>   * @drv: struct cpuidle_driver for the platform
> diff --git a/include/linux/cpuidle.h b/include/linux/cpuidle.h
> index 6038448..5ab7183 100644
> --- a/include/linux/cpuidle.h
> +++ b/include/linux/cpuidle.h
> @@ -183,6 +183,10 @@ static inline int cpuidle_wrap_enter(struct cpuidle_device *dev,
>  
>  #endif
>  
> +#ifdef CONFIG_ARCH_NEEDS_CPU_IDLE_COUPLED
> +void cpuidle_coupled_parallel_barrier(struct cpuidle_device *dev, atomic_t *a);
> +#endif

Why exactly is the extra Kconfig option necessary?

> +
>  /******************************
>   * CPUIDLE GOVERNOR INTERFACE *
>   ******************************/

Rafael
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