linux-kernel - Re: [Patch] idle governor: Avoid lock acquisition to read pm

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Message-Id: <201102112022.25637.rjw@sisk.pl>
Date:	Fri, 11 Feb 2011 20:22:25 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Tim Chen <tim.c.chen@...ux.intel.com>
Cc:	mark gross <markgross@...gnar.org>,
	James Bottomley <James.Bottomley@...e.de>,
	David Alan Gilbert <linux@...blig.org>,
	linux-kernel@...r.kernel.org, Len <len.brown@...el.com>,
	Andi Kleen <ak@...ux.intel.com>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Len Brown <lenb@...nel.org>,
	Linux PM mailing list <linux-pm@...ts.linux-foundation.org>
Subject: Re: [Patch] idle governor: Avoid lock acquisition to read pm_qos before entering idle

On Friday, February 11, 2011, Tim Chen wrote:
> On Fri, 2011-02-11 at 00:17 +0100, Rafael J. Wysocki wrote:
> 
> > > +static inline int pm_qos_set_value(struct pm_qos_object *o, s32 value)
> > > +{
> > > +	o->value = value;
> > > +}
> > 
> > This requires a fixup, the above function is supposed to return an int.
> > 
> > Also, please add a comment about the requisite 32-bitness as suggested by James.
> > 
> > Thanks,
> > Rafael
> 
> I've fixed the above function, and also included a comment warning
> people not to change target value to 64 bit.  

Thanks, but I'd put that comment before the structure definition with the
"NOTE:" prefix and I think you should explain the reason why that field
shouldn't be changed to 64-bits.

Thanks,
Rafael


> Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>
> diff --git a/include/linux/pm_qos_params.h b/include/linux/pm_qos_params.h
> index 77cbddb..a7d87f9 100644
> --- a/include/linux/pm_qos_params.h
> +++ b/include/linux/pm_qos_params.h
> @@ -16,6 +16,10 @@
>  #define PM_QOS_NUM_CLASSES 4
>  #define PM_QOS_DEFAULT_VALUE -1
>  
> +#define PM_QOS_CPU_DMA_LAT_DEFAULT_VALUE	(2000 * USEC_PER_SEC)
> +#define PM_QOS_NETWORK_LAT_DEFAULT_VALUE	(2000 * USEC_PER_SEC)
> +#define PM_QOS_NETWORK_THROUGHPUT_DEFAULT_VALUE	0
> +
>  struct pm_qos_request_list {
>  	struct plist_node list;
>  	int pm_qos_class;
> diff --git a/kernel/pm_qos_params.c b/kernel/pm_qos_params.c
> index aeaa7f8..7b6333b 100644
> --- a/kernel/pm_qos_params.c
> +++ b/kernel/pm_qos_params.c
> @@ -58,6 +58,7 @@ struct pm_qos_object {
>  	struct blocking_notifier_head *notifiers;
>  	struct miscdevice pm_qos_power_miscdev;
>  	char *name;
> +	s32 target_value;	/* Do not change this to 64 bit */
>  	s32 default_value;
>  	enum pm_qos_type type;
>  };
> @@ -70,7 +71,8 @@ static struct pm_qos_object cpu_dma_pm_qos = {
>  	.requests = PLIST_HEAD_INIT(cpu_dma_pm_qos.requests, pm_qos_lock),
>  	.notifiers = &cpu_dma_lat_notifier,
>  	.name = "cpu_dma_latency",
> -	.default_value = 2000 * USEC_PER_SEC,
> +	.target_value = PM_QOS_CPU_DMA_LAT_DEFAULT_VALUE,
> +	.default_value = PM_QOS_CPU_DMA_LAT_DEFAULT_VALUE,
>  	.type = PM_QOS_MIN,
>  };
>  
> @@ -79,7 +81,8 @@ static struct pm_qos_object network_lat_pm_qos = {
>  	.requests = PLIST_HEAD_INIT(network_lat_pm_qos.requests, pm_qos_lock),
>  	.notifiers = &network_lat_notifier,
>  	.name = "network_latency",
> -	.default_value = 2000 * USEC_PER_SEC,
> +	.target_value = PM_QOS_NETWORK_LAT_DEFAULT_VALUE,
> +	.default_value = PM_QOS_NETWORK_LAT_DEFAULT_VALUE,
>  	.type = PM_QOS_MIN
>  };
>  
> @@ -89,7 +92,8 @@ static struct pm_qos_object network_throughput_pm_qos = {
>  	.requests = PLIST_HEAD_INIT(network_throughput_pm_qos.requests, pm_qos_lock),
>  	.notifiers = &network_throughput_notifier,
>  	.name = "network_throughput",
> -	.default_value = 0,
> +	.target_value = PM_QOS_NETWORK_THROUGHPUT_DEFAULT_VALUE,
> +	.default_value = PM_QOS_NETWORK_THROUGHPUT_DEFAULT_VALUE,
>  	.type = PM_QOS_MAX,
>  };
>  
> @@ -132,6 +136,16 @@ static inline int pm_qos_get_value(struct pm_qos_object *o)
>  	}
>  }
>  
> +static inline s32 pm_qos_read_value(struct pm_qos_object *o)
> +{
> +	return o->target_value;
> +}
> +
> +static inline void pm_qos_set_value(struct pm_qos_object *o, s32 value)
> +{
> +	o->value = target_value;
> +}
> +
>  static void update_target(struct pm_qos_object *o, struct plist_node *node,
>  			  int del, int value)
>  {
> @@ -156,6 +170,7 @@ static void update_target(struct pm_qos_object *o, struct plist_node *node,
>  		plist_add(node, &o->requests);
>  	}
>  	curr_value = pm_qos_get_value(o);
> +	pm_qos_set_value(o, curr_value);
>  	spin_unlock_irqrestore(&pm_qos_lock, flags);
>  
>  	if (prev_value != curr_value)
> @@ -190,18 +205,11 @@ static int find_pm_qos_object_by_minor(int minor)
>   * pm_qos_request - returns current system wide qos expectation
>   * @pm_qos_class: identification of which qos value is requested
>   *
> - * This function returns the current target value in an atomic manner.
> + * This function returns the current target value.
>   */
>  int pm_qos_request(int pm_qos_class)
>  {
> -	unsigned long flags;
> -	int value;
> -
> -	spin_lock_irqsave(&pm_qos_lock, flags);
> -	value = pm_qos_get_value(pm_qos_array[pm_qos_class]);
> -	spin_unlock_irqrestore(&pm_qos_lock, flags);
> -
> -	return value;
> +	return pm_qos_read_value(pm_qos_array[pm_qos_class]);
>  }
>  EXPORT_SYMBOL_GPL(pm_qos_request);
>  
> 
> 
> 
> 

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