Re: [Patch v2] idle governor: Avoid lock acquisition to read pm_qos before entering idle

[Rafael J. Wysocki]

On Friday, February 11, 2011, Tim Chen wrote:
> Thanks to the reviews and comments by Rafael, James, Mark and Andi.
> Here's version 2 of the patch incorporating your comments and also some
> update to my previous patch comments.
> 
> I noticed that before entering idle state, the menu idle governor will
> look up the current pm_qos target value according to the list of qos
> requests received.  This look up currently needs the acquisition of a
> lock to access the list of qos requests to find the qos target value,
> slowing down the entrance into idle state due to contention by multiple
> cpus to access this list.  The contention is severe when there are a lot
> of cpus waking and going into idle.  For example, for a simple workload
> that has 32 pair of processes ping ponging messages to each other, where
> 64 cpu cores are active in test system, I see the following profile with
> 37.82% of cpu cycles spent in contention of pm_qos_lock:
> 
> -     37.82%          swapper  [kernel.kallsyms]          [k]
> _raw_spin_lock_irqsave
>    - _raw_spin_lock_irqsave
>       - 95.65% pm_qos_request             
>            menu_select                                             
>            cpuidle_idle_call                      
>          - cpu_idle                                                     
>               99.98% start_secondary
> 
> A better approach will be to cache the updated pm_qos target value so
> reading it does not require lock acquisition as in the patch below.  
> With this patch the contention for pm_qos_lock is removed and I saw a
> 2.2X increase in throughput for my message passing workload. 

Thanks, the patch is going to be merged through the Len Brown's idle tree.

Rafael


> Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> 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..6a8fad8 100644
> --- a/kernel/pm_qos_params.c
> +++ b/kernel/pm_qos_params.c
> @@ -53,11 +53,17 @@ enum pm_qos_type {
>  	PM_QOS_MIN		/* return the smallest value */
>  };
>  
> +/*
> + * Note: The lockless read path depends on the CPU accessing
> + * target_value atomically.  Atomic access is only guaranteed on all CPU
> + * types linux supports for 32 bit quantites
> + */
>  struct pm_qos_object {
>  	struct plist_head requests;
>  	struct blocking_notifier_head *notifiers;
>  	struct miscdevice pm_qos_power_miscdev;
>  	char *name;
> +	s32 target_value;	/* Do not change to 64 bit */
>  	s32 default_value;
>  	enum pm_qos_type type;
>  };
> @@ -70,7 +76,8 @@ 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 +86,8 @@ 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 +97,8 @@ 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 +141,16 @@ 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->target_value = value;
> +}
> +
>  static void update_target(struct pm_qos_object *o, struct plist_node *node,
>  			  int del, int value)
>  {
> @@ -156,6 +175,7 @@ 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 +210,11 @@ 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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