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Date:	Sat, 29 Dec 2012 11:03:03 +0100
From:	Daniel Lezcano <daniel.lezcano@...aro.org>
To:	rafael.j.wysocki@...el.com, rja@....com
CC:	linux-pm@...r.kernel.org, pdeschrijver@...dia.com,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cpuidle - fix lock contention in the idle path


Hi Russ,

Is it possible you try this patch on your 2048 cpus ?

Thanks

  -- Daniel

On 12/26/2012 11:01 AM, Daniel Lezcano wrote:
> The commit bf4d1b5ddb78f86078ac6ae0415802d5f0c68f92 introduces
> a lock in the cpuidle_get_cpu_driver function. This function
> is used in the idle_call function.
> 
> The problem is the contention with a large number of cpus because
> they try to access the idle routine at the same time.
> 
> The lock could be safely removed because of how is used the
> cpuidle api. The cpuidle_register_driver is called first but
> until the cpuidle_register_device is not called we don't
> enter in the cpuidle idle call function because the device
> is not enabled.
> 
> The cpuidle_unregister_driver function, leading the a NULL driver,
> is not called before the cpuidle_unregister_device.
> 
> This is how is used the cpuidle api from the different drivers.
> 
> However, a cleanup around the lock and a proper refcounting
> mechanism should be used to ensure the consistency in the api,
> like cpuidle_unregister_driver should failed if its refcounting
> is not 0.
> 
> These modifications will need some code reorganization and rewrite
> which does not fit with a fix.
> 
> The following patch is a hot fix by returning to the initial behavior
> by removing the lock when getting the driver.
> 
> Signed-off-by: Daniel Lezcano <daniel.lezcano@...aro.org>
> ---
>  drivers/cpuidle/driver.c |    8 +-------
>  1 file changed, 1 insertion(+), 7 deletions(-)
> 
> diff --git a/drivers/cpuidle/driver.c b/drivers/cpuidle/driver.c
> index 3af841f..c2b281a 100644
> --- a/drivers/cpuidle/driver.c
> +++ b/drivers/cpuidle/driver.c
> @@ -235,16 +235,10 @@ EXPORT_SYMBOL_GPL(cpuidle_get_driver);
>   */
>  struct cpuidle_driver *cpuidle_get_cpu_driver(struct cpuidle_device *dev)
>  {
> -	struct cpuidle_driver *drv;
> -
>  	if (!dev)
>  		return NULL;
>  
> -	spin_lock(&cpuidle_driver_lock);
> -	drv = __cpuidle_get_cpu_driver(dev->cpu);
> -	spin_unlock(&cpuidle_driver_lock);
> -
> -	return drv;
> +	return __cpuidle_get_cpu_driver(dev->cpu);
>  }
>  EXPORT_SYMBOL_GPL(cpuidle_get_cpu_driver);
>  
> 


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