linux-kernel - Re: [PATCH] cpuidle - fix lock contention in the idle path

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Date:	Fri, 04 Jan 2013 07:27:24 +0100
From:	Daniel Lezcano <daniel.lezcano@...aro.org>
To:	rafael.j.wysocki@...el.com
CC:	Russ Anderson <rja@....com>, linux-pm@...r.kernel.org,
	pdeschrijver@...dia.com, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, rja@...ricas.sgi.com
Subject: Re: [PATCH] cpuidle - fix lock contention in the idle path

On 01/02/2013 10:13 PM, Russ Anderson wrote:
> On Wed, Dec 26, 2012 at 11:01:48AM +0100, 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.
> 
> I agree.
> 
>> The following patch is a hot fix by returning to the initial behavior
>> by removing the lock when getting the driver.
> 
> The patch fixes the problem.  Verified on a system with 1024 cpus.
> Thanks.
> 
>> Signed-off-by: Daniel Lezcano <daniel.lezcano@...aro.org>
> 
> Reported-by: Russ Anderson <rja@....com>
> Acked-by: Russ Anderson <rja@....com>

Hi Rafael,

could you consider this patch for merging ?

Thanks
  -- Daniel


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