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

From: Daniel Lezcano
Date: Fri Jan 04 2013 - 01:50:51 EST

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@xxxxxxxxxx>
> Reported-by: Russ Anderson <rja@xxxxxxx>
> Acked-by: Russ Anderson <rja@xxxxxxx>

Hi Rafael,

could you consider this patch for merging ?

-- Daniel

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