linux-kernel - Re: [PATCH 19/21] cpuidle: create list of registered drivers

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Date:	Mon, 30 Sep 2013 20:37:47 +0200
From:	Daniel Lezcano <daniel.lezcano@...aro.org>
To:	paulmck@...ux.vnet.ibm.com
CC:	Viresh Kumar <viresh.kumar@...aro.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Lists linaro-kernel <linaro-kernel@...ts.linaro.org>,
	Patch Tracking <patches@...aro.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 19/21] cpuidle: create list of registered drivers

On 09/28/2013 11:33 PM, Paul E. McKenney wrote:
> On Thu, Sep 26, 2013 at 10:19:14AM +0200, Daniel Lezcano wrote:
>> On 09/26/2013 08:17 AM, Viresh Kumar wrote:
>>> On 26 September 2013 04:00, Daniel Lezcano <daniel.lezcano@...aro.org> wrote:
>>>> If you introduce a list, you will have to introduce a lock to protect
>>>> it.
>>>
>>> I missed it, should have added that :)
>>>
>>>> This lock will be in the fast path cpuidle_idle_call with the
>>>> get_driver function and conforming to the comment: "NOTE: no locks or
>>>> semaphores should be used here".
>>>>
>>>> A lock has been introduced in this function already and the system hangs
>>>> with 1024 cpus.
>>>
>>> Hmm... I see.. I didn't knew about this expectation.. What about a rcu
>>> read/write lock? As far as I know its too lightweight... Can we have that
>>> in fast path?
>>
>> Nope, we can't use rcu in the idle path :)
>>
>> http://lists.infradead.org/pipermail/linux-arm-kernel/2012-February/083054.html
>
> But you should be able to use SRCU in the idle path, if that helps.

Interesting, thanks for the pointer.

   -- Daniel


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