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

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Date:	Sat, 28 Sep 2013 14:33:43 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Daniel Lezcano <daniel.lezcano@...aro.org>
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 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.

							Thanx, Paul

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