From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755453Ab3ADX5j (ORCPT ); Fri, 4 Jan 2013 18:57:39 -0500 Received: from hydra.sisk.pl ([212.160.235.94]:60561 "EHLO hydra.sisk.pl" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754749Ab3ADX5f (ORCPT ); Fri, 4 Jan 2013 18:57:35 -0500 From: "Rafael J. Wysocki" To: Daniel Lezcano , Russ Anderson Cc: linux-pm@vger.kernel.org, pdeschrijver@nvidia.com, akpm@linux-foundation.org, linux-kernel@vger.kernel.org, rja@americas.sgi.com Subject: Re: [PATCH] cpuidle - fix lock contention in the idle path Date: Sat, 05 Jan 2013 01:03:11 +0100 Message-ID: <3077900.eue4T2ZoF8@vostro.rjw.lan> User-Agent: KMail/4.9.5 (Linux/3.8.0-rc2+; KDE/4.9.5; x86_64; ; ) In-Reply-To: <50E6764C.9060608@linaro.org> References: <1356516108-11191-1-git-send-email-daniel.lezcano@linaro.org> <20130102211314.GA29447@sgi.com> <50E6764C.9060608@linaro.org> MIME-Version: 1.0 Content-Transfer-Encoding: 7Bit Content-Type: text/plain; charset="utf-8" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Friday, January 04, 2013 07:27:24 AM Daniel Lezcano wrote: > 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 > > > > Reported-by: Russ Anderson > > Acked-by: Russ Anderson > > Hi Rafael, > > could you consider this patch for merging ? Yes, I've taken it already. I'll include it into the next pull request. Thanks, Rafael -- I speak only for myself. Rafael J. Wysocki, Intel Open Source Technology Center.