From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1030197Ab3DSLrk (ORCPT ); Fri, 19 Apr 2013 07:47:40 -0400 Received: from 173-166-109-252-newengland.hfc.comcastbusiness.net ([173.166.109.252]:52187 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S968126Ab3DSLrj (ORCPT ); Fri, 19 Apr 2013 07:47:39 -0400 Message-ID: <1366372036.24945.1.camel@laptop> Subject: Re: [PATCH Resend v6] sched: fix wrong rq's runnable_avg update with rt tasks From: Peter Zijlstra To: Vincent Guittot Cc: linux-kernel@vger.kernel.org, linaro-kernel@lists.linaro.org, mingo@kernel.org, pjt@google.com, rostedt@goodmis.org, fweisbec@gmail.com, efault@gmx.de Date: Fri, 19 Apr 2013 13:47:16 +0200 In-Reply-To: <1366302867-5055-1-git-send-email-vincent.guittot@linaro.org> References: <1366302867-5055-1-git-send-email-vincent.guittot@linaro.org> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.6.2-0ubuntu0.1 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 2013-04-18 at 18:34 +0200, Vincent Guittot wrote: > The current update of the rq's load can be erroneous when RT tasks are > involved > > The update of the load of a rq that becomes idle, is done only if the avg_idle > is less than sysctl_sched_migration_cost. If RT tasks and short idle duration > alternate, the runnable_avg will not be updated correctly and the time will be > accounted as idle time when a CFS task wakes up. > > A new idle_enter function is called when the next task is the idle function > so the elapsed time will be accounted as run time in the load of the rq, > whatever the average idle time is. The function update_rq_runnable_avg is > removed from idle_balance. > > When a RT task is scheduled on an idle CPU, the update of the rq's load is > not done when the rq exit idle state because CFS's functions are not > called. Then, the idle_balance, which is called just before entering the > idle function, updates the rq's load and makes the assumption that the > elapsed time since the last update, was only running time. > > As a consequence, the rq's load of a CPU that only runs a periodic RT task, > is close to LOAD_AVG_MAX whatever the running duration of the RT task is. > > A new idle_exit function is called when the prev task is the idle function > so the elapsed time will be accounted as idle time in the rq's load. Acked-by: Peter Zijlstra Thanks Vince!