From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757375AbZEDPI3 (ORCPT ); Mon, 4 May 2009 11:08:29 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1757121AbZEDPIG (ORCPT ); Mon, 4 May 2009 11:08:06 -0400 Received: from casper.infradead.org ([85.118.1.10]:44135 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756980AbZEDPIF (ORCPT ); Mon, 4 May 2009 11:08:05 -0400 Subject: [PATCH] sched: rt: document the risk of small values in the bandwidth settings From: Peter Zijlstra To: Ingo Molnar Cc: Thomas Gleixner , linux-kernel Content-Type: text/plain Content-Transfer-Encoding: 7bit Date: Mon, 04 May 2009 17:07:58 +0200 Message-Id: <1241449678.7620.4931.camel@twins> Mime-Version: 1.0 X-Mailer: Evolution 2.26.1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Thomas noted that we should disallow sysctl_sched_rt_runtime == 0 for (! RT_GROUP) since the root group always has some RT tasks in it. Further, update the documentation to inspire clue. Signed-off-by: Peter Zijlstra Acked-by: Thomas Gleixner --- Documentation/scheduler/sched-rt-group.txt | 18 ++++++++++++++++++ kernel/sched.c | 7 +++++++ 2 files changed, 25 insertions(+), 0 deletions(-) diff --git a/Documentation/scheduler/sched-rt-group.txt b/Documentation/scheduler/sched-rt-group.txt index 5ba4d3f..cfd51c0 100644 --- a/Documentation/scheduler/sched-rt-group.txt +++ b/Documentation/scheduler/sched-rt-group.txt @@ -4,6 +4,7 @@ CONTENTS ======== +0. WARNING 1. Overview 1.1 The problem 1.2 The solution @@ -14,6 +15,23 @@ CONTENTS 3. Future plans +0. WARNING +========== + +Fiddling with these settings can result in an unusable system, the knobs are +root only and assume root knows what he's on about. + +Most notable: + + * very small values in sched_rt_period_us can result in an unstable + system when the period is smaller than either the available hrtimer + resolution, or the time it takes to handle the budget refresh itself. + + * very small values in sched_rt_runtime_us can result in an unstable + system when the runtime is so small the system has difficulty making + forward progress (NOTE: the migration thread and kstopmachine both + are real-time processes). + 1. Overview =========== diff --git a/kernel/sched.c b/kernel/sched.c index 11f6da4..c5ef114 100644 --- a/kernel/sched.c +++ b/kernel/sched.c @@ -10021,6 +10021,13 @@ static int sched_rt_global_constraints(void) if (sysctl_sched_rt_period <= 0) return -EINVAL; + /* + * There's always some RT tasks in the root group + * -- migration, kstopmachine etc.. + */ + if (sysctl_sched_rt_runtime == 0) + return -EBUSY; + spin_lock_irqsave(&def_rt_bandwidth.rt_runtime_lock, flags); for_each_possible_cpu(i) { struct rt_rq *rt_rq = &cpu_rq(i)->rt;