From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757927AbZEDPdb (ORCPT ); Mon, 4 May 2009 11:33:31 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754591AbZEDPdV (ORCPT ); Mon, 4 May 2009 11:33:21 -0400 Received: from rcsinet11.oracle.com ([148.87.113.123]:33888 "EHLO rgminet11.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752586AbZEDPdU (ORCPT ); Mon, 4 May 2009 11:33:20 -0400 Message-ID: <49FF0AFF.8030303@oracle.com> Date: Mon, 04 May 2009 08:34:23 -0700 From: Randy Dunlap Organization: Oracle Linux Engineering User-Agent: Thunderbird 2.0.0.6 (X11/20070801) MIME-Version: 1.0 To: Peter Zijlstra CC: Ingo Molnar , Thomas Gleixner , linux-kernel Subject: Re: [PATCH] sched: rt: document the risk of small values in the bandwidth settings References: <1241449678.7620.4931.camel@twins> In-Reply-To: <1241449678.7620.4931.camel@twins> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit X-Source-IP: abhmt007.oracle.com [141.146.116.16] X-Auth-Type: Internal IP X-CT-RefId: str=0001.0A010201.49FF0A9F.023D,ss=1,fgs=0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Peter Zijlstra wrote: > 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. I know that my English is limited, but I don't know what this means: "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; > > -- -- ~Randy