From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id D2377C3A5A6 for ; Mon, 26 Aug 2019 16:29:56 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id B3A4720874 for ; Mon, 26 Aug 2019 16:29:56 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732745AbfHZQ34 (ORCPT ); Mon, 26 Aug 2019 12:29:56 -0400 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:36874 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1728559AbfHZQ34 (ORCPT ); Mon, 26 Aug 2019 12:29:56 -0400 Received: from pps.filterd (m0098419.ppops.net [127.0.0.1]) by mx0b-001b2d01.pphosted.com (8.16.0.27/8.16.0.27) with SMTP id x7QGOOSs026361; Mon, 26 Aug 2019 12:29:45 -0400 Received: from pps.reinject (localhost [127.0.0.1]) by mx0b-001b2d01.pphosted.com with ESMTP id 2umjea1buv-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Mon, 26 Aug 2019 12:29:45 -0400 Received: from m0098419.ppops.net (m0098419.ppops.net [127.0.0.1]) by pps.reinject (8.16.0.27/8.16.0.27) with SMTP id x7QGRWLI035418; Mon, 26 Aug 2019 12:29:44 -0400 Received: from ppma03dal.us.ibm.com (b.bd.3ea9.ip4.static.sl-reverse.com [169.62.189.11]) by mx0b-001b2d01.pphosted.com with ESMTP id 2umjea1bug-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Mon, 26 Aug 2019 12:29:44 -0400 Received: from pps.filterd (ppma03dal.us.ibm.com [127.0.0.1]) by ppma03dal.us.ibm.com (8.16.0.27/8.16.0.27) with SMTP id x7QGObQO008923; Mon, 26 Aug 2019 16:29:44 GMT Received: from b01cxnp23034.gho.pok.ibm.com (b01cxnp23034.gho.pok.ibm.com [9.57.198.29]) by ppma03dal.us.ibm.com with ESMTP id 2ujvv6schf-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Mon, 26 Aug 2019 16:29:44 +0000 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp23034.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x7QGThcW51904928 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Mon, 26 Aug 2019 16:29:43 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 263ABB2067; Mon, 26 Aug 2019 16:29:43 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 0152FB2064; Mon, 26 Aug 2019 16:29:42 +0000 (GMT) Received: from paulmck-ThinkPad-W541 (unknown [9.70.82.154]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Mon, 26 Aug 2019 16:29:42 +0000 (GMT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 8CFC216C22D8; Mon, 26 Aug 2019 09:29:45 -0700 (PDT) Date: Mon, 26 Aug 2019 09:29:45 -0700 From: "Paul E. McKenney" To: Sebastian Andrzej Siewior Cc: Joel Fernandes , Scott Wood , linux-rt-users@vger.kernel.org, linux-kernel@vger.kernel.org, Thomas Gleixner , Peter Zijlstra , Juri Lelli , Clark Williams Subject: Re: [PATCH RT v2 2/3] sched: migrate_enable: Use sleeping_lock to indicate involuntary sleep Message-ID: <20190826162945.GE28441@linux.ibm.com> Reply-To: paulmck@linux.ibm.com References: <20190821231906.4224-1-swood@redhat.com> <20190821231906.4224-3-swood@redhat.com> <20190823162024.47t7br6ecfclzgkw@linutronix.de> <433936e4c720e6b81f9b297fefaa592fd8a961ad.camel@redhat.com> <20190824031014.GB2731@google.com> <20190826152523.dcjbsgyyir4zjdol@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190826152523.dcjbsgyyir4zjdol@linutronix.de> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-08-26_08:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1906280000 definitions=main-1908260162 Sender: linux-rt-users-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-rt-users@vger.kernel.org On Mon, Aug 26, 2019 at 05:25:23PM +0200, Sebastian Andrzej Siewior wrote: > On 2019-08-23 23:10:14 [-0400], Joel Fernandes wrote: > > On Fri, Aug 23, 2019 at 02:28:46PM -0500, Scott Wood wrote: > > > On Fri, 2019-08-23 at 18:20 +0200, Sebastian Andrzej Siewior wrote: > > > > > > > > this looks like an ugly hack. This sleeping_lock_inc() is used where we > > > > actually hold a sleeping lock and schedule() which is okay. But this > > > > would mean we hold a RCU lock and schedule() anyway. Is that okay? > > > > > > Perhaps the name should be changed, but the concept is the same -- RT- > > > specific sleeping which should be considered involuntary for the purpose of > > > debug checks. Voluntary sleeping is not allowed in an RCU critical section > > > because it will break the critical section on certain flavors of RCU, but > > > that doesn't apply to the flavor used on RT. Sleeping for a long time in an > > > RCU critical section would also be a bad thing, but that also doesn't apply > > > here. > > > > I think the name should definitely be changed. At best, it is super confusing to > > call it "sleeping_lock" for this scenario. In fact here, you are not even > > blocking on a lock. > > > > Maybe "sleeping_allowed" or some such. > > The mechanism that is used here may change in future. I just wanted to > make sure that from RCU's side it is okay to schedule here. Good point. The effect from RCU's viewpoint will be to split any non-rcu_read_lock() RCU read-side critical section at this point. This alrady happens in a few places, for example, rcu_note_context_switch() constitutes an RCU quiescent state despite being invoked with interrupts disabled (as is required!). The __schedule() function just needs to understand (and does understand) that the RCU read-side critical section that would otherwise span that call to rcu_node_context_switch() is split in two by that call. However, if this was instead an rcu_read_lock() critical section within a PREEMPT=y kernel, then if a schedule() occured within stop_one_task(), RCU would consider that critical section to be preempted. This means that any RCU grace period that is blocked by this RCU read-side critical section would remain blocked until stop_one_cpu() resumed, returned, and so on until the matching rcu_read_unlock() was reached. In other words, RCU would consider that RCU read-side critical section to span the call to stop_one_cpu() even if stop_one_cpu() invoked schedule(). On the other hand, within a PREEMPT=n kernel, the call to schedule() would split even an rcu_read_lock() critical section. Which is why I asked earlier if sleeping_lock_inc() and sleeping_lock_dec() are no-ops in !PREEMPT_RT_BASE kernels. We would after all want the usual lockdep complaints in that case. Does that help, or am I missing the point? Thanx, Paul