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=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_MUTT autolearn=unavailable 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 A1E40C48BE3 for ; Sat, 22 Jun 2019 00:26:13 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 7666A2084E for ; Sat, 22 Jun 2019 00:26:13 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726159AbfFVA0M (ORCPT ); Fri, 21 Jun 2019 20:26:12 -0400 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:32720 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726080AbfFVA0M (ORCPT ); Fri, 21 Jun 2019 20:26:12 -0400 Received: from pps.filterd (m0098393.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.27/8.16.0.27) with SMTP id x5M0MKTQ088575 for ; Fri, 21 Jun 2019 20:26:11 -0400 Received: from e13.ny.us.ibm.com (e13.ny.us.ibm.com [129.33.205.203]) by mx0a-001b2d01.pphosted.com with ESMTP id 2t95vuq6ek-1 (version=TLSv1.2 cipher=AES256-GCM-SHA384 bits=256 verify=NOT) for ; Fri, 21 Jun 2019 20:26:10 -0400 Received: from localhost by e13.ny.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! 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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Sat, 22 Jun 2019 01:26:05 +0100 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 x5M0Q4pD48890310 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Sat, 22 Jun 2019 00:26:04 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id D1AE5B2066; Sat, 22 Jun 2019 00:26:04 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id B45B1B205F; Sat, 22 Jun 2019 00:26:04 +0000 (GMT) Received: from paulmck-ThinkPad-W541 (unknown [9.70.82.26]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Sat, 22 Jun 2019 00:26:04 +0000 (GMT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 405CC16C1D2E; Fri, 21 Jun 2019 17:26:06 -0700 (PDT) Date: Fri, 21 Jun 2019 17:26:06 -0700 From: "Paul E. McKenney" To: Scott Wood Cc: Sebastian Andrzej Siewior , Peter Zijlstra , Steven Rostedt , Thomas Gleixner , Juri Lelli , Clark Williams , linux-rt-users@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [RFC PATCH RT 3/4] rcu: unlock special: Treat irq and preempt disabled the same Reply-To: paulmck@linux.ibm.com References: <20190619011908.25026-1-swood@redhat.com> <20190619011908.25026-4-swood@redhat.com> <20190620211005.GW26519@linux.ibm.com> <20190620222505.GB26519@linux.ibm.com> <5d24d1243849d9f8f6884348e1ceafcc3b7314fd.camel@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5d24d1243849d9f8f6884348e1ceafcc3b7314fd.camel@redhat.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 19062200-0064-0000-0000-000003F14F3E X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00011305; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000286; SDB=6.01221412; UDB=6.00642601; IPR=6.01002550; MB=3.00027415; MTD=3.00000008; XFM=3.00000015; UTC=2019-06-22 00:26:08 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19062200-0065-0000-0000-00003DFB814F Message-Id: <20190622002606.GL26519@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-06-21_16:,, 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=8.0.1-1810050000 definitions=main-1906220002 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jun 20, 2019 at 06:08:19PM -0500, Scott Wood wrote: > On Thu, 2019-06-20 at 15:25 -0700, Paul E. McKenney wrote: > > On Thu, Jun 20, 2019 at 04:59:30PM -0500, Scott Wood wrote: > > > On Thu, 2019-06-20 at 14:10 -0700, Paul E. McKenney wrote: > > > > On Tue, Jun 18, 2019 at 08:19:07PM -0500, Scott Wood wrote: > > > > > [Note: Just before posting this I noticed that the invoke_rcu_core > > > > > stuff > > > > > is part of the latest RCU pull request, and it has a patch that > > > > > addresses this in a more complicated way that appears to deal with > > > > > the > > > > > bare irq-disabled sequence as well. > > > > > > > > Far easier to deal with it than to debug the lack of it. ;-) > > > > > > > > > Assuming we need/want to support such sequences, is the > > > > > invoke_rcu_core() call actually going to result in scheduling any > > > > > sooner? resched_curr() just does the same setting of need_resched > > > > > when it's the same cpu. > > > > > ] > > > > > > > > Yes, invoke_rcu_core() can in some cases invoke the scheduler sooner. > > > > Setting the CPU-local bits might not have effect until the next > > > > interrupt. > > > > > > Maybe I'm missing something, but I don't see how (in the non-use_softirq > > > case). It just calls wake_up_process(), which in resched_curr() will > > > set > > > need_resched but not do an IPI-to-self. > > > > The common non-rt case will be use_softirq. Or are you referring > > specifically to this block of code in current -rcu? > > > > } else if (exp && irqs_were_disabled && !use_softirq && > > !t->rcu_read_unlock_special.b.deferred_qs) { > > // Safe to awaken and we get no help from enabling > > // irqs, unlike bh/preempt. > > invoke_rcu_core(); > > Yes, that one. If that block is removed the else path should be sufficient, > now that an IPI-to-self has been added. I will give it a try and let you know what happens. > Also, shouldn't the IPI-to-self be conditioned on irqs_were_disabled? > Besides that being the problem the IPI was meant to address, if irqs are > enabled the IPI is likely to happen before preempt is re-enabled and thus it > won't accomplish anything. Plus if preempt is disabled, the later preempt_enable() will check (ditto for local_bh_enable()). Unless the preempt_enable() is instead a preempt_enable_no_resched(), of course. :-/ Thanx, Paul