From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752175AbeESC1x (ORCPT ); Fri, 18 May 2018 22:27:53 -0400 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:55230 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751117AbeESC1t (ORCPT ); Fri, 18 May 2018 22:27:49 -0400 Date: Fri, 18 May 2018 19:29:18 -0700 From: "Paul E. McKenney" To: Joel Fernandes Cc: rostedt@goodmis.org, byungchul.park@lge.com, mathieu.desnoyers@efficios.com, Josh Triplett , Lai Jiangshan , linux-kernel@vger.kernel.org, kernel-team@android.com Subject: Re: Tasks RCU vs Preempt RCU Reply-To: paulmck@linux.vnet.ibm.com References: <20180518183623.GA163151@joelaf.mtv.corp.google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180518183623.GA163151@joelaf.mtv.corp.google.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18051902-2213-0000-0000-000002A83448 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00009050; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000261; SDB=6.01034362; UDB=6.00528968; IPR=6.00813490; MB=3.00021194; MTD=3.00000008; XFM=3.00000015; UTC=2018-05-19 02:27:47 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18051902-2214-0000-0000-00005A2BE355 Message-Id: <20180519022918.GV3803@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-05-18_10:,, 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=1011 lowpriorityscore=0 impostorscore=0 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1709140000 definitions=main-1805190025 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, May 18, 2018 at 11:36:23AM -0700, Joel Fernandes wrote: > Hi, > > I was thinking about tasks-RCU and why its needed. Since preempt-RCU allows > tasks to be preempted in read-sections, can we not just reuse that mechanism > for the trampolines since we track all preempted tasks so we would wait on > all tasks preempted within a trampoline? > > I am trying to understand what will _not_ work if we did that.. I'm guessing > the answer is that that would mean the trampoline has to be wrapped with > rcu_read_{lock,unlock} which may add some overhead, but please let me know > if I'm missing something else.. > > The advantage I guess is possible elimination of an RCU variant, and also > possibly eliminating the tasks RCU thread that monitors.. Anyway I was > thinking more in terms of the effort of reduction of the RCU flavors etc and > reducing complexity ideas. The problem is that if they are preempted while executing in a trampoline, RCU-preempt doesn't queue them nor does it wait on them. And the problem with wrapping them with rcu_read_{lock,unlock} is that there would be a point before the trampoline executed rcu_read_lock() but while it was on the trampoline. Nothing good comes from this. ;-) Thanx, Paul