From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752529AbeETAsK (ORCPT ); Sat, 19 May 2018 20:48:10 -0400 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:56484 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1752199AbeETAsI (ORCPT ); Sat, 19 May 2018 20:48:08 -0400 Date: Sat, 19 May 2018 17:49:38 -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> <20180519022918.GV3803@linux.vnet.ibm.com> <20180519225905.GB134184@joelaf.mtv.corp.google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180519225905.GB134184@joelaf.mtv.corp.google.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18052000-0048-0000-0000-0000026F9CEF X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00009053; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000261; SDB=6.01034809; UDB=6.00529233; IPR=6.00813937; MB=3.00021205; MTD=3.00000008; XFM=3.00000015; UTC=2018-05-20 00:48:06 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18052000-0049-0000-0000-0000452E423D Message-Id: <20180520004938.GZ3803@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-05-19_15:,, 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 impostorscore=0 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1709140000 definitions=main-1805200006 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, May 19, 2018 at 03:59:05PM -0700, Joel Fernandes wrote: > On Fri, May 18, 2018 at 07:29:18PM -0700, Paul E. McKenney wrote: > > 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. > > Not if they are wrapped with rcu_read_lock and rcu_read_unlock? From what I > can see, you are preparing a list of blocked tasks that would keep the grace period > from finishing in rcu_preempt_ctxt_queue? But being on the ->blkd_tasks list doesn't necessarily block the current grace period. Only those tasks on that list that are also referenced by ->gp_tasks (or that follow some task referenced by ->gp_tasks) will block the current grace period. This is be design -- otherwise, an endless stream of tasks blocking in their RCU read-side critical sections could prevent the current grace period from ever ending. > > 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. ;-) > > Yes, I see what you're saying. The data being protected and freed in this > case is the code so relying on it to do the rcu_read_lock seems infeasible. > Conceptually atleast, I feel this can be fixed by cleverly implementing > trampolines such that the rcu_read_lock isn't done during the trampoline > execution. But I am not very experienced with how the trampolines work to say > definitely whether it is or isn't possible or worth it. But atleast I felt it > was a worthwhile food for thought ;) I suggested to Steven that the rcu_read_lock() and rcu_read_unlock() might be outside of the trampoline, but this turned out to be infeasible. Not that I remember why! ;-) > I actually want to trace out the trampoline executing as it pertains to RCU, > with your latest rcu/dev.. I think it will be fun :) Cool! In addition, if you are interested, it might be worth looking for fields in rcu_dynticks, rcu_data, rcu_node, and rcu_state that are no longer actually used. It might also be worth looking for RCU macros that are no longer used. I found a few by accident, so there are probably more... Thanx, Paul