From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S968155AbdEWUAo (ORCPT ); Tue, 23 May 2017 16:00:44 -0400 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:35833 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751746AbdEWUAi (ORCPT ); Tue, 23 May 2017 16:00:38 -0400 Date: Tue, 23 May 2017 13:00:35 -0700 From: "Paul E. McKenney" To: Steven Rostedt Cc: Ingo Molnar , linux-kernel@vger.kernel.org, Peter Zijlstra , Thomas Gleixner Subject: Re: Use case for TASKS_RCU Reply-To: paulmck@linux.vnet.ibm.com References: <20170515182354.GA25440@linux.vnet.ibm.com> <20170516062233.tyz7ze7ilmbkxtjc@gmail.com> <20170516122354.GB3956@linux.vnet.ibm.com> <20170519062331.52dhungzvcsdxdgo@gmail.com> <20170519133550.GD3956@linux.vnet.ibm.com> <20170519100421.27298063@gandalf.local.home> <20170519102331.0d5a8536@gandalf.local.home> <20170519190609.GE3956@linux.vnet.ibm.com> <20170523000036.GA13506@linux.vnet.ibm.com> <20170523153939.7122e892@vmware.local.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170523153939.7122e892@vmware.local.home> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 17052320-0036-0000-0000-000002107345 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00007107; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000212; SDB=6.00864620; UDB=6.00429234; IPR=6.00644401; BA=6.00005369; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000; ZP=6.00000000; ZH=6.00000000; ZU=6.00000002; MB=3.00015555; XFM=3.00000015; UTC=2017-05-23 20:00:36 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17052320-0037-0000-0000-00004072BB36 Message-Id: <20170523200035.GW3956@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-05-23_07:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 suspectscore=0 malwarescore=0 phishscore=0 adultscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1703280000 definitions=main-1705230102 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, May 23, 2017 at 03:39:39PM -0400, Steven Rostedt wrote: > On Mon, 22 May 2017 17:00:36 -0700 > "Paul E. McKenney" wrote: > > > > > > > Hmmm... The goal is to make sure that any task that was preempted > > > or running at a given point in time passes through a voluntary > > > context switch (or userspace execution, or, ...). > > > > > > What is the simplest way to get this job done? To Ingo's point, I > > > bet that there is a simpler way than the current TASKS_RCU > > > implementation. > > > > > > Ingo, if I make it fit into 100 lines of code, would you be OK with > > > it? I probably need a one-line hook at task-creation time and > > > another at task-exit time, if that makes a difference. > > > > And please see below for such a patch, which does add (just barely) > > fewer than 100 lines net. > > > > Unfortunately, it does not work, as I should have known ahead of time > > from the dyntick-idle experience. Not all context switches go through > > context_switch(). :-/ > > Wait. What context switch doesn't go through a context switch? Or do > you mean a user/kernel context switch? I mean that putting printk() before and after the call to context_switch() can show tasks switching out twice without switching in and vice versa. No sign of lost printk()s, and I also confirmed this behavior using a flag in task_struct. One way that this can happen on some architectures is via the "helper" mechanism, where the task sleeps normally, but where a later interrupt or exception takes on its context "behind the scenes" in the arch code. This is what messed up my attempt to use a simple interrupt-nesting counter for RCU dynticks some years back. What I counted on there was that the idle loop would never do that sort of thing, so I could zero the count when entering idle from process context. But I have not yet found a similar trick for counting voluntary context switches. I also tried making context_switch() look like a momentary quiescent state, but of course that means that tasks that block forever also block the grace period forever. At which point, I need to scan the task list to find them. And that pretty much brings me back to the current RCU-tasks implementation. :-/ Thanx, Paul > -- Steve > > > > > I believe this is fixable, more or less like dyntick-idle's > > half-interrupts were fixable, but it will likely be a few days. Not > > clear whether the result will be simpler than current TASKS_RCU, but > > there is only one way to find out. ;-) > > >