From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on archive.lwn.net X-Spam-Level: X-Spam-Status: No, score=-6.0 required=5.0 tests=MAILING_LIST_MULTI, RCVD_IN_DNSWL_HI,SPF_HELO_NONE,SPF_NONE autolearn=ham autolearn_force=no version=3.4.2 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by archive.lwn.net (Postfix) with ESMTP id 2B4BA7D90D for ; Thu, 29 Aug 2019 16:54:42 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727736AbfH2Qyl (ORCPT ); Thu, 29 Aug 2019 12:54:41 -0400 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:19772 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1727483AbfH2Qyl (ORCPT ); Thu, 29 Aug 2019 12:54:41 -0400 Received: from pps.filterd (m0098414.ppops.net [127.0.0.1]) by mx0b-001b2d01.pphosted.com (8.16.0.27/8.16.0.27) with SMTP id x7TGrDRS033344; Thu, 29 Aug 2019 12:54:09 -0400 Received: from pps.reinject (localhost [127.0.0.1]) by mx0b-001b2d01.pphosted.com with ESMTP id 2uphbyue43-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Thu, 29 Aug 2019 12:54:09 -0400 Received: from m0098414.ppops.net (m0098414.ppops.net [127.0.0.1]) by pps.reinject (8.16.0.27/8.16.0.27) with SMTP id x7TGs82j036591; Thu, 29 Aug 2019 12:54:08 -0400 Received: from ppma01dal.us.ibm.com (83.d6.3fa9.ip4.static.sl-reverse.com [169.63.214.131]) by mx0b-001b2d01.pphosted.com with ESMTP id 2uphbyue3r-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Thu, 29 Aug 2019 12:54:08 -0400 Received: from pps.filterd (ppma01dal.us.ibm.com [127.0.0.1]) by ppma01dal.us.ibm.com (8.16.0.27/8.16.0.27) with SMTP id x7TGqJr0006466; Thu, 29 Aug 2019 16:54:07 GMT Received: from b01cxnp22033.gho.pok.ibm.com (b01cxnp22033.gho.pok.ibm.com [9.57.198.23]) by ppma01dal.us.ibm.com with ESMTP id 2unb3t6vbm-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Thu, 29 Aug 2019 16:54:07 +0000 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp22033.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x7TGs7hb55378288 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Thu, 29 Aug 2019 16:54:07 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id E8A83B2067; Thu, 29 Aug 2019 16:54:06 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id C6F79B2065; Thu, 29 Aug 2019 16:54:06 +0000 (GMT) Received: from paulmck-ThinkPad-W541 (unknown [9.85.151.154]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Thu, 29 Aug 2019 16:54:06 +0000 (GMT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 71A0816C12C2; Thu, 29 Aug 2019 09:54:07 -0700 (PDT) Date: Thu, 29 Aug 2019 09:54:07 -0700 From: "Paul E. McKenney" To: Andy Lutomirski Cc: Joel Fernandes , LKML , Frederic Weisbecker , Jonathan Corbet , Josh Triplett , Android Kernel Team , Lai Jiangshan , "open list:DOCUMENTATION" , Mathieu Desnoyers , Mauro Carvalho Chehab , rcu@vger.kernel.org, Steven Rostedt Subject: Re: [RFC v1 2/2] rcu/tree: Remove dynticks_nmi_nesting counter Message-ID: <20190829165407.GT4125@linux.ibm.com> Reply-To: paulmck@kernel.org References: <20190828211904.GX26530@linux.ibm.com> <20190828214241.GD75931@google.com> <20190828220108.GC26530@linux.ibm.com> <20190828221444.GA100789@google.com> <20190828231247.GE26530@linux.ibm.com> <20190829015155.GB100789@google.com> <20190829034336.GD4125@linux.ibm.com> <20190829144355.GE63638@google.com> <20190829160946.GP4125@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: 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=2019-08-29_07:,, 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=973 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1906280000 definitions=main-1908290179 Sender: linux-doc-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-doc@vger.kernel.org On Thu, Aug 29, 2019 at 09:21:46AM -0700, Andy Lutomirski wrote: > On Thu, Aug 29, 2019 at 9:10 AM Paul E. McKenney wrote: > > > > On Thu, Aug 29, 2019 at 10:43:55AM -0400, Joel Fernandes wrote: > > > > [ . . . ] > > > > > Paul, do we also nuke rcu_eqs_special_set()? Currently I don't see anyone > > > using it. And also remove the bottom most bit of dynticks? > > > > > > Also what happens if a TLB flush broadcast is needed? Do we IPI nohz or idle > > > CPUs are the moment? > > > > > > All of this was introduced in: > > > b8c17e6664c4 ("rcu: Maintain special bits at bottom of ->dynticks counter") > > > > Adding Andy Lutomirski on CC. > > > > Andy, is this going to be used in the near term, or should we just get > > rid of it? > > Let's get rid of it. I'm not actually convinced it *can* be used as designed. > > For those who forgot the history or weren't cc'd on all of it: I had > this clever idea about how we could reduce TLB flushes. I implemented > some of it (but not the part that would have used this RCU feature), > and it exploded in nasty and subtle ways. This caused me to learn > that speculative TLB fills were a problem that I had entirely failed > to account for. Then PTI happened and thoroughly muddied the water. Yeah, PTI was quite annoying. Still is, from what I can see. :-/ > So I think we should just drop this :( OK, thank you! I will put a tag into -rcu marking its removal in case it should prove useful whenever for whatever. Joel, would you like to remove this, or would you rather that I did? It is in code you are working with right now, so if I do it, I need to wait until yours is finalized. Which wouldn't be a problem. Thanx, Paul