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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT autolearn=ham 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 9100BC3279B for ; Tue, 10 Jul 2018 18:49:46 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 597E120870 for ; Tue, 10 Jul 2018 18:49:46 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 597E120870 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=linux.vnet.ibm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2388115AbeGJSah (ORCPT ); Tue, 10 Jul 2018 14:30:37 -0400 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:51972 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2388073AbeGJSag (ORCPT ); Tue, 10 Jul 2018 14:30:36 -0400 Received: from pps.filterd (m0098410.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.22/8.16.0.22) with SMTP id w6AHdCHT054902 for ; Tue, 10 Jul 2018 13:40:12 -0400 Received: from e17.ny.us.ibm.com (e17.ny.us.ibm.com [129.33.205.207]) by mx0a-001b2d01.pphosted.com with ESMTP id 2k501vcd7g-1 (version=TLSv1.2 cipher=AES256-GCM-SHA384 bits=256 verify=NOT) for ; Tue, 10 Jul 2018 13:40:11 -0400 Received: from localhost by e17.ny.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Tue, 10 Jul 2018 13:40:10 -0400 Received: from b01cxnp22036.gho.pok.ibm.com (9.57.198.26) by e17.ny.us.ibm.com (146.89.104.204) with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Tue, 10 Jul 2018 13:40:07 -0400 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp22036.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id w6AHe6XJ37814360 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=FAIL); Tue, 10 Jul 2018 17:40:06 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 4DCB4B2064; Tue, 10 Jul 2018 13:39:40 -0400 (EDT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 2F0FCB205F; Tue, 10 Jul 2018 13:39:40 -0400 (EDT) Received: from paulmck-ThinkPad-W541 (unknown [9.70.82.159]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Tue, 10 Jul 2018 13:39:40 -0400 (EDT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 9516C16C19F1; Tue, 10 Jul 2018 10:42:25 -0700 (PDT) Date: Tue, 10 Jul 2018 10:42:25 -0700 From: "Paul E. McKenney" To: Joel Fernandes Cc: Joel Fernandes , Mathieu Desnoyers , Alexei Starovoitov , Daniel Colascione , Alexei Starovoitov , linux-kernel , Tim Murray , Daniel Borkmann , netdev , fengc@google.com Subject: Re: [RFC] Add BPF_SYNCHRONIZE bpf(2) command Reply-To: paulmck@linux.vnet.ibm.com References: <20180707015616.25988-1-dancol@google.com> <20180707025426.ssxipi7hsehoiuyo@ast-mbp.dhcp.thefacebook.com> <20180707203340.GA74719@joelaf.mtv.corp.google.com> <951478560.1636.1531083278064.JavaMail.zimbra@efficios.com> <20180710051347.GA180724@joelaf.mtv.corp.google.com> <20180710164212.GY3593@linux.vnet.ibm.com> <20180710165744.GA99146@joelaf.mtv.corp.google.com> <20180710171229.GZ3593@linux.vnet.ibm.com> <20180710172957.GA103636@joelaf.mtv.corp.google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180710172957.GA103636@joelaf.mtv.corp.google.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18071017-0040-0000-0000-0000044CAF0B X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00009346; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000266; SDB=6.01059399; UDB=6.00543707; IPR=6.00837319; MB=3.00022090; MTD=3.00000008; XFM=3.00000015; UTC=2018-07-10 17:40:10 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18071017-0041-0000-0000-00000852D2C0 Message-Id: <20180710174225.GA3593@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-07-10_06:,, 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=729 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1806210000 definitions=main-1807100188 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jul 10, 2018 at 10:29:57AM -0700, Joel Fernandes wrote: > On Tue, Jul 10, 2018 at 10:12:29AM -0700, Paul E. McKenney wrote: > [..] > > > > > The other question I have is about the whole "nohz-full doesn't work" thing. > > > > > I didn't fully understand why. RCU is already tracking the state of nohz-full > > > > > CPUs because the rcu dynticks code in (kernel/rcu/tree.c) monitors > > > > > transitions to and from usermode even if the timer tick is turned off. So why > > > > > would it not work? > > > > > > > > In the nohz_full case, there is no need for sys_membarrier()'s call to > > > > synchronize_sched() to interact directly with the nohz_full CPU. It > > > > can instead look at the target CPU's dyntick-idle state, and that state > > > > would potentially have been set in the dim distant past, thus having > > > > no effect on the target CPU's current execution. > > > > > > In nohz-idle case though, there's nothing to promote the barrier() to > > > smp_mb() if you were to purely look at the dynticks-idle state on the > > > nohz-full CPU executing in user mode? > > > > > > So then it makes sense to me now that nohz-full needs something to IPI that > > > CPU inorder to enforce the needed memory barrier and pure synchronize_sched() > > > wouldn't work. So then makes me think the expedited versions of > > > synchronize_sched should be able to do the job but I could off on a different > > > track.. > > > > The problem is that the expedited versions also check the dyntick-idle > > state and don't touch idle (or nohz_full usermode) CPUs. This is by > > design for the battery-powered embedded use case. ;-) > > Oh ok! ;) > > I guess there's also a MEMBARRIER_CMD_GLOBAL_EXPEDITED which seems to IPI > CPUs (I'm guessing regardless of dynticks state) and execute smp_mb within > the IPI so userspace can fallback to using that incase MEMBARRIER_CMD_GLOBAL > returns -EINVAL. Yes, and this avoids IPIing idle CPUs via the ->mm checks. But it will IPI nohz_full CPUs in that same process, as it must for correctness. Thanx, Paul