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=-1.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,URG_BIZ,URIBL_BLOCKED,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 2E4EAC5CFE7 for ; Wed, 11 Jul 2018 16:47:41 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id A3A1920843 for ; Wed, 11 Jul 2018 16:47:40 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org A3A1920843 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 S2389948AbeGKQwt (ORCPT ); Wed, 11 Jul 2018 12:52:49 -0400 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:32926 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1733026AbeGKQwt (ORCPT ); Wed, 11 Jul 2018 12:52:49 -0400 Received: from pps.filterd (m0098421.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.22/8.16.0.22) with SMTP id w6BGj76G130007 for ; Wed, 11 Jul 2018 12:47:38 -0400 Received: from e15.ny.us.ibm.com (e15.ny.us.ibm.com [129.33.205.205]) by mx0a-001b2d01.pphosted.com with ESMTP id 2k5k41qcmh-1 (version=TLSv1.2 cipher=AES256-GCM-SHA384 bits=256 verify=NOT) for ; Wed, 11 Jul 2018 12:47:37 -0400 Received: from localhost by e15.ny.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Wed, 11 Jul 2018 12:47:37 -0400 Received: from b01cxnp23032.gho.pok.ibm.com (9.57.198.27) by e15.ny.us.ibm.com (146.89.104.202) with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Wed, 11 Jul 2018 12:47:34 -0400 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp23032.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id w6BGlXll14811478 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=FAIL); Wed, 11 Jul 2018 16:47:33 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 3A07DB205F; Wed, 11 Jul 2018 12:47:33 -0400 (EDT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 09C9AB2064; Wed, 11 Jul 2018 12:47:33 -0400 (EDT) Received: from paulmck-ThinkPad-W541 (unknown [9.70.82.159]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Wed, 11 Jul 2018 12:47:32 -0400 (EDT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id CCA4916C3EC5; Wed, 11 Jul 2018 09:49:52 -0700 (PDT) Date: Wed, 11 Jul 2018 09:49:52 -0700 From: "Paul E. McKenney" To: David Woodhouse Cc: Peter Zijlstra , mhillenb@amazon.de, linux-kernel Subject: Re: [RFC] Make need_resched() return true when rcu_urgent_qs requested Reply-To: paulmck@linux.vnet.ibm.com References: <20180709163432.GV3593@linux.vnet.ibm.com> <1531162254.26547.3.camel@infradead.org> <20180709203441.GE3593@linux.vnet.ibm.com> <1531168538.26547.5.camel@infradead.org> <20180709204248.GF3593@linux.vnet.ibm.com> <1531169145.26547.8.camel@infradead.org> <20180709210532.GH3593@linux.vnet.ibm.com> <20180709220823.GA18045@linux.vnet.ibm.com> <1531319025.8759.57.camel@infradead.org> <20180711144303.GQ3593@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20180711144303.GQ3593@linux.vnet.ibm.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18071116-0068-0000-0000-0000031684F4 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00009352; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000266; SDB=6.01059861; UDB=6.00543984; IPR=6.00837781; MB=3.00022105; MTD=3.00000008; XFM=3.00000015; UTC=2018-07-11 16:47:36 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18071116-0069-0000-0000-000044FE940E Message-Id: <20180711164952.GA29994@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-07-11_04:,, 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=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1806210000 definitions=main-1807110179 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jul 11, 2018 at 07:43:03AM -0700, Paul E. McKenney wrote: > On Wed, Jul 11, 2018 at 03:23:45PM +0100, David Woodhouse wrote: > > > > > > On Mon, 2018-07-09 at 15:08 -0700, Paul E. McKenney wrote: > > > index f9c0ca2ccf0c..3350ece366ab 100644 > > > --- a/kernel/rcu/tree.c > > > +++ b/kernel/rcu/tree.c > > > @@ -2839,6 +2839,15 @@ void rcu_check_callbacks(int user) > > >                 rcu_bh_qs(); > > >         } > > >         rcu_preempt_check_callbacks(); > > > +       /* The load-acquire pairs with the store-release setting to true. */ > > > +       if (smp_load_acquire(this_cpu_ptr(&rcu_dynticks.rcu_urgent_qs))) { > > > +               /* Idle and userspace execution already are quiescent states. */ > > > +               if (rcu_is_cpu_rrupt_from_idle() && !user) { > > > > if (idle && !user) seems tautological and... illogical. > > > > If I make it 'if (!rcu_is_cpu_rrput_from_idle() && !user)' it seems to > > work better. Ripping out my debugging printks now to check that's still > > true... > > Right you are! I will step away for a bit to put a paper bag over > my head... > > > (Also, isn't userspace execution only a quiescent state if NO_HZ_FULL?) > > Userspace execution is a quiescent state in all cases. However, you > are quite right that NO_HZ_FULL makes a difference, namely, it allows > one CPU to reliably determine whether or not some other CPU is > currently executing either in userspace or in idle. > > Without NO_HZ_FULL, CPUs can only detect their own userspace execution. > Which is what is happening here because rcu_check_callbacks() is being > invoked from the scheduling-clock interrupt, which is where the "user" > parameter comes from. > > So the above code can reliably detect the usermode-execution quiescent > state because it is always running on the CPU in question. And here is an updated v4.15 patch with Marius's Reported-by and David's fix to my lost exclamation point. Thanx, Paul ------------------------------------------------------------------------ commit 83c4beae36f2a2b38c1a0fa80538af7ce2477823 Author: Paul E. McKenney Date: Mon Jul 9 13:47:30 2018 -0700 rcu: Make need_resched() respond to urgent RCU-QS needs The per-CPU rcu_dynticks.rcu_urgent_qs variable communicates an urgent need for an RCU quiescent state from the force-quiescent-state processing within the grace-period kthread to context switches and to cond_resched(). Unfortunately, such urgent needs are not communicated to need_resched(), which is sometimes used to decide when to invoke cond_resched(), for but one example, within the KVM vcpu_run() function. As of v4.15, this can result in synchronize_sched() being delayed by up to ten seconds, which can be problematic, to say nothing of annoying. This commit therefore checks rcu_dynticks.rcu_urgent_qs from within rcu_check_callbacks(), which is invoked from the scheduling-clock interrupt handler. If the current task is not an idle task and is not executing in usermode, a context switch is forced, and either way, the rcu_dynticks.rcu_urgent_qs variable is set to false. If the current task is an idle task, then RCU's dyntick-idle code will detect the quiescent state, so no further action is required. Similarly, if the task is executing in usermode, other code in rcu_check_callbacks() and its called functions will report the corresponding quiescent state. Reported-by: Marius Hillenbrand Reported-by: David Woodhouse Suggested-by: Peter Zijlstra Signed-off-by: Paul E. McKenney [ paulmck: Backported to v4.15. Probably applies elsewhere. ] diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index f9c0ca2ccf0c..de2f91cb2a0c 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -2839,6 +2839,15 @@ void rcu_check_callbacks(int user) rcu_bh_qs(); } rcu_preempt_check_callbacks(); + /* The load-acquire pairs with the store-release setting to true. */ + if (smp_load_acquire(this_cpu_ptr(&rcu_dynticks.rcu_urgent_qs))) { + /* Idle and userspace execution already are quiescent states. */ + if (!rcu_is_cpu_rrupt_from_idle() && !user) { + set_tsk_need_resched(current); + set_preempt_need_resched(); + } + __this_cpu_write(rcu_dynticks.rcu_urgent_qs, false); + } if (rcu_pending()) invoke_rcu_core(); if (user)