From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754242AbdCOW6f (ORCPT ); Wed, 15 Mar 2017 18:58:35 -0400 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:35275 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754056AbdCOW6V (ORCPT ); Wed, 15 Mar 2017 18:58:21 -0400 Date: Wed, 15 Mar 2017 15:58:17 -0700 From: "Paul E. McKenney" To: Peter Zijlstra Cc: linux-kernel@vger.kernel.org, mingo@redhat.com, fweisbec@gmail.com Subject: Re: [PATCH] clock: Fix smp_processor_id() in preemptible bug Reply-To: paulmck@linux.vnet.ibm.com References: <20170308215306.GA8776@linux.vnet.ibm.com> <20170309152420.GC3343@twins.programming.kicks-ass.net> <20170309153114.GU30506@linux.vnet.ibm.com> <20170309183732.GB13748@linux.vnet.ibm.com> <20170313124621.GA3328@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170313124621.GA3328@twins.programming.kicks-ass.net> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 17031522-0036-0000-0000-000001AFD4B9 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00006789; HX=3.00000240; KW=3.00000007; PH=3.00000004; SC=3.00000206; SDB=6.00834367; UDB=6.00409742; IPR=6.00612024; BA=6.00005214; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000; ZP=6.00000000; ZH=6.00000000; ZU=6.00000002; MB=3.00014664; XFM=3.00000013; UTC=2017-03-15 22:58:18 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17031522-0037-0000-0000-00003EECF6DE Message-Id: <20170315225817.GA14170@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-03-15_11:,, 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=8.0.1-1702020001 definitions=main-1703150174 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Mar 13, 2017 at 01:46:21PM +0100, Peter Zijlstra wrote: > On Thu, Mar 09, 2017 at 10:37:32AM -0800, Paul E. McKenney wrote: > > And it does pass light testing. I will hammer it harder this evening. > > > > So please send a formal patch! > > Changed it a bit... > > --- > Subject: sched/clock: Some clear_sched_clock_stable() vs hotplug wobbles > > Paul reported two independent problems with clear_sched_clock_stable(). > > - if we tickle it during hotplug (even though the sched_clock was > already marked unstable) we'll attempt to schedule_work() and > this explodes because RCU isn't watching the new CPU quite yet. > > - since we run all of __clear_sched_clock_stable() from workqueue > context, there's a preempt problem. > > Cure both by only doing the static_branch_disable() from a workqueue, > and only when it's still stable. > > This leaves the problem what to do about hotplug actually wrecking TSC > though, because if it was stable and now isn't, then we will want to run > that work, which then will prod RCU the wrong way. Bloody hotplug. Would it help to do the same trick tglx applied to the hot-unplug path, that is IPIing some other CPU to schedule the workqueue? Thanx, Paul > Reported-by: "Paul E. McKenney" > Signed-off-by: Peter Zijlstra (Intel) > --- > kernel/sched/clock.c | 17 ++++++++++++----- > 1 file changed, 12 insertions(+), 5 deletions(-) > > diff --git a/kernel/sched/clock.c b/kernel/sched/clock.c > index a08795e21628..fec0f58c8dee 100644 > --- a/kernel/sched/clock.c > +++ b/kernel/sched/clock.c > @@ -141,7 +141,14 @@ static void __set_sched_clock_stable(void) > tick_dep_clear(TICK_DEP_BIT_CLOCK_UNSTABLE); > } > > -static void __clear_sched_clock_stable(struct work_struct *work) > +static void __sched_clock_work(struct work_struct *work) > +{ > + static_branch_disable(&__sched_clock_stable); > +} > + > +static DECLARE_WORK(sched_clock_work, __sched_clock_work); > + > +static void __clear_sched_clock_stable(void) > { > struct sched_clock_data *scd = this_scd(); > > @@ -160,11 +167,11 @@ static void __clear_sched_clock_stable(struct work_struct *work) > scd->tick_gtod, gtod_offset, > scd->tick_raw, raw_offset); > > - static_branch_disable(&__sched_clock_stable); > tick_dep_set(TICK_DEP_BIT_CLOCK_UNSTABLE); > -} > > -static DECLARE_WORK(sched_clock_work, __clear_sched_clock_stable); > + if (sched_clock_stable()) > + schedule_work(&sched_clock_work); > +} > > void clear_sched_clock_stable(void) > { > @@ -173,7 +180,7 @@ void clear_sched_clock_stable(void) > smp_mb(); /* matches sched_clock_init_late() */ > > if (sched_clock_running == 2) > - schedule_work(&sched_clock_work); > + __clear_sched_clock_stable(); > } > > void sched_clock_init_late(void) >