From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751154AbdBFGXb convert rfc822-to-8bit (ORCPT ); Mon, 6 Feb 2017 01:23:31 -0500 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:48001 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1750937AbdBFGX3 (ORCPT ); Mon, 6 Feb 2017 01:23:29 -0500 Content-Type: text/plain; charset=us-ascii Mime-Version: 1.0 (Mac OS X Mail 10.2 \(3259\)) Subject: Re: [tip:sched/core] sched/core: Add debugging code to catch missing update_rq_clock() calls From: Sachin Sant In-Reply-To: <20170203155453.GA23760@linux.vnet.ibm.com> Date: Mon, 6 Feb 2017 11:53:10 +0530 Cc: Peter Zijlstra , Matt Fleming , "linuxppc-dev@lists.ozlabs.org" , Mike Galbraith , LKML , "linux-next@vger.kernel.org" , Ross Zwisler Content-Transfer-Encoding: 8BIT References: <20170130213435.GM31613@codeblueprint.co.uk> <33118425-8548-475A-8DED-9A2424C0BCCC@linux.vnet.ibm.com> <1485863317.4498.2.camel@gmx.de> <20170202155506.GX6515@twins.programming.kicks-ass.net> <3BAB5673-2BF8-46E0-8018-6A0F3DEEC9F7@linux.vnet.ibm.com> <20170203085301.GA6515@twins.programming.kicks-ass.net> <1486126774.4277.6.camel@gmx.de> <20170203133748.GB6515@twins.programming.kicks-ass.net> <20170203154457.GJ30506@linux.vnet.ibm.com> <20170203155453.GA23760@linux.vnet.ibm.com> To: paulmck@linux.vnet.ibm.com X-Mailer: Apple Mail (2.3259) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 17020606-0024-0000-0000-000003989E00 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17020606-0025-0000-0000-00001112B9C1 Message-Id: <45DC157B-EA1A-4E1C-9449-8DD317837FCB@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-02-06_02:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 suspectscore=1 malwarescore=0 phishscore=0 adultscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1612050000 definitions=main-1702060064 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org >>> I've seen it on tip. It looks like hot unplug goes really slow when >>> there's running tasks on the CPU being taken down. >>> >>> What I did was something like: >>> >>> taskset -p $((1<<1)) $$ >>> for ((i=0; i<20; i++)) do while :; do :; done & done >>> >>> taskset -p $((1<<0)) $$ >>> echo 0 > /sys/devices/system/cpu/cpu1/online >>> >>> And with those 20 tasks stuck sucking cycles on CPU1, the unplug goes >>> _really_ slow and the RCU stall triggers. What I suspect happens is that >>> hotplug stops participating in the RCU state machine early, but only >>> tells RCU about it really late, and in between it gets suspicious it >>> takes too long. >>> >>> I've yet to dig through the RCU code to figure out the exact sequence of >>> events, but found the above to be fairly reliable in triggering the >>> issue. > >> If you send me the full splat from the dmesg and the RCU portions of >> .config, I will take a look. Is this new behavior, or a new test? > I have sent the required files to you via separate email. > If new behavior, I would be most suspicious of these commits in -rcu which > recently entered -tip: > > 19e4d983cda1 rcu: Place guard on rcu_all_qs() and rcu_note_context_switch() actions > 913324b1364f rcu: Eliminate flavor scan in rcu_momentary_dyntick_idle() > fcdcfefafa45 rcu: Pull rcu_qs_ctr into rcu_dynticks structure > 0919a0b7e7a5 rcu: Pull rcu_sched_qs_mask into rcu_dynticks structure > caa7c8e34293 rcu: Make rcu_note_context_switch() do deferred NOCB wakeups > 41e4b159d516 rcu: Make rcu_all_qs() do deferred NOCB wakeups > b457a3356a68 rcu: Make call_rcu() do deferred NOCB wakeups > > Does reverting any of these help? I tried reverting the above commits. That does not help. I can still recreate the issue. Thanks -Sachin