From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx0a-001b2d01.pphosted.com (mx0a-001b2d01.pphosted.com [148.163.156.1]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 3vH9s440SjzDqG9 for ; Tue, 7 Feb 2017 02:10:47 +1100 (AEDT) Received: from pps.filterd (m0098410.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.20/8.16.0.20) with SMTP id v16F61tM011217 for ; Mon, 6 Feb 2017 10:10:45 -0500 Received: from e35.co.us.ibm.com (e35.co.us.ibm.com [32.97.110.153]) by mx0a-001b2d01.pphosted.com with ESMTP id 28dccdad01-1 (version=TLSv1.2 cipher=AES256-SHA bits=256 verify=NOT) for ; Mon, 06 Feb 2017 10:10:45 -0500 Received: from localhost by e35.co.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Mon, 6 Feb 2017 08:10:44 -0700 Date: Mon, 6 Feb 2017 07:10:48 -0800 From: "Paul E. McKenney" To: Sachin Sant Cc: Peter Zijlstra , Matt Fleming , "linuxppc-dev@lists.ozlabs.org" , Mike Galbraith , LKML , "linux-next@vger.kernel.org" , Ross Zwisler Subject: Re: [tip:sched/core] sched/core: Add debugging code to catch missing update_rq_clock() calls Reply-To: paulmck@linux.vnet.ibm.com References: <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> <45DC157B-EA1A-4E1C-9449-8DD317837FCB@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <45DC157B-EA1A-4E1C-9449-8DD317837FCB@linux.vnet.ibm.com> Message-Id: <20170206151048.GK30506@linux.vnet.ibm.com> List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Mon, Feb 06, 2017 at 11:53:10AM +0530, Sachin Sant wrote: > > >>> 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. Thank you for testing, Sachin! Could you please try building and testing with CONFIG_RCU_BOOST=y? You will need to enable CONFIG_RCU_EXPERT=y to see this Kconfig option. Thanx, Paul