From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756119AbdIRQYT (ORCPT ); Mon, 18 Sep 2017 12:24:19 -0400 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:36258 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1754643AbdIRQYR (ORCPT ); Mon, 18 Sep 2017 12:24:17 -0400 Date: Mon, 18 Sep 2017 09:24:12 -0700 From: "Paul E. McKenney" To: Steven Rostedt Cc: Neeraj Upadhyay , josh@joshtriplett.org, mathieu.desnoyers@efficios.com, jiangshanlai@gmail.com, linux-kernel@vger.kernel.org, sramana@codeaurora.org, prsood@codeaurora.org, pkondeti@codeaurora.org, markivx@codeaurora.org, peterz@infradead.org Subject: Re: Query regarding synchronize_sched_expedited and resched_cpu Reply-To: paulmck@linux.vnet.ibm.com References: <8f33e48e-ac6d-2c88-e16f-20b698c06292@codeaurora.org> <20170917010015.GW3521@linux.vnet.ibm.com> <20170918111105.15f687da@gandalf.local.home> <20170918160125.GL3521@linux.vnet.ibm.com> <20170918121213.312c82b0@gandalf.local.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170918121213.312c82b0@gandalf.local.home> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 17091816-0052-0000-0000-000002634DD9 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00007757; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000230; SDB=6.00918862; UDB=6.00461609; IPR=6.00699089; BA=6.00005595; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000; ZP=6.00000000; ZH=6.00000000; ZU=6.00000002; MB=3.00017196; XFM=3.00000015; UTC=2017-09-18 16:24:14 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17091816-0053-0000-0000-0000520C13B6 Message-Id: <20170918162412.GM3521@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-09-18_06:,, 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=8.0.1-1707230000 definitions=main-1709180233 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Sep 18, 2017 at 12:12:13PM -0400, Steven Rostedt wrote: > On Mon, 18 Sep 2017 09:01:25 -0700 > "Paul E. McKenney" wrote: > > > > sched: Make resched_cpu() unconditional > > > > The current implementation of synchronize_sched_expedited() incorrectly > > assumes that resched_cpu() is unconditional, which it is not. This means > > that synchronize_sched_expedited() can hang when resched_cpu()'s trylock > > fails as follows (analysis by Neeraj Upadhyay): > > > > o CPU1 is waiting for expedited wait to complete: > > sync_rcu_exp_select_cpus > > rdp->exp_dynticks_snap & 0x1 // returns 1 for CPU5 > > IPI sent to CPU5 > > > > synchronize_sched_expedited_wait > > ret = swait_event_timeout( > > rsp->expedited_wq, > > sync_rcu_preempt_exp_done(rnp_root), > > jiffies_stall); > > > > expmask = 0x20 , and CPU 5 is in idle path (in cpuidle_enter()) > > > > o CPU5 handles IPI and fails to acquire rq lock. > > > > Handles IPI > > sync_sched_exp_handler > > resched_cpu > > returns while failing to try lock acquire rq->lock > > need_resched is not set > > > > o CPU5 calls rcu_idle_enter() and as need_resched is not set, goes to > > idle (schedule() is not called). > > > > o CPU 1 reports RCU stall. > > > > Given that resched_cpu() is used only by RCU, this commit fixes the > > assumption by making resched_cpu() unconditional. > > Probably want to run this with several workloads with lockdep enabled > first. As soon as I work through the backlog of lockdep complaints that appeared in the last merge window... :-( sparse_irq_lock, I am looking at you!!! ;-) Thanx, Paul