From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from smtp.codeaurora.org ([198.145.29.96]:46148 "EHLO smtp.codeaurora.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725951AbeHFKpm (ORCPT ); Mon, 6 Aug 2018 06:45:42 -0400 Date: Mon, 6 Aug 2018 14:07:36 +0530 From: Pavan Kondeti To: Sodagudi Prasad Cc: Peter Zijlstra , Thomas Gleixner , Sebastian Andrzej Siewior , isaacm@codeaurora.org, matt@codeblueprint.co.uk, mingo@kernel.org, linux-kernel@vger.kernel.org, gregkh@linuxfoundation.org, stable@vger.kernel.org Subject: Re: [PATCH] stop_machine: Disable preemption after queueing stopper threads Message-ID: <20180806083736.GA2661@codeaurora.org> References: <1531856129-9871-1-git-send-email-isaacm@codeaurora.org> <20180724062350.nlem2suuy5wlxpts@linutronix.de> <20180730112140.GH2494@hirez.programming.kicks-ass.net> <109d0e70606ccd34861a80525d6d11aa@codeaurora.org> <20180730210728.GQ2494@hirez.programming.kicks-ass.net> <52a34ed365cd560457e9abf5877c5b37@codeaurora.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <52a34ed365cd560457e9abf5877c5b37@codeaurora.org> Sender: stable-owner@vger.kernel.org List-ID: Hi Prasad, On Wed, Aug 01, 2018 at 01:07:03AM -0700, Sodagudi Prasad wrote: > On 2018-07-30 14:07, Peter Zijlstra wrote: > >On Mon, Jul 30, 2018 at 10:12:43AM -0700, Sodagudi Prasad wrote: > >>How about including below change as well? Currently, there is > >>no way to > >>identify thread migrations completed or not. When we observe > >>this issue, > >>the symptom was work queue lock up. It is better to have some > >>timeout here > >>and induce the bug_on. > > > >You'd trigger the soft-lockup or hung-task detector I think. And > >if not, > >we ought to look at making it trigger at least one of those. > > > >>There is no way to identify the migration threads stuck or not. > > > >Should be pretty obvious from the splat generated by the above, no? > Hi Peter and Thomas, > > Thanks for your support. > I have another question on this flow and retry mechanism used in > this cpu_stop_queue_two_works() function using the global variable > stop_cpus_in_progress. > > This variable is getting used in various paths, such as task > migration, set task affinity, and CPU hotplug. > > For example cpu hotplug path, stop_cpus_in_progress variable getting > set with true with out checking. > takedown_cpu() > --stop_machine_cpuslocked() > ---stop_cpus() > ---__stop_cpus() > ----queue_stop_cpus_work() > setting stop_cpus_in_progress to true directly. > > But in the task migration path only, the stop_cpus_in_progress > variable is used for retry. > > I am thinking that stop_cpus_in_progress variable lead race > conditions, where CPU hotplug and task migration happening > simultaneously. Please correct me If my understanding wrong. > The stop_cpus_in_progress variable is to guard against out of order queuing. The stopper locks does not protect this when cpu_stop_queue_two_works() and stop_cpus() are executing in parallel. stop_one_cpu_{nowait} functions are called to handle affinity change and load balance. Since we are queuing the work only on 1 CPU, stop_cpus_in_progress variable protection is not needed. Thanks, Pavan -- Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc. Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project.