From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from smtp.codeaurora.org ([198.145.29.96]:39666 "EHLO smtp.codeaurora.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1733173AbeHAJvc (ORCPT ); Wed, 1 Aug 2018 05:51:32 -0400 MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII; format=flowed Content-Transfer-Encoding: 7bit Date: Wed, 01 Aug 2018 01:07:03 -0700 From: Sodagudi Prasad To: Peter Zijlstra Cc: Thomas Gleixner , Sebastian Andrzej Siewior , isaacm@codeaurora.org, matt@codeblueprint.co.uk, mingo@kernel.org, linux-kernel@vger.kernel.org, gregkh@linuxfoundation.org, pkondeti@codeaurora.org, stable@vger.kernel.org Subject: Re: [PATCH] stop_machine: Disable preemption after queueing stopper threads In-Reply-To: <20180730210728.GQ2494@hirez.programming.kicks-ass.net> 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> Message-ID: <52a34ed365cd560457e9abf5877c5b37@codeaurora.org> Sender: stable-owner@vger.kernel.org List-ID: 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. -Thanks, Prasad > >> --- a/kernel/stop_machine.c >> +++ b/kernel/stop_machine.c >> @@ -290,6 +290,7 @@ int stop_two_cpus(unsigned int cpu1, unsigned int >> cpu2, >> cpu_stop_fn_t fn, void * >> struct cpu_stop_done done; >> struct cpu_stop_work work1, work2; >> struct multi_stop_data msdata; >> + int ret; >> >> msdata = (struct multi_stop_data){ >> .fn = fn, >> @@ -312,7 +313,10 @@ int stop_two_cpus(unsigned int cpu1, unsigned int >> cpu2, >> cpu_stop_fn_t fn, void * >> if (cpu_stop_queue_two_works(cpu1, &work1, cpu2, &work2)) >> return -ENOENT; >> >> - wait_for_completion(&done.completion); >> + ret = wait_for_completion_timeout(&done.completion, >> msecs_to_jiffies(1000)); >> + if (!ret) >> + BUG_ON(1); >> + > > That's a random timeout, which if you spuriously trigger it, will take > down your machine. That seems like a cure worse than the disease. -- The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, Linux Foundation Collaborative Project