From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933836AbaCSLvV (ORCPT ); Wed, 19 Mar 2014 07:51:21 -0400 Received: from mx1.redhat.com ([209.132.183.28]:13599 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933380AbaCSLvU (ORCPT ); Wed, 19 Mar 2014 07:51:20 -0400 Message-ID: <532984A9.8080001@redhat.com> Date: Wed, 19 Mar 2014 07:51:05 -0400 From: Prarit Bhargava User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:10.0.12) Gecko/20130108 Thunderbird/10.0.12 MIME-Version: 1.0 To: Igor Mammedov CC: linux-kernel@vger.kernel.org, tglx@linutronix.de, mingo@redhat.com, hpa@zytor.com, bp@suse.de, paul.gortmaker@windriver.com, JBeulich@suse.com, drjones@redhat.com, toshi.kani@hp.com, x86@kernel.org, riel@redhat.com, gong.chen@linux.intel.com Subject: Re: [PATCH 0/3] x86: fix hang when AP bringup is too slow References: <1394720720-8484-1-git-send-email-imammedo@redhat.com> <53283A3F.6040302@redhat.com> <20140318194951.17fd61ea@thinkpad> In-Reply-To: <20140318194951.17fd61ea@thinkpad> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 03/18/2014 02:49 PM, Igor Mammedov wrote: > On Tue, 18 Mar 2014 08:21:19 -0400 > Prarit Bhargava wrote: > >> >> >> On 03/13/2014 10:25 AM, Igor Mammedov wrote: >>> Hang is observed on virtual machines during CPU hotplug, >>> especially in big guests with many CPUs. (It happens more >>> often if host is over-committed). >>> >> >> Hey Igor, I like this better than the previous version. Thanks for taking into >> account the possible races in this code. >> >> A quick question on system behaviour. As you know I've been more concerned >> lately with error handling, etc., through the cpu hotplug code as we've seen >> several customer reports of silent failures or cascading failures in the cpu >> hotplug code when users have been attempting to perform physical hotplug. >> >> After your patches have been applied, in theory the following can happen: >> >> The master CPU is completing the AP cpu's bring up. The AP cpu is doing (sorry >> for the cut-and-paste), >> >> void cpu_init(void) >> { >> int cpu = smp_processor_id(); >> struct task_struct *curr = current; >> struct tss_struct *t = &per_cpu(init_tss, cpu); >> struct thread_struct *thread = &curr->thread; >> >> /* >> * wait till the master CPU completes it's STARTUP sequence, >> * and decides to wait till this AP boots >> */ >> while (!cpumask_test_cpu(cpu, cpu_callout_mask)) { >> cpu_relax(); >> if (per_cpu(x86_cpu_to_apicid, cpu) == BAD_APICID) >> halt(); >> } >> >> and is spinning on cpu_relax(). Suppose something goes wrong and the softlockup >> watchdog fires on the AP cpu: >> >> 1. Can it? :) ie) will the softlockup fire at this point of the AP init? Okay, >> I'm being really lazy and not looking at the code ;) > It shouldn't, CPU is in pristine state and just came from boot trampoline at > this point without interrupts configured yet. Okay, not a big problem. > >> >> 2. Is there anything we can do in this code to notify the user of a problem? >> Even a pr_crit() here I think would help to indicate what went wrong; it might >> be useful for future debugging in this area to have some sort of output. I >> think a WARN() or BUG() is necessary here as there are several calls to cpu_init(). > Do you mean something like this: > > + if (per_cpu(x86_cpu_to_apicid, cpu) == BAD_APICID) { > + WARN(1); > + halt(); > + } Yeah, maybe WARN_ON(1, "some comment") though. > >> >> 3. Change this comment: >> >> * wait till the master CPU completes it's STARTUP sequence, >> * and decides to wait till this AP boots >> >> to >> >> /* wait for the master CPU to complete this cpu's STARTUP. */ ? > well, that is not quite the same as above, comment should underline that > AP waits for ACK from master CPU before continuing with this AP initialization. > > How about: > /* wait for ACK from master CPU before continuing with AP initialization */ Awesome :) P. > >> >> Apologies for the late review, >> >> P. > >