From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751455AbaDUNl5 (ORCPT ); Mon, 21 Apr 2014 09:41:57 -0400 Received: from mx1.redhat.com ([209.132.183.28]:8932 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751367AbaDUNlz (ORCPT ); Mon, 21 Apr 2014 09:41:55 -0400 Date: Mon, 21 Apr 2014 15:41:54 +0200 From: Oleg Nesterov To: Don Zickus Cc: Aaron Tomlin , mingo@redhat.com, mingo@kernel.com, linux-kernel@vger.kernel.org Subject: [PATCH 1/4 v2] nmi: Provide the option to issue an NMI back trace to every cpu but current Message-ID: <20140421134154.GA13766@redhat.com> References: <20140415075650.GG2195@atomlin.usersys.redhat.com> <20140415170202.GA7948@redhat.com> <20140415172649.GA10152@redhat.com> <20140421132100.GW8488@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140421132100.GW8488@redhat.com> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 04/21, Don Zickus wrote: > > On Tue, Apr 15, 2014 at 07:26:49PM +0200, Oleg Nesterov wrote: > > On 04/15, Oleg Nesterov wrote: > > > > > > Looking at https://lkml.org/lkml/2014/4/4/469... It seems that 2/4 can be > > > simplified, you can simply remove smp_processor_id() from backtrace_mask > > > if !include_self and use apic->send_IPI_mask(backtrace_mask). But this is > > > minor, I won't insist. > > > > And in fact, I do not understand why arch_trigger_all_cpu_backtrace() doesn't > > disable preemption. OK, probably we can simply ignore the race with cpu hotplug. > > > > But it seems that your patch makes the things worse. Lets look at, say, > > numachip_send_IPI_mask_allbutself(). The usage of smp_processor_id() is > > obviously racy but perhaps we do not care again. But we do not want a warning > > from debug_smp_processor_id(). > > Good point. I forgot that going from all cpus down to allbutself, > preemption now matters. I am not sure it actually matters wrt "show other CPU's traces". If the preemption is possible then the caller can be preempted even before it sends ipi. OTOH I think it does matter anyway, even without your patch, otherwise the usage of cpu_online_mask is racy and we can hit the "Wait for up to 10 seconds" case. Btw... /* Wait for up to 10 seconds for all CPUs to do the backtrace */ for (i = 0; i < 10 * 1000; i++) { if (cpumask_empty(to_cpumask(backtrace_mask))) break; mdelay(1); } OK, but perhaps we should clear backtrace_mask if we return due to timeout. > does disabling preemption help in the cpu > hotplug case? Yes. But I'd suggest to change your patch to use get_cpu() instead of preempt_disable/smp_processor_id. And I think it would be better to not discuss this off-list, I added lkml. Oleg.