From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753969AbcBOTDi (ORCPT ); Mon, 15 Feb 2016 14:03:38 -0500 Received: from foss.arm.com ([217.140.101.70]:47494 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752242AbcBOTDg (ORCPT ); Mon, 15 Feb 2016 14:03:36 -0500 Subject: Re: Crashes in arm qemu emulations due to 'cpufreq: governor: Replace timers with utilization ...' To: "Rafael J. Wysocki" References: <20160215170527.GA24453@roeck-us.net> <56C21DD0.40508@arm.com> Cc: Guenter Roeck , Viresh Kumar , "Rafael J. Wysocki" , linux-next@vger.kernel.org, Linux Kernel Mailing List , "linux-arm-kernel@lists.infradead.org" , "linux-pm@vger.kernel.org" , Peter Zijlstra From: Marc Zyngier Organization: ARM Ltd Message-ID: <56C22105.6050900@arm.com> Date: Mon, 15 Feb 2016 19:03:33 +0000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Icedove/38.5.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 15/02/16 18:54, Rafael J. Wysocki wrote: > On Mon, Feb 15, 2016 at 7:49 PM, Marc Zyngier wrote: >> On 15/02/16 18:41, Rafael J. Wysocki wrote: >>> On Mon, Feb 15, 2016 at 6:05 PM, Guenter Roeck wrote: >>>> Rafael, >>> >>> Hi, >>> >>> Thanks for the report! >>> >>>> I see crashes in various arm qemu tests due to 'cpufreq: governor: Replace >>>> timers with utilization update callbacks' with next-20160215. An example >>>> crash log and bisect results are attached below. >>>> >>>> Please let me know if there is anything I can do to help tracking down >>>> the problem. >>> >>> It looks like we've uncovered some nastiness in the arch ARM code (see below). >>> >>> [cut] >>> >>>> [ 1.340000] Unable to handle kernel NULL pointer dereference at virtual address 00000000 >>>> [ 1.340000] pgd = c0204000 >>>> [ 1.340000] [00000000] *pgd=00000000 >>>> [ 1.340000] Internal error: Oops: 80000005 [#1] SMP ARM >>>> [ 1.340000] Modules linked in: >>>> [ 1.340000] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.5.0-rc4-next-20160215 #1 >>>> [ 1.340000] Hardware name: Generic OMAP3-GP (Flattened Device Tree) >>>> [ 1.340000] task: cb060000 ti: cb05a000 task.ti: cb05a000 >>>> [ 1.340000] PC is at 0x0 >>>> [ 1.340000] LR is at arch_send_call_function_single_ipi+0x34/0x38 >>> >>> Since this is ARM, arch_send_call_function_single_ipi() looks like this: >>> >>> void arch_send_call_function_single_ipi(int cpu) >>> { >>> smp_cross_call(cpumask_of(cpu), IPI_CALL_FUNC_SINGLE); >>> } >>> >>> so I'm not sure how the NULL pointer deref is possible even. >>> >>> The only thing coming to mind would be that cpumask_of(cpu) triggers >>> this, but I'm not sure how exactly that can happen. >>> >>> I need help from somebody who knows how this low-level stuff works on ARM. >> >> Given that OMAP3 is a UP system, there is zero chance that it has >> registered the magic hook that delivers IPIs (its interrupt controller >> is not even capable of doing so). >> >> I don't really know the context, but IPIs on a UP system seem at best odd. > > That would explain it, thanks. > > So it looks like we should always use irq_work_queue() on UP even if > CONFIG_SMP is set, shouldn't we? Something like that, yes. CONFIG_SMP is not an indication of an SMP system anymore (we've even dropped the config option on arm64). Hopefully num_possible_cpus() is reliable enough to let you do the right thing... Thanks, M. -- Jazz is not dead. It just smells funny...