From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753578AbdK0Tg4 convert rfc822-to-8bit (ORCPT ); Mon, 27 Nov 2017 14:36:56 -0500 Received: from mx1.redhat.com ([209.132.183.28]:59840 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753086AbdK0Tgz (ORCPT ); Mon, 27 Nov 2017 14:36:55 -0500 Subject: Re: [RFC PATCH 2/2] lib: debugobjects: touch watchdog to avoid softlockup when !CONFIG_PREEMPT To: Yang Shi , tglx@linutronix.de Cc: linux-kernel@vger.kernel.org References: <1510947833-116482-1-git-send-email-yang.s@alibaba-inc.com> <1510947833-116482-2-git-send-email-yang.s@alibaba-inc.com> <553877ed-843e-a59d-3a76-2e90ce192ab1@alibaba-inc.com> <1a6f92f5-2c12-1276-916b-fd2848899e5c@redhat.com> From: Waiman Long Organization: Red Hat Message-ID: Date: Mon, 27 Nov 2017 14:36:52 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8BIT Content-Language: en-US X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.32]); Mon, 27 Nov 2017 19:36:55 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/27/2017 01:52 PM, Yang Shi wrote: > > > On 11/27/17 10:18 AM, Waiman Long wrote: >> On 11/27/2017 12:54 PM, Yang Shi wrote: >>> Hi Waiman, >>> >>> The second patch of this series. >>> >>> Thanks, >>> Yang >>> >>> >>> On 11/17/17 11:43 AM, Yang Shi wrote: >>>> There are nested loops on debug objects free path, sometimes it may >>>> take >>>> over hundred thousands of loops, then cause soft lockup with >>>> !CONFIG_PREEMPT >>>> occasionally, like below: >>>> >>>> ... >>>> >>>> The code path might be called in either atomic or non-atomic context, >>>> so touching softlockup watchdog instead of calling cond_resched() >>>> which >>>> might fall asleep. However, it is unnecessary to touch the watchdog >>>> every loop, so just touch the watchdog at every 10000 (best estimate) >>>> loops. >>>> >>>> Signed-off-by: Yang Shi >> >> I do have some concern about suppressing the soft lockup warning >> entirely. If the system feels unresponsive for a certain period of time >> (e.g. 22s), most users would like to know what is going on. It can be a >> custom message with less scary warning. Alternatively, some opt-out > > I'm not sure if it is necessary for debug code since the > unresponsiveness is introduced by debug config and is expected > somehow, so the user is supposed to know what they are doing, and it > sounds preferred to disregard the soft lockup message reported by > object debug for the most time. > Yes, it may be normal to cause the soft lockup while running some kind of stress tests. However, it isn't normal for a real user application to cause the lockup on a debug kernel. That is why I am suggesting some kind of opt-out mechanism so that one can turn off the soft lockup warning for stress testing. > We do have some other examples which suppress soft lockup completely > in kernel, i.e. kdb debug, printing some verbose debug or error > message, some slow driver code, etc. > >> mechanism can be added to explicitly disable soft lookup warning for >> debugobjs is OK as long as it is not the default. > If we really want to some opt-out, we should be able to add a proc > knob to disable soft lockup as the patch does, but not default. If > this is too overkilling, we may just add some comment in the Kconfig > help text to tell users the side effect. You may add a new debugfs file under the debug_objects sub-directory for suppressing the soft lockup message. Cheers, Longman