From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 51C01C282DD for ; Tue, 23 Apr 2019 20:56:34 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 2A17621773 for ; Tue, 23 Apr 2019 20:56:34 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727812AbfDWU4U (ORCPT ); Tue, 23 Apr 2019 16:56:20 -0400 Received: from mga14.intel.com ([192.55.52.115]:9121 "EHLO mga14.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726083AbfDWU4U (ORCPT ); Tue, 23 Apr 2019 16:56:20 -0400 X-Amp-Result: UNKNOWN X-Amp-Original-Verdict: FILE UNKNOWN X-Amp-File-Uploaded: False Received: from orsmga007.jf.intel.com ([10.7.209.58]) by fmsmga103.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 23 Apr 2019 13:56:18 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.60,387,1549958400"; d="scan'208";a="133774408" Received: from romley-ivt3.sc.intel.com ([172.25.110.60]) by orsmga007.jf.intel.com with ESMTP; 23 Apr 2019 13:56:17 -0700 Date: Tue, 23 Apr 2019 13:48:10 -0700 From: Fenghua Yu To: Thomas Gleixner Cc: Ingo Molnar , Borislav Petkov , H Peter Anvin , Paolo Bonzini , Dave Hansen , Ashok Raj , Peter Zijlstra , Ravi V Shankar , Xiaoyao Li , Christopherson Sean J , Kalle Valo , Michael Chan , linux-kernel , x86 , kvm@vger.kernel.org, netdev@vger.kernel.org, linux-wireless@vger.kernel.org Subject: Re: [PATCH v7 15/21] x86/split_lock: Add a sysfs interface to enable/disable split lock detection during run time Message-ID: <20190423204809.GD18776@romley-ivt3.sc.intel.com> References: <1555536851-17462-1-git-send-email-fenghua.yu@intel.com> <1555536851-17462-16-git-send-email-fenghua.yu@intel.com> <20190418005711.GB18776@romley-ivt3.sc.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-wireless-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-wireless@vger.kernel.org On Thu, Apr 18, 2019 at 08:41:30AM +0200, Thomas Gleixner wrote: > On Wed, 17 Apr 2019, Fenghua Yu wrote: > > On Thu, Apr 18, 2019 at 12:47:24AM +0200, Thomas Gleixner wrote: > > > On Wed, 17 Apr 2019, Fenghua Yu wrote: > > > > > > > The interface /sys/device/system/cpu/split_lock_detect is added > > > > to allow user to control split lock detection and show current split > > > > lock detection setting. > > > > > > > > Writing [yY1] or [oO][nN] to the file enables split lock detection and > > > > writing [nN0] or [oO][fF] disables split lock detection. Split lock > > > > detection is enabled or disabled on all CPUs. > > > > > > > > Reading the file returns current global split lock detection setting: > > > > 0: disabled > > > > 1: enabled > > > > > > Again, You explain WHAT this patch does and still there is zero > > > justification why this sysfs knob is needed at all. I still do not see any > > > reason why this knob should exist. > > > > An important application has split lock issues which are already discovered > > and need to be fixed. But before the issues are fixed, sysadmin still wants to > > run the application without rebooting the system, the sysfs knob can be useful > > to turn off split lock detection. After the application is done, split lock > > detection will be enabled again through the sysfs knob. > > Are you sure that you are talking about the real world? I might buy the > 'off' part somehow, but the 'on' part is beyond theoretical. > > Even the 'off' part is dubious on a multi user machine. I personally would > neither think about using the sysfs knob nor about rebooting the machine > simply because I'd consider a lock operation accross a cacheline an malicious > DoS attempt. Why would I allow that? > > So in reality the sysadmin will either move the workload to a machine w/o > the #AC magic or just tell the user to fix his crap. > > > Without the sysfs knob, sysadmin has to reboot the system with kernel option > > "no_split_lock_detect" to run the application before the split lock issues > > are fixed. > > > > Is this a valid justification why the sysfs knob is needed? If it is, I can > > add the justification in the next version. > > Why has this information not been in the changelog right away? I'm really > tired of asking the same questions and pointing you to > Documentation/process over and over. So should I remove the sysfs knob patches in the next version? Or add the following justification and still keep the sysfs knob patches? "To workaround or debug a split lock issue, the administrator may need to disable or enable split lock detection during run time without rebooting the system." Thanks. -Fenghua