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=-5.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=no 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 D7122C4361B for ; Fri, 18 Dec 2020 04:06:07 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 9E9FD233A2 for ; Fri, 18 Dec 2020 04:06:07 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732037AbgLREFv (ORCPT ); Thu, 17 Dec 2020 23:05:51 -0500 Received: from mga01.intel.com ([192.55.52.88]:50611 "EHLO mga01.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726851AbgLREFv (ORCPT ); Thu, 17 Dec 2020 23:05:51 -0500 IronPort-SDR: +Vxzu50+9Acaorum5Si1d0lAYaQ3N4DQnaEAtiA3lGP86piOurtx4rVBJAPI+gtzK70qrNnsEa waC/LfErMccg== X-IronPort-AV: E=McAfee;i="6000,8403,9838"; a="193782108" X-IronPort-AV: E=Sophos;i="5.78,429,1599548400"; d="scan'208";a="193782108" Received: from orsmga002.jf.intel.com ([10.7.209.21]) by fmsmga101.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 17 Dec 2020 20:05:10 -0800 IronPort-SDR: H7Ptrfhyn4Q0/MQ290Py5RH1IOQkdFP9uA3u+tCGsO9MgVxuu6W0YKL6rjeHqs7mJDR3X8q6uC hP2pWdLfD4iQ== X-IronPort-AV: E=Sophos;i="5.78,429,1599548400"; d="scan'208";a="353728334" Received: from iweiny-desk2.sc.intel.com (HELO localhost) ([10.3.52.147]) by orsmga002-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 17 Dec 2020 20:05:10 -0800 Date: Thu, 17 Dec 2020 20:05:09 -0800 From: Ira Weiny To: Dave Hansen Cc: Thomas Gleixner , Ingo Molnar , Borislav Petkov , Andy Lutomirski , Peter Zijlstra , Dave Hansen , Fenghua Yu , x86@kernel.org, linux-kernel@vger.kernel.org, Andrew Morton , linux-doc@vger.kernel.org, linux-nvdimm@lists.01.org, linux-mm@kvack.org, linux-kselftest@vger.kernel.org, Dan Williams , Greg KH Subject: Re: [PATCH V3 10/10] x86/pks: Add PKS test code Message-ID: <20201218040509.GD1563847@iweiny-DESK2.sc.intel.com> References: <20201106232908.364581-1-ira.weiny@intel.com> <20201106232908.364581-11-ira.weiny@intel.com> <570ead2a-ff41-e730-d61d-0f59c67b1903@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <570ead2a-ff41-e730-d61d-0f59c67b1903@intel.com> User-Agent: Mutt/1.11.1 (2018-12-01) Precedence: bulk List-ID: X-Mailing-List: linux-kselftest@vger.kernel.org On Thu, Dec 17, 2020 at 12:55:39PM -0800, Dave Hansen wrote: > On 11/6/20 3:29 PM, ira.weiny@intel.com wrote: > > + /* Arm for context switch test */ > > + write(fd, "1", 1); > > + > > + /* Context switch out... */ > > + sleep(4); > > + > > + /* Check msr restored */ > > + write(fd, "2", 1); > > These are always tricky. What you ideally want here is: > > 1. Switch away from this task to a non-PKS task, or > 2. Switch from this task to a PKS-using task, but one which has a > different PKS value Or both... > > then, switch back to this task and make sure PKS maintained its value. > > *But*, there's no absolute guarantee that another task will run. It > would not be totally unreasonable to have the kernel just sit in a loop > without context switching here if no other tasks can run. > > The only way you *know* there is a context switch is by having two tasks > bound to the same logical CPU and make sure they run one after another. Ah... We do that. ... + CPU_ZERO(&cpuset); + CPU_SET(0, &cpuset); + /* Two processes run on CPU 0 so that they go through context switch. */ + sched_setaffinity(getpid(), sizeof(cpu_set_t), &cpuset); ... I think this should be ensuring that both the parent and the child are running on CPU 0. At least according to the man page they should be. A child created via fork(2) inherits its parent's CPU affinity mask. Perhaps a better method would be to synchronize the 2 threads more to ensure that we are really running at the 'same time' and forcing the context switch. > This just gets itself into a state where it *CAN* context switch and > prays that one will happen. Not sure what you mean by 'This'? Do you mean that running on the same CPU will sometimes not force a context switch? Or do you mean that the sleeps could be badly timed and the 2 threads could run 1 after the other on the same CPU? The latter is AFAICT the most likely case. > > You can also run a bunch of these in parallel bound to a single CPU. > That would also give you higher levels of assurance that *some* context > switch happens at sleep(). I think more cycles is a good idea for sure. But I'm more comfortable with forcing the test to be more synchronized so that it is actually running in the order we think/want it to be. > > One critical thing with these tests is to sabotage the kernel and then > run them and make *sure* they fail. Basically, if you screw up, do they > actually work to catch it? I'll try and come up with a more stressful test. Ira