From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757137AbcHYUYF (ORCPT ); Thu, 25 Aug 2016 16:24:05 -0400 Received: from mail-by2nam03on0112.outbound.protection.outlook.com ([104.47.42.112]:3040 "EHLO NAM03-BY2-obe.outbound.protection.outlook.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1755510AbcHYUYB (ORCPT ); Thu, 25 Aug 2016 16:24:01 -0400 Authentication-Results: spf=none (sender IP is ) smtp.mailfrom=waiman.long@hpe.com; Message-ID: <57BF4FFD.5060901@hpe.com> Date: Thu, 25 Aug 2016 16:07:25 -0400 From: Waiman Long User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:10.0.12) Gecko/20130109 Thunderbird/10.0.12 MIME-Version: 1.0 To: Dave Hansen CC: Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , , , Borislav Petkov , Andy Lutomirski , Prarit Bhargava , Scott J Norton , Douglas Hatch , Randy Wright Subject: Re: [PATCH v6] x86/hpet: Reduce HPET counter read contention References: <1471049968-63172-1-git-send-email-Waiman.Long@hpe.com> <57BF3DE6.1090005@intel.com> In-Reply-To: <57BF3DE6.1090005@intel.com> Content-Type: text/plain; charset="windows-1252"; format=flowed Content-Transfer-Encoding: 7bit X-Originating-IP: [72.71.243.247] X-ClientProxiedBy: BLUPR0401CA0027.namprd04.prod.outlook.com (10.162.114.165) To TU4PR84MB0318.NAMPRD84.PROD.OUTLOOK.COM (10.162.186.28) X-MS-Office365-Filtering-Correlation-Id: 7adeb2aa-3f27-433f-4754-08d3cd2371e0 X-Microsoft-Exchange-Diagnostics: 1;TU4PR84MB0318;2:/X/v5tioDEHelMtmWxk6dOCKNu9G+oIvFyRXOkbJUNE1JJ6ZOWJihXrNPey1AgfbURn0SQGJSe397S1h5Oha1z3F1KkBsd5MWbe+WSxhxHtDQ3ArJNaZiobWLApnqz0XrbeP9WUnafSD3+1JD7Jz7LpboyiJ/IAhbMAJ8NgN4u11hZ0f4eb0ochtOZxiSz4w;3:fxGx2Uh7Y2t0AFI5pZbXaR/Tv4jdbxxYy3mSpj6a83umG1FJQ+bkUUrZsH3noSqEjLsp3FlkBHlfBPplgtqmfDXh1kM7JLA8GXxGiVdrVoI0fBJ2b9DYnEfMIh+L7g3D X-Microsoft-Antispam: UriScan:;BCL:0;PCL:0;RULEID:;SRVR:TU4PR84MB0318; 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It is explicitly assumed that arch_spinlock_t >> + * is 32 bits in size. > This requirement forces us to give up all of the goodness of lockdep. > Is this strictly a performance optimization or is there some function > requirement behind it as well? Yes, it is mostly performance optimization. Using a full spinlock will require additional synchronization code like a memory barrier to prevent race between the lock and HPET value with respect to the readers. It is a simple lock that won't have additional locks nested inside. So I wonder if there is any value in having the lockdep functionality for this lock. Cheers, Longman