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Sat, 19 Sep 2020 14:55:35 +0000 Received: from BY5PR11MB4241.namprd11.prod.outlook.com ([fe80::3581:4d13:b613:eb81]) by BY5PR11MB4241.namprd11.prod.outlook.com ([fe80::3581:4d13:b613:eb81%7]) with mapi id 15.20.3391.019; Sat, 19 Sep 2020 14:55:35 +0000 Subject: Re: [PATCH v2] sched, mm: Optimize current_gfp_context() To: Waiman Long Cc: Andrew Morton , Peter Zijlstra , Ingo Molnar , Mathieu Desnoyers , Michel Lespinasse , linux-kernel@vger.kernel.org References: <20200618212936.9776-1-longman@redhat.com> <20200811152921.1d43e34ad7aff928c6c1267f@linux-foundation.org> From: "Xu, Yanfei" Message-ID: Date: Sat, 19 Sep 2020 22:55:28 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.7.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit X-ClientProxiedBy: HKAPR04CA0004.apcprd04.prod.outlook.com (2603:1096:203:d0::14) To BY5PR11MB4241.namprd11.prod.outlook.com (2603:10b6:a03:1ca::13) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from [128.224.162.160] (60.247.85.82) by HKAPR04CA0004.apcprd04.prod.outlook.com (2603:1096:203:d0::14) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.3391.11 via Frontend Transport; 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X-MS-Exchange-AntiSpam-MessageData: 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 X-OriginatorOrg: windriver.com X-MS-Exchange-CrossTenant-Network-Message-Id: 86209909-b0bf-4302-5ccd-08d85cac1067 X-MS-Exchange-CrossTenant-AuthSource: BY5PR11MB4241.namprd11.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 19 Sep 2020 14:55:35.6042 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 8ddb2873-a1ad-4a18-ae4e-4644631433be X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: iqcNffNiERkMOsGWydShBs+anmUtj/AprhwpGpDXpOx1XVSkuC2aZQfUFoim2KO2G+pPHeSAuW06Bnc1V+Dq+A== X-MS-Exchange-Transport-CrossTenantHeadersStamped: BY5PR11MB4226 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 9/18/20 11:18 PM, Waiman Long wrote: > On 9/18/20 2:44 AM, Xu, Yanfei wrote: >> Hi Waiman, >> >> On 8/12/20 6:29 AM, Andrew Morton wrote: >>> On Thu, 18 Jun 2020 17:29:36 -0400 Waiman Long >>> wrote: >>> >>>> The current_gfp_context() converts a number of PF_MEMALLOC_* >>>> per-process >>>> flags into the corresponding GFP_* flags for memory allocation. In >>>> that function, current->flags is accessed 3 times. That may lead to >>>> duplicated access of the same memory location. >>>> >> I have a puzzle about this comment, what's the meaning about "That may >> lead to duplicated access of the same memory location". After using >> variable 'pflags', will it not duplicated access the same memory >> location? >> Looking forward to your reply :) > > That condition usually won't happen on a non-debug kernel. However, if > certain debugging capability is turned on, access to current will be > compiled into a bunch of checks and memory accesses. So if current is > used multiple times, the same set of codes will be duplicated the same > number of times slowing down the operation and increasing code size. By > accessing current once, we avoid this overhead in a debug kernel. > > Cheers, > Longman > Hah， got it. Thanks for your detailed explain！ cheers， Yanfei